1 Introduction

Traffic air pollution has increase due to the growth of automobiles, transportation services and unplanned urbanization [1,2,3,4]. In addition, the usage of poorly maintained old cars, second-hand vehicles, the limited development of urban road infrastructure and poor road infrastructure in low-income countries contribute to high levels of traffic air pollution [5, 6]. Carbon monoxide emitted from road traffic is a major contributor to air pollution and remains a major public health problem in urban areas [7,8,9]. When vehicles encounter frequent stop-and-go, increase commuting time, fuel consumption and pollutant emissions has increased [10].

The concentration of carbon monoxide near a city street is the product of vehicle engine emissions and produced as a result of incomplete combustion in the vehicle engine. Road traffic is one of the primary sources of air quality deterioration and the unprecedented growth in private modes of transportation and poor traffic flow management system make the vehicular emissions worst [11,12,13]. Carbon monoxide is an odorless, colorless, non–irritating and tasteless gaseous pollutant emitted by anthropogenic activities and natural sources [7, 14]. In addition, the sources are also classified as mobile and stationary sources, which are responsible for the emission of 165 thousand tons of carbon monoxide per year, with 97% of carbon monoxide being emitted by motor vehicles [2, 7].

A high concentration of carbon monoxide is considered to be very toxic to humans as it can lead to acute poisoning, which promotes death by asphyxiation. The interaction of hemoglobin with carbon monoxide is 240 times greater than with oxygen. Therefore, carboxyhemoglobin is formed instead of oxyhemoglobin. The acute effects of carbon monoxide poisoning are well known. Healthy people develop headaches when the COHb concentration reaches 10%; tinnitus and drowsiness occur in 20%; nausea, vomiting and weakness in 20–30%; clouding of consciousness and coma in about 35%; and death occurs in about 50% [7, 15]. Exposure to high levels of carbon monoxide has resulted in a variety of health effects including, but not limited to, stroke, lung cancer, asthma, cardiac arrest, cardiovascular, blood and central nervous system and respiratory diseases [3, 9, 16, 17]. Carbon monoxide has a high affinity for hemoglobin, reduces the oxygen supply to the brain and spreads to other organs. The carboxyhemoglobin content in the blood indicates the health consequences of carbon monoxide exposure [14, 17]. It is worth noting that 80% of the world’s population lives in areas where the air pollution limits set by the WHO are exceeded. Every year, 4.2 million people die as a result of exposure to outdoor air [18, 19].

The Ethiopian government has adopted a number of strategies to reduce pollutant emissions in the transport sector, such as promoting non-motorised transport, switching from single-mode transport, e.g. private vehicles, to multi-mode transport, e.g. buses, and banning the import of leaded gasoline. In addition, the government has blended 5% ethanol into gasoline to reduce emissions and plans to blend gasoline with 25% ethanol in the future [20, 21]. The study addresses exposure to traffic-related air pollution and attempts to tackle traffic-related emissions. Air pollution monitoring is crucial to fully understand the impact on health. It provides urban planners and policy makers with important insights to mitigate the negative effects of air pollution on the population. This study is thus instrumental in filling the information gaps on traffic-related air pollution in sub-Saharan African cities such as Hawassa city, Ethiopia.

Data on carbon monoxide concentrations remains limited. Traffic-related air pollution is poorly characterized in African countries as very few studies have been conducted on air pollution. Air quality monitoring in Sub-Saharan African (SSA) cities is virtually non-existent and there is no real text to regulate pollutant emissions, forcing the populations [22, 23]. For example, few studies in low-income and middle-income countries have measured roadside concentrations of particulate matter and carbon monoxide, and most studies have reported exceedances of WHO limits [1, 2, 5, 24]. Moreover, there is limited data on carbon monoxide concentration in Ethiopia. To the best of the researcher’s knowledge, there are only two publications in Ethiopia that deal exclusively with carbon monoxide concentration. These shows that the concentration at roadsides and near roads is above 50% of the WHO recommended limits [5, 25]. The aim of this study was to investigate the concentration levels as well as the temporal and spatial variations of carbon monoxide in Hawassa City and to correlate them with factors such as temperature, relative humidity, traffic flow, and road routing in Hawassa City.

Previous work has shown that carbon monoxide concentrations vary along different types of roads and at different times. For example, carbon monoxide concentrations were higher on roads with traffic light than on asphalted main roads, followed by roads with low traffic volumes [5, 14, 26]. There is also a statistically significant difference between high and low traffic volume roads in terms of carbon monoxide concentration, as described in previous studies by [5, 15, 27, 28]. Carbon monoxide concentrations were higher in the morning than in the afternoon, as found in previous researches by [15, 29,30,31]. Studies have shown that temperature [32], relative humidity [30], traffic flow [32], and road routing were positively correlated with carbon monoxide concentration [28].

2 Methods and material

2.1 Study setting

The study was conducted in Hawassa city, the capital of Sidama region, which is 273 km away from Addis Ababa, the capital of Ethiopia. The city administration is divided into 8 sub-cities and 32 kebeles (kebeles are the lowest administrative units in Ethiopia). According to the Ethiopian Central Statistical Authority, the administrative area of Hawassa is 15,720 ha with a total population of 351,469. The geographical location of Hawassa (latitude and longitude 07°29N and 38°52′ E respectively) with an altitude of 1708 m above sea level. Hawassa has a long rainy season (March-October with average monthly rainfall between 85 and 133 mm). The average annual rainfall is 950 mm with a relative humidity of 60% and the annual temperature ranges between 9 and 29 °C.

According to the Hawassa Municipality's Transport and Road Development Office, the city has a total road network of 1983 km. Of these, 152 km are paved, 620 km are gravel, 511 km are dry, 240 km are red ash and 460 km are cobblestone. The total area of the entire road is 10.26 km2. Of this, 36% is asphalt, 48% is gravel and the remaining 16% is cobblestone. The public transport service consists of auto-rickshaws (bajaj) with 11 rickshaw associations, minibuses and City buses, but not public vehicles used by private, non-government and government organizations for passenger transport. There are 3300 rickshaws (bajaj), 568 minibuses and 17 City buses that are legally registered and serve the communities, but vehicles that are not legally registered predominate in the city.

2.2 Study design and period

A cross-sectional study was conducted to investigate the temporal analysis of carbon monoxide concentration at different road types and its relationship with influencing factors in Hawassa city, Ethiopia, during the study period from March 20, 2023 to April 14, 2023.

2.3 Sample size determination and sampling techniques

The sample size was determined using purposive sampling techniques. Generally, three main monitoring sites were purposively selected based on the average traffic volume and peak traffic hours (7:00 to 9:00 am and 4:00 to 5:30 pm) in Hawassa city: High traffic volume roads, including traffic lights roads (5) and main paved roads (7), and low traffic volume roads, including gravel, cobblestone, dry weather and red ash roads with a total of 12 monitoring sites. This resulted in a total of 24 monitoring sites [8]. To characterize the temporal variations of the measurements, sampling was conducted in two phases, with one  hour measurements taken during each of the morning and afternoon peak hours (7:00 to 9:00 am and 4:30 to 5:30 pm). A one-hour measurement was carried out at each location in the morning and afternoon sections.

After measuring the carbon monoxide concentration at each measurement site at 3 min intervals adjusted to one hour, the mean carbon monoxide concentrations during 15 min, 30 min and one hour at each measurement site were calculated as descriptive statistics using the following formula [28]. This allowed a comprehensive assessment of carbon monoxide in the different road types and at peak times and provided valuable insights into the impact of traffic on air pollution in Hawassa City.

$${\text{Average concentrations of CO}} = \frac{{summation\,of\,CO\,concentration\,recorded\,within\,15\,min}}{{5\left( {number\,of\,record\,within\,15\,min} \right)~~}}$$
$$\mathrm{Average\, concentrations\, of\, CO\, }=\frac{Summation\, of\, CO\, concentration\, within\, 30\, min}{\begin{array}{c} \\ 10\left(number\, of\, record\, within\, 30\, min\right)\end{array}}$$
$$\mathrm{Average\, concentrations\, of\, CO}=\frac{Summation\, of\, CO\, concentration\, within\, 60\, min}{20\left(number\, of\, record\, within\, 60\, min\right)}$$

2.4 Operational definition

High traffic volume areas were defined as areas where the average daily traffic volume is greater than or equal to 18,000 vehicles (750 vehicles/hour) and low traffic volume areas refer to areas where the average daily traffic volume is less than or equal to 2800 vehicles (117 vehicles/hour)[8].

2.5 Data collection tools and procedure

The carbon monoxide concentration data was monitored using Aero-Qual series 500 portable monitors placed 1.5 m above the floor to avoid any form of obstruction. The Aero-Qual series is placed in the middle of the roads facing the carbon monoxide sources. The Aero-Qual Series 500 is a lightweight, easy to use contaminant gas detector for determining the outdoor and indoor air quality in an environment. The Aero-Qual Series 200 and 300 are available, but the Aero-Qual Series 500 is more advanced. The operating ranges of the monitor are (− 400–1240c and 0–100%) for temperature and relative humidity respectively. It can measure carbon monoxide concentration in the range of 0–1000 ppm and gives instant, minimum, maximum and average readings depending on the adjustments. The model had a site ID, a monitor ID and an alarm to indicate complete registration. A field monitoring checklist was used to collect the data, which was adapted from previous literature. Monitors were set to record data at 3 min intervals for 1 hour, and the average values were documented on the registration data sheet every 3 min. The recording data sheet included the name of the sampling site with ID and the time [33,34,35,36,37].

2.6 Data quality control

The quality of the data was ensured by the careful use of measuring equipment, compliance with the manufacturer’s guidelines and the use of trained field technicians. The data collectors held master’s degrees in public health with expertise in environmental health and underwent a comprehensive 3 day training program that included the introduction and operation of the portable carbon monoxide monitors. Before measurements began, we carried out an assessment of the Aero-Qual series calibration procedures provided by the manufacturers. The sensors had factory calibration procedures that the manufacturer had calibrated every year. The carbon monoxide sensors have a factory calibration procedure that is reportedly valid for annual factory calibration with a certificate that provides the ultimate assurance [36].

2.7 Data processing and analysis

The data were entered in EpiData version 3.1 and analyzed using the Statistical Package for the Social Sciences (SPSS) version 26. Descriptive statistics were used to summarize the data such as mean, maximum, and minimum. A Kendall’s tau-b correlation coefficient analysis was performed to assess the correlation of carbon monoxide concentrations with factors such as temperature, relative humidity, traffic flow, and road routing.

3 Results of the study

3.1 Temperature and relative humidity

The average ambient temperature for all road types was between 24.45 and 26.8 °C with a mean temperature of 25.5 °C. Similarly, the average relative humidity ranged between 49.5 and 64.5% with a mean relative humidity of 57.05% (Table 1) [32].

Table 1 Metrological data of measurement sites in Hawassa City, 2023
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3.2 The average concentration of carbon monoxide for different road types

The average carbon monoxide concentration on main roads with asphalt, traffic lights and low traffic volume roads including dry weather roads, red ash, gravel and cobblestone ranged from 1.96 to 6.75 ppm, 3.10–7.87 ppm and 0.95–2.52 ppm, respectively (Table 2).

Table 2 Distribution of carbon monoxide concentrations among paved main roads, traffic-light roads and low-traffic roads in Hawassa City, 2023
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The average carbon monoxide concentration on high traffic volume roads, including asphalt main roads and roads with traffic lights, ranged from 1.96 to 7.87 ppm with a mean concentration of 5.0824 ± 1.6 ppm, while the mean carbon monoxide concentration on low traffic volume roads, consisting of dry weather roads, red ash, gravel and cobblestone roads, was 1.62 ± 0.53 ppm.

3.3 Spatial variations in carbon monoxide concentration

The average carbon monoxide concentration during the 15 min, 30 min and 1 hour measurements was 3.48, 3.50 and 3.44 ppm respectively. The average carbon monoxide concentration at sampling site five (S5) was 9.41 ppm, while it was 1.04 ppm at sampling site (S16) during 15 min measurement (Table 3).

Table 3 the distribution of carbon monoxide concentration on each road in Hawassa City, 2023
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The average carbon monoxide concentrations on main roads with asphalt, traffic lights, dry weather, red ash, gravel and cobblestones were 4.80, 5.38, 1.08, 1.64, 1.76 and 1.99 ppm, respectively (Table 4).

Table 4 Average carbon monoxide concentrations by road type in Hawassa City, 2023
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3.4 Temporal variations of CO concentrations

As figured out from (Fig. 1), the average concentration of CO on one-hour measurements was 4.5 ppm in the morning and 2.2 ppm in the afternoon respectively.

Fig. 1
figure 1

Variations of carbon monoxide concentration by morning and afternoon

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3.5 Correlations of carbon monoxide concentration with related factors

Relative humidity, traffic volume, average temperature, and road routing were positively correlated with carbon monoxide concentration. However, the temperature in the afternoon showed no correlation with the carbon monoxide concentration (Table 5) [32, 38, 39].

Table 5 Kendall's tau-b correlation coefficient between carbon monoxide concentration and related factors in Hawassa city, Ethiopia, 2023
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4 Discussion

4.1 Concentration of carbon monoxide on roads

The average total concentration of carbon monoxide on traffic light roads (5.38 ppm) was higher than on paved main roads (4.87 ppm), followed by low-traffic congestion roads (1.62 ppm). The result of this study is consistent with a study from Iran [40] which stated that the concentration of carbon monoxide was up to 150 ppm when the traffic flow is high and it is up to 11 ppm and sometimes up to 4.5 ppm on commercial areas or low exposure area, and Malawi [41], and the present study is also in agreement with the results of a local study from Addis Ababa, Ethiopia [34]. The reason for this discrepancy is that vehicles are released at a traffic light at the same time, causing traffic congestion. During “this time”, “they release” a higher concentration of carbon monoxide than at other times. However, the results of this study report are below the limits recommended by the WHO [1]. The reason for this could be that carbon monoxide is naturally dissolved in the atmospheric air within a minute and does not remain in the environment for long.

This study confirms that there was a highly significant difference between high-traffic and low-traffic congestion roads in terms of carbon monoxide concentration (Z = − 3.868, P = 0.000) with (95% CI P < 0.05). In this study, the carbon monoxide concentration was 4.80 ± 1.45 ppm on paved roads, 5.38 ± 1.77 ppm on traffic light roads and 1.619 ± 0.53 ppm on low-traffic roads. The finding of this study is supported by the study conducted in the USA [42] and Norway [27], which indicates that the carbon monoxide concentration varies depending on the type of road, the presence of traffic lights and the characteristics of the road. The reason for this discrepancy is that vehicles start at a traffic light at the same time, causing traffic congestion. During this time, they release a higher concentration of carbon monoxide. During this time, they released a higher concentration of carbon monoxide than at other times. Another reason could be that asphalted main roads are more congested than roads with low traffic volumes because drivers prefer to drive on main roads, even if there are other roads as an alternative.

In the current study, the concentration of carbon monoxide was higher in the morning than in the afternoon. Accordingly, the mean concentration of carbon monoxide was 4.5 ± 2.87 ppm in the morning and 2.2019 ± 1.58 ppm in the afternoon. The results of this study are consistent with a study in the USA [42], a study in Malaysia [43], a review in Ethiopia [44], and a study in Nigeria [35]. The reason behind the difference might be that carbon monoxide in the morning is less dissolved in the atmospheric air than afternoon times due to climate variability.

4.2 Correlation between carbon monoxide concentrations and related factors

According to this study, average temperature was positively correlated with carbon monoxide concentration (r = 0.315). During this study, the temperature varied between 22.2 and 25.8 °C in the morning and 24–26.7 °C in the afternoon. The morning temperature was positively correlated with the concentration of carbon monoxide (r = 0.475). The result of this study is consistent with the results of the studies done in Dire Dawa, Ethiopia [37]. The reason for the positive correlation could be due to two factors based on the results of this study. The first reason is that the traffic volume in the morning was high compared to the traffic volume in the afternoon as shown by the average traffic flow counts. Secondly, the temperature in the morning is lower than the temperature in the afternoon due to climate variations. Therefore, the carbon monoxide can stay in the atmosphere longer compared to the afternoon time. The third reason could be that the temperature in the morning is more humid compared to the afternoon temperature and these humid temperatures have a lower scattering effect on the carbon monoxide concentration in the air [5, 45].

In the present study, relative humidity was positively correlated with the concentration of carbon monoxide (r = 0.412). The relative humidity ranged between 57.0 and 68.0% and 42–62% in the morning and afternoon, respectively. The result of this study is consistent with a study conducted in the USA [46] and Malaysia [8]. The reason for this is that a humid environment poorly supports the dissolution of carbon monoxide concentrations. In this study, the road routing was positively correlated with the concentration of carbon monoxide (r = 0.478). The result of this study is consistent with the study conducted in Ghana [47]. The positive correlation indicates that traffic flow increases when the number of routes on which vehicles travel increases. This is because these routes were intentionally built in the area where traffic flow was high. So there should be alternative routes for the vehicles to reduce the traffic flow on roads with multiple routes. In this study, traffic flow was also positively correlated with carbon monoxide concentration (r = 0.616). This result is consistent with similar studies in the USA [42], and in Ethiopia [37]. The positive correlation shows that the carbon monoxide concentration in the urban environment is released into the environment by motor vehicles.

5 Conclusion

The concentration as well as the temporal fluctuations of carbon monoxide was investigated. Based on the results, we concluded that carbon monoxide concentrations were higher on traffic light roads than on paved main roads, followed by low-traffic congested roads. In addition, there was a difference in carbon monoxide concentration on low-traffic roads, which include dry-weather, cobblestone, red ash and gravel roads. In terms of time, the average carbon monoxide concentration peaked in the morning but was decline in the afternoon period. Temperature, relative humidity, traffic flow and road conditions were positively correlated with carbon monoxide concentration. Carbon monoxide concentration levels on paved main roads and low-traffic roads were low compared to the World Health Organization (WHO) guideline values, but concentrations on traffic light roads were close to the guideline values. In general, the hourly cycle of carbon monoxide concentration has a peak in the morning and the maximum concentration values occur in the morning hours due to the busy roads.