1 Introduction

Urban areas across the globe are struggling with a growing problem of increasing air pollution, which presents significant health hazards and environmental challenges (Molina & Molina, 2004). Air pollution provides a substantial risk to global health, leading to millions of deaths from respiratory diseases caused by pollutants. The swift progress has resulted in a concerning deterioration in the air's quality. Lahore, a prominent urban center in Pakistan, experiences significant air pollution as a result of rapid industrial growth, high levels of vehicle emissions, population expansion and various other human activities (Anjum et al., 2021). The increase in emissions from vehicles, discharge from industrial sources, burning of fossil fuels, poor management of petroleum products and refineries (Goodsite et al., 2021), and other urban activities has significantly raised the concentrations of airborne contaminants. This has resulted in significant health hazards for the residents and the surrounding ecology. Air pollution, particularly in cities, is widely recognized as a significant and urgent concern in contemporary times. Exposure to air pollution is linked to an annual occurrence of over 8.34 million deaths (Lelieveld et al., 2023). Inhalation and skin contact are the principal routes of metal intake in humans since air is the primary medium of interaction. Ambient, anthropogenic, and various routes of exposure all play a role in risk assessment (Morawska et al., 2013). Approximately 90% of individuals worldwide, particularly those residing in urban areas, are exposed to air that does not meet the air quality levels deemed satisfactory by the World Health Organization (WHO) per the guidelines for maintaining respiratory health (WHO, 2005). According to research, a significant majority of the global population, around 99%, is exposed to air quality that falls below the established guidelines set by the World Health Organization (WHO, 2006). Given the well-established link between air pollution and severe health outcomes, like the millions of annual deaths attributed to it, robust and continuous monitoring systems become essential. Spotty data from limited air quality stations hinders ability to comprehensively understand the spatial and temporal distribution of pollutants, especially in rapidly developing cities. This lack of comprehensive data makes it difficult to establish effective public health interventions and mitigation strategies. Conventional air quality monitoring stations are often expensive and limited in their spatial coverage. These methods, while effective, typically necessitate the use of advanced and costly equipment, which restricts their widespread and uninterrupted implementation (Sokhi et al., 2022). However, air quality monitoring stations, although essential for collecting centralized data, frequently have constraints in terms of spatial coverage. Installing and maintaining these stations is costly, and their stationary positions may not accurately measure the detailed fluctuations in pollution levels around a city. The scarcity of data poses challenges for academics and policymakers in identifying specific areas with high pollution levels, monitoring long-term patterns, and formulating successful measures to reduce pollution. There is an increasing demand for new biomonitoring techniques that can effectively overcome these constraints.

Optimal bio-monitors should be easily accessible in urban settings, economically viable to deploy, and capable of generating data on a localized level. Biomonitoring with biological materials provides a novel and economically efficient alternative (Chaudhuri & Roy, 2024). Bioindicators refer to living species or biological characteristics that offer insights into the state of environmental well-being and the existence of contaminants. Numerous bioindicators have been utilized in the past (Conti & Cecchetti, 2001) to assess and track levels of air pollution, as lichens and mosses possess the capability to gather certain pollutants from the atmosphere, rendering them valuable for monitoring heavy metals and other types of airborne contaminants (Demková et al., 2017). Along with these, many insects such as honey bees and their products have also been used to indicate environmental pollution or specific types of pollutants in the air, such as heavy metals. The occurrence of specific diseases, such as asthma or respiratory problems, might serve as a bioindicator of substandard air quality, particularly in urban regions (Nel, 2005). Spider silk, specifically, has attracted attention as a possible biomonitoring tool because of its exceptional characteristics (Stojanowska et al., 2021). Spiders are abundant in metropolitan settings, and their webs naturally ensnare a diverse range of airborne pollutants. Spider silk has notable properties such as exceptional tensile strength, long-lasting endurance, and the capacity to ensnare particulate matter (PM), heavy metals, and organic compounds (Batin, 2024). Spider webs are found everywhere in these habitats, with webs created by different spider species covering flora and structures around the metropolis. These spider webs have a significant surface area that allows them to capture particles in the air, which makes them potentially efficient in accumulating heavy metals and other contaminants. Spider webs offer several advantages: they are ubiquitous, readily available in urban environments, and possess a high surface area for capturing airborne particles.

This study evaluates the use of spider web silk as a biomonitoring tool for airborne heavy metals, which could provide a cost-effective, readily available, and sustainable alternative to conventional techniques that spider webs are a potential tool. However, it also examines the viability and potential of utilizing spider web silk as a biomonitoring instrument to evaluate airborne contaminants in urban regions of Lahore. Furthermore, portable devices use to quantify PM2.5, PM10, and several other contaminants at each of the designated locations for data collection. The objective of this dual monitoring system is to assess the feasibility and efficacy of incorporating spider web silk in conjunction with traditional portable sensors and to show that spider web silk may be used as an effective and supplementary tool in urban air quality monitoring programs by analyzing data from spider webs.

Mostly cities lack a proper network of air quality stations to support decision-making in public health. Biomonitoring tools are an alternative to instrumental data that may increase the spatial resolution and even the records span like using tree rings. It has the potential to completely transform the way air quality is monitored in Lahore and other urban areas, to obtain a more thorough comprehension of the spatial arrangement of heavy metals in Lahore's urban environment by using a network of spider webs as bio-monitors. Subsequently, this data can be utilized to enlighten public health initiatives, focus mitigation strategies on the most contaminated regions, and eventually ensure the well-being of Lahore's inhabitants. The fast-paced development of several economic sectors resulted in social and economic advances followed by a significant decline in air quality. The hypothesis posits that spider web silk in metropolitan Lahore contains substantial amounts of heavy metals, which are directly related to the levels of particulate matter (PM) and other ambient air pollutants such as CO TVOCs, detected at the same sites using portable instruments. These findings indicate that spider web silk can be utilized as a reliable biomonitoring method to evaluate the amounts of airborne pollutants spider silk has the potential to efficiently catch and reflect the levels of airborne contaminants. The aim of this study is: to determine the precise levels of several heavy metals, such as arsenic, lead, cadmium, mercury, copper, chromium and nickel in spider web silk obtained from various urban areas; to establish a correlation between these concentrations and the amounts of particulate matter (PM) and other selected ambient air pollutants recorded at the same locations using portable instruments; to evaluate the practicality and dependability of utilizing spider web silk as an alternate approach for monitoring air quality in situations with limited resources. This investigation can improve the management of air quality and safeguard public health in Lahore and other urban places globally. Promoting biomonitoring advocates for a shift away from the production and utilization of machinery, batteries, and related equipment for air quality assessment. This transition would help minimize the carbon footprint and environmental impact associated with such technologies (Shang & Luo, 2021). Embracing biomonitoring methods like spider web silk, which offer a natural and sustainable alternative, ensures consistent air pollution capture while reducing reliance on conventional instrumentation like remote sensing etc (Mushtaq et al., 2024; Varughese et al., 2023). This approach aligns with the broader goal of minimizing environmental harm and promoting eco-friendly practices in air quality monitoring endeavors.

2 Materials and Methods

2.1 Study Area

This research was conducted in Lahore, Pakistan, covering an area of approximately 1772 km2. Lahore is a densely populated urban area, ranking as the 26th largest in the world, with a population exceeding 13 million people (PBS, 2023). The city experiences high but variable levels of air pollution, exhibiting spatio-temporal changes. It is situated between 31°15' and 31°45' North and 74°01' and 74°39' East. The average annual rainfall is 838.8 mm, with a mean annual minimum temperature of 17.8 °C and a maximum temperature of 30.8 °C (PMD, 2023). Monsoon rains typically start in the last week of June and persist until the end of September. Severe dust storms, accompanied by a significant influx of dust particles into the air, occur during the summer months. Geographically, Lahore is generally flat, sloping towards the south and southwest at a gentle gradient of 1:3000, with an average elevation of 217 m.a.s.l. To the north of Lahore, the Ravi River flows from India, originating in the Himalayas. With a population density of 7339/km2, the Lahore area can be categorized into dominant land use types, including built-up areas, vegetation/agricultural land, barren land, and water bodies (Fahad et al., 2021). The built-up area of Lahore has increased from 35,900 hectares to 65,180 hectares from 2010 to 2021 (Basheer & Waseem, 2022).

2.2 Sampling

Sites for sampling were chosen based on the diversity of urban land use, encompassing residential areas, industrial and commercial zones, government and administrative divisions, educational institutes, parks, graveyards, and transportation infrastructure. A total of 18 sites across Lahore were selected for measuring ambient air pollution (Fig. 1). As a border city, Lahore faces significant air pollution challenges, particularly due to transboundary inputs. This issue intensifies during the crop residue burning season, influenced by weather parameters such as wind speed and direction.

Fig. 1
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Location map showing sampling sites of spider silk and ambient air pollutant measurements in urban Lahore

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2.2.1 Spider Web Silk

The sampling campaigns were conducted during the summer season (May to August). Spider web silk samples were collected from open to semi-covered locations, such as trees, buildings, and sheds, allowing for sufficient air circulation. Sampling activities were limited to clear sunny days, as precipitation can influence the dispersion of particulate matter and spider silk, affecting this air quality bio-indicator.

Spider webs were sampled at a vertical height of 4 to 7 feet 1.2192m to 2.1336 above ground level to better represent ambient air pollutants. The collection method involved using a wooden stick, transferring the webs to a glass jar, and securing them with aluminum foil, following the sampling procedure reported by Goßmann et al. (2022). Field data sheets and maps recorded sampling site characteristics (GPS location, human activities, dominant land use, etc.) and weather parameters (temperature, wind speed, etc.). Additionally, physical characteristics of spider webs, such as size, form, and density, were determined. These physical features have the potential to influence the silk's ability to trap and retain contaminants (Górka et al., 2018). This study did not include an analysis of spider species types and diversity, as only abandoned web silk material was sampled. Spider webs typically have a lifespan ranging from weeks to months, influenced by factors like protein type and environmental conditions. The degradation and decomposition of web silk may take days to weeks (Arai et al., 2004). The samples were transported to the laboratory and stored in a refrigerator until further analysis.

2.2.2 Ambient Air Quality

All locations where spider silk samples were collected also underwent air quality sampling using portable air quality measuring devices (Aeroqual 500 series, Aeroqual Ltd, New Zealand, and IGEREES, Model: WP6930S, VSON Technology Co., Ltd., China). The instruments were calibrated before sampling, following the procedures outlined in the manuals. These instruments were positioned in close proximity to the spider web sampling sites. Once the instrument readings stabilized, air pollutant levels were recorded on field sheets for later processing and analysis. Ambient air quality sampling specifically took place on clear days. Instrumental measurements of pollutants (PM10, PM2.5, HCHO, CO2, and H2S) were part of the dual monitoring system, complementing the spider web silk materials for biomonitoring of airborne heavy metals.

2.3 Laboratory Analysis

Spider silk samples were oven-dried at 70°C for 48 hours. The samples were then weighed using an analytical balance, and approximately 0.01 gram of each sample was taken for digestion on a hot plate with acids. A mixture of 5 mL nitric acid (HNO3 concentration ~ 65%) and 3 mL hydrogen peroxide (H2O2 concentration ~ 30%) was added to the sample and heated at 180°C for 20 minutes using a microwave system. This process was repeated twice (220 °C for 20 minutes) to achieve the total digestion of web silk and adsorbed dust particles. A clear liquid was obtained at this stage of sample digestion. The samples were transferred into a 25 mL volumetric flask containing deionized water to bring it to a specific volume. The analytical procedures followed were as described by Stojanowska et al. (2021).

For the measurement of concentrations of selected trace elements (As, Cd, Cr, Cu, Hg, Ni, and Pb), an Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES, 700 Series, Agilent Technologies, Inc.) instrument was used. QA/QC procedures included procedural blanks, duplicates, and Standard Reference Material (SRM). Analytical-grade chemicals and reagents were used in the laboratory analysis. The detection limits and wavelengths of elements' determination by ICP-OES for the experimental conditions employed in this study are shown in Table 1.

Table 1 The experimental conditions of elements’ measurement by ICP-OES in the study
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The QA/QC process facilitated the identification of specific heavy metals and metalloids present in the sample, as well as the quantification of their respective concentrations.

The collected web silk samples were mounted on the stub and sputter coated with gold to obtain a desired quality image by the scanning electron microscope (SEM) and energy dispersive spectrometer (EDS; JEOL-JSM 5310) analysis.

2.4 GIS and Statistical Analysis

The data gathered from field observations and laboratory sample analyses were employed in creating spatial distribution maps of the study area using the Geographic Information System (GIS) tool within ArcGIS. This GIS tool facilitated the integration of geospatial data, interpolation, and the visualization of spatial variation trends, along with hotspot identification throughout urban areas (Loukili et al., 2022). For statistical analysis and graphical visualizations, RStudio, a robust statistical computing environment, was utilized. Principal Component Analysis (PCA) was employed to detect variable clusters and determine associations among the measured parameters.

3 Results

3.1 Spider Silk Metal Contents

Field data and laboratory analysis of spider silk collected from various parts of the study area are presented in the material (Table 2). The obtained results are discussed in the following sections, along with minimum, maximum, and average values.

Table 2 Field data and lab analysis of measured parameters in spider web silk samples from the study area
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3.1.1 Arsenic

The concentration of Arsenic (As) in spider silk ranged between 2.65 and 10.05 mg/kg, with an average value of 6.64 mg/kg. The lowest value was found at the sampling site in the Iqbal Town housing area which is a residential area in southwestern Lahore, while the highest value was measured at the location of Thokar Niaz Baig which is a rushy area with heavy traffic and small industries in the study area. Stojanowska et al. (2021) reported a minimum concentration of As at 212 mg/kg and a maximum value of 687 mg/kg in spider webs from a small village in Poland. Spatial variations indicate higher concentrations around industrial areas, with major sources identified as industrial and vehicular emissions (Suvarapu & Baek, 2017). Particulate matter containing potentially toxic elements (PTEs) can accumulate on passive sampler materials, such as spider webs.

3.1.2 Cadmium

The concentration of cadmium (Cd) in spider silk exhibited a range of 0.80 to 218.34 mg/kg, with an average concentration of 15.45 mg/kg. Stojanowska et al. (2021) reported that the cadmium in web silk samples was below the detection limit in Opolskie Voivodship, southwestern Poland. The sampling site of the Bahria Town housing area which is a residential area, registered the lowest value, whereas the location of Mehmood Booti, which is near the industrial zone and landfill site, recorded the highest value. Studies have identified industrial emissions, vehicular emissions, and secondary aerosols as the primary contributors to particulate matter and potentially toxic elements in the atmosphere, compared to residential areas (Suvarapu & Baek, 2016). Cadmium has no known important biological function in higher organisms, including humans; hence, its exposure may cause organ system toxicity.

3.1.3 Chromium

The concentration of Chromium (Cr) in spider silk ranged from 15.65 to 184.24 mg/kg, with an average value of 57.04 mg/kg. A mean value of 51 μg/g of Cr was reported in the spider web of Wroclaw, Southwest Poland (Rybak, 2015). It's worth noting that Sundar Industrial State, which is included in an industrial zone, had the lowest documented value, while Lake City, which is a residential area with an actively constructed site, location documented the highest concentration. Elevated airborne chromium is likely attributed to active construction sites, metal and welding work, poor handling of collected waste, and the burning of fossil fuels for running cranes and other machinery. Similar anthropogenic activities in urban areas have been reported elsewhere (Golbabaei & Khadem, 2015; Rehr et al., 2010).

3.1.4 Copper

The copper (Cu) concentration in the examined spider silk showed a range of values from 15.65 to 184.64 mg/kg, with an average concentration of 102.87 mg/kg. Stojanowska et al. (2021) reported a minimum of 60 mg/kg and a maximum of 136 mg/kg Cu concentration in spider web materials collected in a village in Poland. The Badshahi Mosque area, which is a tourist point, exhibited the lowest concentration among the studied areas, while Lake City, which has an active construction site, demonstrated the highest recorded value. Vehicular emissions, construction site dust, and burning of fuels in construction areas produce gaseous copper (Nagar et al., 2014).

3.1.5 Mercury

The Mercury (Hg) concentration in spider silk ranged from 0.51 to 10.89 mg/kg, with a mean concentration of 2.66 mg/kg. The sampling location of Thokar Niaz Baig, which has heavy traffic and small industry around, registered the lowest value, whereas the residential site of DHA residential society measured the highest concentration. Major sources of airborne mercury include industrial point sources and landfill burning (Nagar et al., 2014). There has been no reporting of the concentration of mercury in spider web material yet. Brown et al. (2015) documented a median UK concentration of total gaseous mercury measured over ten years to be around 2.0 ng/m3, with a slightly higher urban increment of about 0.4 ng/m3 above the background levels. Anthropogenic activities in urban areas produce mercury mostly in the elemental form (Hg(0)), which is the dominant form of mercury in ambient air and can be oxidized to various Hg(II) species by several atmospheric oxidants, such as ozone (Ariya et al., 2015).

3.1.6 Nickel

The nickel (Ni) levels in spider silk material measured a range of values spanning from 8.03 to 135.54 mg/kg, with an average concentration of 31.72 mg/kg. A value of 24 μg/g Ni in spider web material was reported in Wroclaw, Poland (Rybak, 2015), which is significantly lower compared to the study area samples of web silk. It is worth mentioning that the housing area in Bahria Town, Lahore displayed the lowest recorded value, whereas Paragon City, Lahore, which has an active construction site, as a selected sampling location, showed the highest concentration. The presence of nickel concentration in the air is attributed to the burning of agricultural residue, fossil fuels, and vehicular emissions (Sah et al., 2019; Suvarapu & Baek, 2017).

3.1.7 Lead

The lead (Pb) concentration in spider web material showed a range of values varying from 15.12 to 356.41 mg/kg, with a mean concentration of 80.30 mg/kg. The Thokar Niaz Baig, which has heavy traffic and small industries around, sampling site displayed the lowest recorded value, whereas the Mehmood Booti, which is near the industrial zone and landfill site, sampling location registered the highest Pb concentration. Rybak (2015) identified the correlation between Pb and particulate matter by using spiderweb as a biomonitoring tool for airborne pollution in Poland. Spider silk contents measured a minimum value of 173 mg/kg and a maximum value of 2245 mg/kg in Wroclaw, Poland (Stojanowska et al., 2021). Rybak (2015) also identified the sources of lead, mainly emissions from motor vehicles. Elom et al. (2014) reported typical total Pb levels in urban street dust in northern UK cities ranging from 306 to 558 mg/kg, falling in the higher range compared to the web silk contents measured in this study. Although unleaded formulations of petrol have been introduced in Pakistan since 2001, urban air Pb levels continue to be high.

3.2 Spatial Variation Pattern

The spatial distribution map of As shows that the concentration is relatively higher in the northeastern areas of Lahore compared to the southwestern part (Fig. 2(a)). The highest values of As in spider silk were measured in samples collected near the fuel station in the DHA commercial area, while the As concentration in ambient air was unknown. The ambient air quality, according to the WHO (2000) reference level for As, is 6.6 ng/m3. According to NAAQS (2023), the As value in the air should be 0.035 μg/m3 in winter and 0.07 μg/m3 in summer, with an overall standard value of 0.006 μg/m3. The possible sources of this pollutant are industrial operations, vehicular emissions, and other anthropogenic activities. Men et al. (2018) reported that the sources of As in urban areas also include household activities and road dust.

Fig. 2
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Spatial variation maps of measured elements in spider silk collected from various locations of the study area

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The spatial distribution map of Hg shows a similar trend to As, with concentrations substantially higher in the northeastern portions of Lahore compared to the southwestern areas (Fig. 2(b)). The workplace permissible limit for Hg is 0.1 mg/m3 over an eight-hour work shift (NJDH, 2009). The World Health Organization (WHO) suggests a permissible daily limit of 0.2 μg/m3 for elemental mercury vapor in the atmosphere (WHO, 2000). The ambient air quality limits for Hg are 200 ng/m3, substantially lower than the value measured in this study. Possible sources of this pollutant include industrial operations, automobile emissions, and other anthropogenic activities (Streets et al., 2011). The highest values of mercury in spider silk were detected near the fuel station in the DHA commercial area. It is worth noting here that there is no established standard for ambient air quality levels of Hg. It is extremely hazardous to human health and may cause bronchitis and shortness of breath upon repeated exposure.

The spatial distribution map of Cr indicates a significantly higher concentration in the southwestern areas of Lahore compared to the northeastern side (Fig. 2(c)). The study revealed that the highest levels of Cr in spider silk were observed in Shahkam Industries and Lake City, near the construction sites. The ambient air quality threshold for Cr is 0.012 μg/m3, a significantly lower number compared to the measurements obtained in this research. Possible sources of this contaminant include industrial operations, vehicular emissions, and other human activities mainly associated with oil and coal combustion (Cheng et al., 2014). According to NAAQS, NIOSH, and USEPA, the permissible Cr value in the air is 0.354 μg/m3.

The spatial distribution map of Cu reveals a notably greater concentration in the northeastern and southwestern regions of Lahore (Fig. 2(d)). The study found that the spider silk material in Paragon City, located on the northeastern side near building sites, had the greatest levels of Cu. Conversely, the lowest concentration of Cu was detected in the walled city of Lahore, particularly near Badshahi mosque The ambient air quality threshold for Cu is ppm, which is considerably lower than the level found in this investigation (Morakinyo et al., 2021). According to NAAQS, NIOSH and USEPA permissible Cu value in the air is 0.2μg/m3. Potential origins of this pollutant encompass industrial activity, automobile discharges, and various human endeavors. In urban areas, much construction, industrial, and fossil fuel burning is responsible for airborne Cu.

The spatial distribution map of Cd demonstrates a much higher concentration in the north of Lahore compared to all other areas (Fig. 2(e)). In contrast, the research area's remaining locations exhibited the lowest concentration of Cd. The ambient air quality threshold for Cd is 5 ng/m3, which is only slightly lower than the level detected in this study. According to NAAQS, the permissible Cd value in air should be less than 0.026 μg/m3 in winter and less than 0.022 μg/m3 in the summer season. The spider web samples from the Mehmood Booti location, situated north of Lahore in close proximity to a large dump and industrial trash, exhibited the highest concentrations of Cd.

The spatial distribution map of Ni reveals that the concentration is comparatively greater in the northeastern and southwestern regions of Lahore (Fig. 2(f)). Nickel levels in spider silk were found to be highest on the northeastern side at Paragon City as well as on the southwestern side at Lake City near construction sites, whereas the lowest concentration was observed in the residential section of Bahria Town. The ambient air quality reference level by WHO for Ni is 25 ng/m3, a far lower threshold than the number observed in this investigation.

The potential origins of this pollutant include industrial operations, automobile emissions, and other human activities. Men et al. (2018) found that the primary sources of nickel in urban environments are domestic activities and road dust in Beijing, China.

The spatial distribution map of lead (Pb) shows significantly higher concentrations in the north and northeastern parts of Lahore (Fig. 2(g)). The annual ambient air quality threshold for lead is 1.5 μg/m3, and for 24 hours, it is 2 μg/m3—both considerably lower than the levels identified in this study. Potential sources of this contaminant include industrial operations, vehicular emissions, and other human activities. Further research is needed to pinpoint the exact source. Levin et al. (2021) discovered that household activities and road dust are the primary contributors to lead in urban environments. Their study highlighted that the highest lead levels in spider silk were found near the large Mehmood Booti landfill. It has been previously reported (Khan et al., 2020) that areas with higher particulate matter (PM10) in Islamabad, Pakistan, also exhibit elevated levels of lead, posing a significant cancer risk to individuals residing in those polluted areas.

3.3 Selected Air Pollutants for Ambient Air Monitoring

Ambient air refers to the outdoor atmospheric air in its natural state which is different from the indoor air in terms of air pollution in urban areas. Field data measurements and other observations of major air pollutants are given in the material (Table 3). The concentrations of selected major air pollutants are discussed in the following subsections.

Table 3 Ambient air quality data and the field data of spider web sample characteristics
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3.3.1 Particulate Matter (PM2.5)

The field measurement of PM2.5 levels in air has yielded a range from 75.0 to 195.0 μg/m3, with a mean value of 119 μg/m3. Distinctive variations were seen among the studied locations, with the DHA housing area demonstrating the lowest recorded concentration, while Shahkam Industries demonstrated the highest concentration. The PM2.5 levels of 93.0 ± 49.9 μg/m3 have been reported in Lahore in 2007 and 2008 (Rasheed et al., 2015). Lodhi et al. (2009) reported the sources of PM2.5 in urban air to be vehicular emissions, industrial activities, residential combustion, and construction activities originating dust particles.

3.3.2 Particulate Matter (PM10)

The determination of particulate matter (PM10) concentration in the air has produced a range of values spanning from 87 to 226 μg/m3, with an average level of 150.35 μg/m3. The field measurements from sampled locations exhibited variances with the DHA housing area displaying the lowest recorded concentration, whilst Sundar Industrial State recorded the highest concentration. Alam et al. (2014) found the PM10 levels variation from 254 to 555 μg/m3 with an average of 406 ± 87 μg/m3 in Lahore.

3.3.3 Formaldehyde

The measurements of atmospheric formaldehyde (HCHO) contents have shown a range of values spanning from 0.000 to 0.02 μmg/m3, with an average measurement of 0.003 μg/m3. The analyzed locations exhibited discernible variations, with the Civil Courts, Valencia Housing Society, Thokar Niaz Baig, Lake City Housing Society, Miani Sahib Graveyard, Bahria Housing Society, Iqbal Town, and Shahkam Industries locations all registering a concentration of zero (below the detection limit). Conversely, Johar Town displayed the highest recorded concentration.

3.3.4 Total Volatile Organic Compounds (TVOC)

The measured levels of ambient air total volatile organic compounds (TVOC) contents are in the range of zero to 0.27 ug/m3, with an average measurement of 0.14 ug/m3. The analyzed locations showed variations, with the Paragon City sampling site registering a concentration of zero. Conversely, Mehmood Booti displayed the highest recorded concentration. The Data Darbar in Lahore had the highest levels of volatile organic compounds (VOCs) at 10 mg/m3 and formaldehyde (HCHO) at 0.99 mg/m3, whilst the G.O.R-1 location had the lowest values at 0.004 mg/m3 and 0.002 mg/m3, respectively. There was a strong positive correlation between PM1.0 and PM10, although PM2.5 also showed a strong negative correlation with relative humidity in the area (p = 0.009) which has been reported by Aslam et al. (2020) in urban areas of Lahore. TVOCs are priority atmospheric pollutants because of their adverse impacts on human health and many of the VOCs are human-made chemicals.

3.3.5 Carbon Monoxide (CO)

The measurement of carbon monoxide concentration in the ambient air recorded a range of values from zero to 54.0 ppm, with an average value of 23.28 ppm. The sampled locations exhibited notable variances, with the DHA housing area, Miani Sahib graveyard, Iqbal town, and Paragon City (all are residential areas) showing the lowest recorded concentration, whereas the Railway Station showed the highest which are agreement with levels reported in earlier studies (Shah & Arooj, 2019). Typical sources include vehicle exhaust, cigarette smoke, and others. In the US, daily mean ambient concentration of CO fluctuate between 0.5 and 2 ppm (Samet et al., 2000).

3.3.6 Hydrogen Sulphide (H2S)

The field measurement of the concentration of hydrogen sulphide in ambient air registered a range from zero to 0.1 ppm. Mehmood Booti and Hudyara drain locations are the only two hot spots that show significant levels of elevated concentrations. The rest of the sampling locations measured levels very low or zero.

3.3.7 Lower Explosive Limit (LEL)

The assessment of LEL levels in the surrounding atmosphere encompassed a range of values spanning from zero to 0.2 %vol. Only Mehmood Booti (Landfill) and Hudyara drains (contains industrial effluents) recorded a notable level of concentration, making them the two primary hotspots. All remaining locations registered levels below the detection limit.

Distribution of Ambient Air Pollutants

The spatial distribution map of TVOCs displays some concentrations in the northwestern and southwestern regions of Lahore, in contrast to the southeastern and northwestern side (Fig. 3(a)). Mehmood Booti which is a land fill area, is showing much greater values of total volatile organic compounds while Paragon Society and other housing societies are showing less amount of emitted TVOCs. The ambient air quality criterion for TVOCs is 0.3mg/m3 for indoor air quality practices, and the measured values lie within the range of the permissible limit set by WHO. Indoor sources of TVOCs Paint, paint strippers, Varnishes and finishes, Caulks and sealants, Adhesives, Flooring, carpet, and pressed wood products while outdoor potential origins of this pollutant encompass Gasoline, diesel emissions, wood burning, oil and gas extraction and processing (ALA, 2023), which is very unfortunate that this type of economic strengthens indicator process have lower ratio in Pakistan. Harmless air is defined as having a total VOC level of less than 100g/m3 (Aslam et al., 2020).

Fig. 3
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Spatial distribution maps measured ambient air levels of major pollutants in the study area

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The spatial distribution map of HCHO exhibits somehow elevated concentrations in the southwestern areas of Lahore, as opposed to the southeastern and northeastern portions (Fig. 3(b)). Thokar Niaz Baig has significantly higher levels of HCHO compared to Paragon Society and other housing societies, which display lower emissions of HCHO levels. The ambient air quality criteria to avoid health concerns, the USEPA suggests maintaining formaldehyde levels in indoor air at 0.4 μg/m3 (EPA, 2023b), observed values in this study show the concentration of HCHO is within the range of ambient air quality guidelines set by USEPA. Possible sources of this contaminant are natural phenomena such as forest fires, as well as being emitted into the atmosphere through industrial activities, incineration, and fuel burning. Additionally, it is generated in the environment by the process of photooxidation of reactive organic gasses (CARB, 2020).

The spatial distribution map of H2S does not show notably higher concentrations in the southeastern and northwestern regions of Lahore, in contrast to the southwestern and northeastern parts (Fig. 3(c)). Mehmood Booti and Hudyara drain exhibits markedly elevated levels of H2S in comparison to Paragon Society and other housing societies, which demonstrate lower emissions of H2S. Kourtidis et al. (2008) reported that as per WHO guidelines for ambient air quality standard for H2S is less than 7μg/m3 for every 30 min, and the measured values of the study are significantly within the range of WHO guidelines. According to USEPA ambient air quality standards, 10ppm is the limit. Decaying vegetation and animal matter is the primary source of H2S emissions in urban areas. Anthropogenic hydrogen sulfide primarily originates as a byproduct of the purification of natural gas and the refinement of crude oil (Malone Rubright et al., 2017).

The spatial distribution map of CO reveals significantly elevated concentrations in the southwestern and northwestern areas of Lahore, as opposed to the southeastern and northeastern areas (Fig. 3(d)). Mehmood Booti, Hudyara drains, Shahkam industries, and Sundar Industrial states indicate somehow higher amounts of CO emissions compared to Paragon society and other DHA housing societies, which exhibit lower or no levels of CO. As per USEPA rules, the concentration of CO in ambient air has to be less than or equal to 9 ppm for 8 hours (EPA, 2023a), which is not far away from the reported values of CO of the sampling site. Possible sources of this pollution include industrial activities, vehicle emissions, and various human activities. The main source of CO production is incomplete or partial combustion of fossil fuels which contain carbon as their structural backbone such as gasoline, wood, and natural gas.

The spatial distribution map of particulate matter PM2.5 indicates notably higher concentrations in the southwestern and northwestern regions of Lahore, in contrast to the southeastern and northeastern sections (Fig. 3(e)). Mehmood Booti, Hudyara drainage, Shahkam Industries, and Sundar Industrial States demonstrate significantly elevated levels of PM2.5 emissions in comparison to Paragon Society and other DHA housing societies, which display lower concentrations of PM2.5. The ambient air quality standard according to WHO (WHO, 2000) for PM2.5 is 5 μg/m3 annually, which is significantly lower than the measurement mentioned in this query for specific locations. Potential contributors to this pollution encompass industrial operations, automobile discharges, and diverse human endeavors (Klimont et al., 2017).

The spatial distribution map of particulate matter PM10 reveals significantly elevated concentrations in the southwestern and northwestern areas of Lahore, in contrast to the southeastern and northeastern sections (Fig. 3(f)). The ambient air quality standard for PM10 is 15 μg/m3 annually (WHO, 2000), a value considerably lower than the number indicated in this inquiry for specific areas. The sources of this pollution include industrial activities, vehicle emissions, and various human activities (Lenschow et al., 2001). Mehmood Booti, Hudyara drainage, Shahkam industries, and Sundar industrial states exhibit notably higher levels of PM10 emissions compared to Paragon society and other DHA housing societies, which show lower concentrations of PM10 (Colbeck et al., 2011).

3.4 Atmospheric Parameters and Web Types

The measurement of air temperature in the study area recorded values ranging from 25 to 30 0C, with an average value of 30.9 0C. The measured wind varied between 1 km/hr to 23 km/hr. The measured air humidity varied between 30 and 80 % vol.

Four types of spider webs were found in this study, including cob, sheet, orb, and funnel shapes (material: Table S2 and S3). The majority of web types were sheet structures hanging between tree branches or corners of the building's ceiling. The strands in the sheet type were found to be woven horizontally and were dense in shape and structure.

During sample collection, spiders were absent, and the spider species were not identified. Nadeem et al. (2023) reported and documented the spider species in Punjab, Pakistan, with the highest occurrence rates belonging to the Araneidae (33.62%), Lycosidae (24.93%), and Oxyopidae (9.40%) families. Figure 2 shows an SEM image of strands of spider web silk along with accumulations of dust particles. The elemental compositions of these particles are dominated by Fe, Ca, K, and S as shown by the EDX (Fig. 4).

Fig. 4
figure 4

Scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX) of spider web silk showing strands and accumulations of dust particles. Major and minor elemental data show the dominance of Fe, Ca, K, and S elements

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4 Discussion

The results of the current investigation showed elevated levels of heavy metals, such as arsenic, copper, chromium, lead, mercury, cadmium, and nickel in the spider web silk samples obtained from urban Lahore. This finding is significant because heavy metals are not commonly found in spiderwebs' natural composition or they are at a very minute level (Mohamed, 2014). So, the presence of high levels of heavy metals in spider silk content suggests that they most likely come from either the environment or from spiders’ prey or food. The minimal amounts of heavy metals found in the spiders' diet are inadequate to explain the high concentrations detected in the silk. This is because spiders mostly feed on prey that is rich in protein, which does not contain large amounts of heavy metals. Thus, it is reasonable to suggest that these high concentrations of heavy metals are deposited onto the spider webs through atmospheric deposition, indicating the levels of local airborne pollution. These findings demonstrate that spider web silk can efficiently trap and detect airborne heavy metals, therefore serving as a helpful biomonitoring tool for evaluating environmental pollution. Additional research is necessary to validate these discoveries and get a deeper comprehension of the processes by which heavy metals accumulate in spider webs.

4.1 Spatial Variation Pattern

The spatial variation data demonstrates that spider webs in regions with intense human activities, such as industrial zones and bustling metropolitan centers, exhibit substantially elevated levels of heavy metals in comparison to those found in residential areas. This pattern demonstrates a clear correlation between the presence of heavy metal pollution in the air and its subsequent buildup in spider webs. In areas characterized by significant industrial production and a large number of vehicles, heavy metals in the air deposit onto spider webs, serving as a reliable indicator of environmental pollution. On the other hand, in residential areas with less industrial activity, the concentrations of heavy metals in spider webs are significantly reduced. This association not only showcases the magnitude of air pollution in various regions but also illustrates the efficacy of spider webs as reliable bioindicators for monitoring airborne heavy metal pollution.

4.2 Principal Component Analysis

Field and lab data of measured parameters indicated three groups depending upon the sources of origin of pollutants (Fig. 5(a & b). The plotted variables accounted for 51.33% variability of determined parameters. The first cluster identified is shown in the PC1 negative and PC2 negative quadrants consisting of Hg, Cr, Ni, and Cu. This cluster of elements seems to be related to sampling locations with active construction sites. Yang et al. (2020) reported that urban construction sites produce gaseous emissions with elevated levels of metals in rapidly growing cities in China.

Fig. 5
figure 5

PCA plot showing various clusters of measured parameters in the study area. Heatmap showing associations among various measured parameters and the field sampling locations in the study area

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The second cluster is plotted in the quadrant with positive PC1 and negative PC2 (Fig. 5) consisting of Cd, LEL, H2S, and Pb. This cluster seems to be related to sampling sites of city landfills and major urban drains. The gaseous emissions from these sources might be contributing to elevated levels in ambient air and spider silk. TVOC plot is closer to the urban drain containing untreated effluent from industry and residential areas.

Sampling locations consisting of residential areas registered concentrations of pollutants relatively lower than the other sampling sites. These included housing societies such as Valencia Town, Bahria Town, and Judicial Colony due to better vegetation cover and parks, low traffic volume, and moderate anthropogenic activities (e.g., industry). Cluster analysis on the measured parameters and the correlation heatmap (Fig. 5) for air pollutants visualizing variance across multiple variables to display patterns in correlations. A significant correlation was set at p < 0.05. The color shade and intensity in the heat map indicate the expression amount of the index. The tree on the map shows the clustering results of different variables.

4.3 Correlation Among Measured Parameters

The correlation plot and heat map show an association among measured parameters (Fig. 6). A few parameters showed a high positive correlation (r=0.94) between them such as H2S and LEL and a negative correlation (r=-0.46) between CO and Hg. The heat map shows the grouping and association among measured parameters and sampling locations in the study area.

Fig. 6
figure 6

Chord diagram showing the pollutant types and levels in association with sampling locations. Various types of anthropogenic activities produce differ types of emissions reflecting urban area characteristics

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For data visualization, the chord diagram served as a useful statistical tool to depict associations of determined pollutants to sampled sites. The Chord diagram (Fig. 6) shows an association between measured pollutants and sampling location. All locations (LSW-4, LSW-6, and LSW-8) contributing the least amount in terms of proportion of pollutant loading consisted of residential areas. The most dominant pollutant at this location was identified as PM10. The sampling sites (LSW-15 and LSW-5) that contributed the highest in terms of air pollution proportion consisted of Pb, Cd, PM10, and PM2.5.

The sampling location (LSW-2) close to construction-related activities registered a significant contribution of Cu in spider silk material reflecting ambient air higher levels. Fuge (2012) reported that construction activities are the major source of airborne Cu levels in urban areas. Particulate matter (PM2.5 and PM10) forms the major proportion of ambient air pollutants among the measured parameters. Hence, dust-related air pollution is a significant environmental concern in urban areas of the metropolis of Lahore. The PM10 levels were found to be 10 times higher than the WHO guideline of 15 μg/m3 annually and the PM2.5 levels were measured 24 times higher than the WHO guideline of 5 μg/m3 annually. This creates an alarming situation for urban dwellers and managers in terms of human health concerns. It is unfortunate that the study area, Lahore, is often classified as the most polluted city in the world. Therefore, this city is important to explore new bio-monitoring tools such as spider web silk along with instrumental measurement for exploring robust and sustainable measurement tools.

This study on using spider web silk as a biomonitoring tool for assessing air quality shows great potential for future applications on a global scale. This research provides a cost-effective and scalable solution to urban air quality monitoring difficulties by utilizing the inherent characteristics of spider silk. The investigation's accomplishments have advantageous implications for several stakeholders, encompassing urban communities, environmental agencies, and the scientific community. In the future, it is important to perform additional research to confirm the effectiveness of this method in other situations. This research should also include long-term monitoring studies and the integration of remote sensing technology. Additionally, efforts should be made to engage and empower the community through capacity development projects. By combining the efforts of multiple disciplines and conducting research together, the use of spider web silk biomonitoring has the ability to enhance the management of air quality and protect public health on a worldwide scale.

4.4 Viability and Applicability

This research showcases the effectiveness of utilizing spider web silk as a supplementary tool in conjunction with traditional portable devices, as seen by the dual monitoring system. Spider webs, which are commonly seen in metropolitan areas, have been discovered to efficiently trap many types of airborne contaminants, such as particulate matter, heavy metals, and organic compounds. The portable equipment offered accurate quantitative measurements of PM2.5 and PM10, acting as a dependable reference point to verify the quantities of pollutants detected by the spider webs.

The comparison of data between spider webs and portable devices revealed a robust association, suggesting that spider silk can serve as a dependable indicator of airborne pollution levels. This correlation confirms the premise that spider web silk is a reliable bio-monitor. Spider webs were highly effective at trapping smaller particles known as PM2.5, which is important due to its substantial effects on health. The spider webs also ensnared a wider range of contaminants, including some that are not typically detected by conventional portable devices, indicating an additional benefit of utilizing biological materials for a thorough evaluation of air quality.

4.5 Wider Ramifications

Although this study specifically examined Lahore, its ramifications have worldwide reach. Urban areas over the globe encounter comparable issues with air pollution, and the exorbitant expense and intricate nature of conventional monitoring techniques frequently restrict their extensive application. Utilizing spider web silk as a biomonitor offers a cost-effective, scalable, and environmentally friendly alternative. This method can be especially beneficial in areas with limited resources for comprehensive air quality monitoring infrastructure.

4.6 Significance for The Environment and Public Health

Expanding the use of spider web silk biomonitoring can greatly improve the management of environmental and public health. Cities can obtain comprehensive and uninterrupted air quality tests by implementing a cost-effective and readily deployable monitoring approach (Hadj Sassi & Chaari Fourati, 2022). This can result in more well-informed policy choices, focused initiatives, and increased public consciousness regarding air pollution. The approach is also in line with worldwide sustainability objectives, advocating for inventive and environmentally friendly solutions to ecological obstacles.

Utilizing spider webs for air quality monitoring can also promote community involvement and environmental instruction (Grossberndt & Liu, 2016). Engaging local communities in the gathering and examination of spider webs can enhance public involvement in environmental monitoring and consciousness. Implementing this collaborative approach can amplify the overall effectiveness of air quality management efforts.

4.7 Constraints and Prospects for Further Investigation

Although the study demonstrates encouraging outcomes, it is important to acknowledge its limitations. The spatial distribution of spider webs and the impact of nearby environmental elements can alter the effectiveness of pollution capture (Rachwał et al., 2018). Subsequent studies should focus on establishing uniform procedures for collecting spider webs and doing more in-depth investigations into the specific contaminants that are most effectively trapped by these webs. Moreover, doing the study in other urban settings worldwide would enhance the credibility of the results and improve the research technique.

The study effectively showed that spider web silk may be used as an efficient biomonitoring method for airborne contaminants in urban areas of Lahore. The theory of the effectiveness of spider silk in capturing and reflecting the concentration of airborne pollutants was confirmed by combining spider web silk with portable devices that measure PM2.5, PM10, and other contaminants. The robust correlation between the data obtained from spiderwebs and portable sensors validates the dependability and effectiveness of this combined monitoring methodology.

The findings provided evidence to support the concept that spider silk is excellent in capturing airborne contaminants. Spider webs obtained from several metropolitan locations in Lahore successfully trapped a wide variety of contaminants, in which this study focused on heavy metals contamination an. The data derived from the spider webs exhibited a substantial connection with those obtained from the portable devices, suggesting that spider web silk can serve as a dependable indicator of airborne pollution levels. Cartographic representations of the arrangement or dispersion of objects or phenomena in a given space. Furthermore, the efficacy of spider web silk as a bio-monitor was confirmed, and spatial distribution maps were created to depict the patterns of airborne pollutants around Lahore. These maps visually depict the quantities of pollutants at various locations, emphasizing areas with elevated levels of PM2.5, PM10, and other harmful substances. The spatial study unveiled clear patterns of pollutant dispersion, which were shaped by factors such as traffic density, industrial activity, and local weather circumstances.

5 Conclusions

Spider silk metal content showed wide spatial variation, which, in most places, was found to be parallel to the ambient air quality pollutant level. Concentrations of Hg, Cd, Cr, and Pb in the silk were found to be higher in the north and northeast directions of the study area which carries high levels of anthropogenic activities including vehicular emissions. Statistical and geospatial analysis revealed three types of anthropogenic activities along which measured data clustered. The first one was found to be the locations of landfills and urban effluent drains that registered higher contents of Pb, Cd, and LEL. The second cluster was dominated by activities such as construction and vehicular-related emissions in Lahore. The third cluster was dominated by housing and residential areas with relatively better vegetation cover and lower traffic volume.

This research demonstrates that spider web silk efficiently traps airborne heavy metals, highlighting the crucial importance of this innovative biomonitoring method. This research not only improves our understanding of how air pollution works and how heavy metals are spread in cities, but it also provides a basis for future investigations and practical applications in monitoring the environment and assessing public health. As, it has been found that spider webs obtained from regions with high levels of human activity, such as industrial zones and busy metropolitan centres, had much greater levels of heavy metals compared to those from residential areas. Simultaneously, the levels of particulate matter were measured at these collection sites using portable devices. The data demonstrated a distinct correlation: areas with high amounts of heavy metals in spider silk also displayed increased levels of particulate matter (PM2.5 and PM10) and other variables such as CO, H2S. The utilization of spider webs in combination with direct measurements of particulate matter highlights the importance of a dual monitoring system in order to obtain a thorough evaluation of air quality. These findings validate the hypothesis and emphasize the capability of spider webs as a convenient and non-intrusive technique for monitoring the environment. They assist in pinpointing areas of pollution and informing specific actions to enhance urban air quality and public health.

The utilization of spider web silk alongside with portable instruments in a dual monitoring system has demonstrated its effectiveness and feasibility in assessing air quality, so, expanding knowledge of biomonitoring techniques beyond conventional approaches. The spatial distribution maps depicted the patterns of airborne contaminants throughout Lahore, offering valuable information for focused actions. The discovery has wider ramifications, indicating that this strategy can be used worldwide, providing a novel, economical, and enduring resolution to the urgent problem of urban air pollution. By doing more extensive study and implementing spider web silk biomonitoring on a larger scale, it has the potential to become a valuable aspect of urban air quality management systems. This can lead to enhanced public health and environmental protection on a global scale. This research provides valuable insights that have significant global significance, especially for urban areas that are dealing with similar air pollution issues.

The significance of this work on a global scale is its ability to be applied to metropolitan areas around the world, particularly in countries with few resources where conventional monitoring techniques may not be feasible. This work presents spider web silk biomonitoring as a viable approach that is both cost-effective and scalable, surpassing geographical limitations. Moreover, this research highlights the significance of employing interdisciplinary methods to tackle environmental problems. A new technique has been created that combines the disciplines of biology, environmental science, and engineering to utilize the inherent characteristics of spider silk for the purpose of improving air quality monitoring.

The dual monitoring system (active and passive) using spider web silk contents and ambient air quality measurements showed promising potential. However, more research is needed to establish the relationship between ambient air pollution and spider silk levels of pollutants. Particulate matter (PMs) seems to be the major pollutant, with levels up to 20 times higher than WHO guidelines, contributing to the deterioration of air quality in Lahore. This research is significant in documenting new techniques as air pollution and smog-related events become more common in urban areas worldwide, including Pakistan. Nevertheless, additional research is required to verify the widespread use of spider web silk contents for airborne metal detection.

In the future, it is important to research to investigate the possibilities of using spider web silk as a biomonitoring tool in various metropolitan contexts around the world. Conducting comparative research in other countries can offer useful insights into the efficacy of this technique in diverse climatic, geographical, and socio-economic contexts. Ultimately, this work enhances scientific understanding and provides effective remedies for actual problems in the real world. By showcasing the international effectiveness of using spider web silk as a biomonitoring tool, it sets the stage for enhanced control of air quality and safeguarding human health on a global scale.