Introduction

A healthy environment is vital for efficient and impactful learning, especially for children vulnerable to air pollution (Masekela and Vanker 2020). Over time, it has been proven that a clean-air environment increases a child’s attention rate and leads to better and improved participation in the learning and development process of the child (Clark et al. 2020; Adaji et al. 2020; Michelot et al. 2013; Agbo et al. 2021). Therefore, it is vital to always ensure that the environment is clean and health-promoting. The environment is categorized into two divisions: indoor and outdoor environments. There have been numerous studies on ambient air pollution/quality. Still, very few studies have been conducted on indoor air quality (IAQ), particularly in children’s public spaces such as day-care centres (DCCs), preschools, nurseries and kindergartens (Annesi-Maesano et al. 2013; Zhang et al. 2021; Manuel et al. 2021). There are many definitions for IAQ (Cincinelli and Martellini 2017); for this context, IAQ has been defined as the air quality within and around a building, which can affect the general well-being of its occupants (Soreanu 2016; USEPA 2022). Two major parameters are used in assessing IAQ, namely, infiltration of outdoor contaminants and thermal conditions such as temperature, relative humidity and airflow (Cincinelli and Martellini 2017; WHO 2021a). Indoor air pollution in children’s learning spaces is associated with types of indoor activities, infiltration of outdoor pollutants into the indoor environment, nature of building structures, interior decorations, emission of pollutants from building materials, cleaning chemicals, geographical conditions and the nature of ventilation system in use (natural, mechanical or a combination of the two) (Branco et al. 2014; Mannan and Al-Ghamdi 2021; Oliveira et al. 2019; Salthammer et al. 2016; Valderrama-Ulloa et al. 2020; WHO 2021b).

Air quality in an indoor environment is critical because it has been scientifically proven that we spend approximately 70–90% of our time indoors (UNICEF 2019). According to WHO, about five hundred thousand (500,000) children under the age of 5 died in 2016 due to respiratory tract diseases induced by indoor air pollution (IAP) (WHO 2017). Given the health relevance of IAQ, unhealthy IAQ has been assessed as the eighth (8th) most critical environmental risk factor and is responsible for 2.7% of death cases globally. Based on the foregoing, the United Nations Sustainable Development Goal (UNSDG) 3.9 focuses on drastically reducing deaths and illnesses caused by air pollution. As a result, there is an urgent need to navigate research interests to this area.

Some of the indoor chemical pollutants that thrive in children learning environments include particulate matter (PMs) (Guak et al. 2021; Sara et al. 2020; Kalimeri et al. 2016), carbon monoxide (CO) (Masekela and Vanker 2020), nitrogen dioxide (NO2) (Holgate et al. 2021; Nunes et al. 2016), (ozone (O3) (Vu et al. 2019; WHO 2021b), sulphur dioxide (SO2) (Kotzias 2021), phthalate esters (PAEs) (Li et al. 2021, Anake and Nnamani 2022), polycyclic aromatic hydrocarbons (Vardoulakis et al. 2020; Wang et al. 2021), benzene (C6H6) (Siwarom et al. 2017; Vu et al. 2019), formaldehyde (HCHO) and volatile organic compounds (VOCs) (Almeida et al. 2011; Zhang et al. 2021). In order to protect public health from the adverse effects of exposure to these indoor chemical pollutants, standards and guidelines values have been provided by governments in different countries and worldwide organisations. Table 1 outlines the criteria for chemical pollutant set limits by the two internationally recognized regulatory bodies across the globe: the United States Environmental and Protection Agency (USEPA) and World Health Organization (WHO).

Table 1 Air quality standards for criteria air pollutants by the USEPA and WHO
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Previous study report shows that the effect of elevated levels of these indoor chemical pollutants on a child is more than that of an average adult (Olaoye et al. 2021; Canha et al. 2016). A review conducted by Zhang et al. (2021) on indoor air pollution levels and its associated environmental and behavioural factors in nurseries was able to highlight the thermal comfort, ventilation rate and exposure of children to measured pollutants (biological and chemical) in nursery environments. Their study examined work done between 1992 and 2018 in nurseries of children in the age bracket of 3 months to 10 years in Europe, Asia and North America except for Africa. Overall, inadequate ventilation evidenced in the increased levels of CO2 above recommended standards was observed. Also, IAQ in nurseries often exceeded current guidelines; as such, the IAQ performance was declared unacceptable. In this article, we have provided a global overview of the predominant indoor chemical pollutant levels monitored in DCCs from reported studies, compare their concentration with available regulations for IAQ and health protection, evaluate the sources and health risk effects of chemical pollutants on children’s health and propose strategies for enhancing IAQ in DCCs. Furthermore, to the best of the authors’ knowledge, this review is the first to provide information on monitored indoor chemical pollutants in DCCs on the African continent.

Materials and methods

Selection of research method

In this study, the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) approach was used to identify eligible studies for inclusion in the review. All English –language reported work on chemical pollutants done globally on IAQ in DCCs from January 2008 to June 2021 was considered. Databases such as Science Direct, Google Scholar, Scopus, SCIVAL and Journal Storage (JSTOR) were used to generate the relevant materials. The keywords used for the search were day-care centres, preschools, kindergartens, indoor air quality, air pollution, criteria pollutants, chemical pollutants, health effects of chemical pollutants, sources of chemical pollutants and SDGs. The search yielded two hundred and seven (207) peer-reviewed journal articles and conference papers. The exclusion criteria defined for this review include microbial pollutants, measurements in settled dust, studies published before January 2008 and after June 2021, studies conducted in other indoor microenvironments (homes, vehicles, offices, laboratories and universities), non-English articles and when sufficient data were not made available or only plots given without figure descriptions. The inclusion criteria focused solely on indoor chemical pollutant(s), studies published between January 2008 and June 2021, indoor studies conducted in preschools, day-care centres, kindergartens and nursery schools, English articles and articles with results indicating at least one of the following measurement values: minimum, maximum, arithmetic mean or median. The studies’ titles and abstracts were screened using the inclusion and exclusion criteria. Based on the appropriate selection criteria, thirty-seven (37) out of two hundred and seven (207) articles were suitable for inclusion, as shown in Fig. 1.

Fig. 1
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PRISMA study flow diagram of IAQ in DCCs

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Results and discussion

Indoor chemical pollutants monitored in children’s DCCs across the continents

Figure 2 represents the geographical distribution of the 37 articles summarized in the current study. Slovenia, Spain, Greece, France, Poland and Portugal are among the European nations that have published research work on IAQ in DCCs. The continent of Asia with published work on IAQ in DCCs includes South Korea, Malaysia, Iran, Singapore and Thailand. North America has two published research works in Canada and the United States of America (USA) while in Africa, Nigeria is the only country with two published research articles. The USEPA and other environmental regulatory bodies in the USA, such as California health care programs, frequently publish articles on indoor air quality in day-care centres. Australia and South America have no published research work on IAQ in DCCs. However, relevant governmental agencies frequently update their websites with information on IAQ in DCCs. To the best of the authors’ knowledge, no information has been viewed or obtained for Antarctica as of the time this review was being written.

Fig. 2
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Continental map depicting the paucity of data on IAQ in day-care centres

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An overview of the 37 reviewed papers’ results for IAQ in day-care centres with locations, chemical pollutants, age of children in the class, measurement device and the number of centres monitored is shown in Table 2. Also, Table 3 shows a glimpse of the concentration of indoor chemical pollutants in DCCs endemic in specific countries and continents reported from 37 studies. As shown in Tables 2 and 3, only a few studies investigated above five parameters of indoor chemical pollutants, and these studies were mostly from the European continent. A breakdown of the studies based on their pollutants of interest includes 10 studies (I indoor pollutant), 1 study (2 indoor pollutants) 5 studies (3 indoor pollutants), 10 studies (4 indoor pollutants), 6 studies (5 indoor pollutants), 2 studies (6 indoor pollutants), 2 studies (7 indoor pollutants) and 1 study (9 indoor pollutants). However, there were variations in the measurement devices used for measuring specific pollutants across the study.

Table 2 A summary of reviewed paper results for indoor air pollutants in day-care centres
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Table 3 Concentration of common chemical pollutants reported across the continents
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From the studies of indoor air quality in preschools, kindergartens and day-care centres conducted across the globe between January 2008 and June 2021, the investigated indoor chemical pollutants identified were particulate matter (total suspended particulate matter (TSP), coarse particulate matter (PM10), fine particulate matter (PM2.5) and ultrafine particulate matter (PM1, PM0.1)) carbon dioxide (CO2), carbon monoxide (CO), formaldehyde (HCHO), volatile organic compounds (VOCs), total volatile organic compounds (TVOCs), nitrogen dioxide (NO2), ozone (O3), benzene (C6H6.), sulphur dioxide (SO2) and radon (Rn). The predominant indoor chemical pollutant was PM (TSP: 3 studies; PM10: 19 studies, PM2.5: 14 studies, PM1: 5 studies and PM0.1: 2 studies). This was followed by CO2 (23 studies), HCHO (14 studies), CO (12 studies), TVOCs (10 studies), VOCs (8 studies), NO2 (8 studies), O3 (7 studies), C6H6 (4 studies), SO2 (2 studies) and Rn (2 studies). In total, 15 pollutants were considered (see Table 3). The decreasing trend among the investigated indoor chemical pollutants globally is as shown: PM (PM10 > PM2.5) > CO2 > HCHO> CO > TVOCs > VOCs = NO2 > O3 > C6H6 > SO2 = Rn.

Particulate matter was the most investigated indoor chemical pollutants in day-care centres in this review. Particulate matter is used as a surrogate indicator of air pollution on a broad scale (Almeida et al. 2011). It has been reported as one of the most researched contaminants because of its effects on a child’s growing brain (Jelili et al. 2020). The concentration ranges for the different classes of particulate matter results shown in Table 3 are coarse particulate matter (PM10) (0.116–1920.71 μg/m3), fine particulate matter (PM2.5) (0.279.2–260.74 μg/m3), ultrafine particulate matter (PM1 and PM0.1) (8.99–78.13 and 30.50–90.50 μg/m3) and total suspended particulate matter (TSP) (15.04–217.33 μg/m3). There are three main categories of particulate matter: PM10, PM2.5 and PM0.1. PM10 particles have a diameter of less than 10 μm, PM2.5 particles are smaller than 2.5 μm, and PM0.1 particles are smaller than 0.1 μm (Anake et al. 2017; Mukherjee and Agrawal 2017). However, the two most frequently investigated categories of particulate matter across the globe in descending order were PM10 (19 studies) > PM2.5 (14 studies). From the result presented, it was observed that the continent of Asia (Iran) had the highest levels of PM10 (1920.71 μg/m3) and PM2.5 (260.74 μg/m3) which was attributed to the occurrence of the Middle Eastern Dust (MED) storm in February 2017 (Harbizadeh et al. 2019). In order of decreasing trend, PM concentrations in Asian children’s day-care centres were Iran > South Korea > Malaysia > Thailand. The second highest continent with respect to PM10 was Africa (Nigeria: 677 μg/m3) and Europe (Poland: 80.94 μg/m3), respectively. The second and third most measured indoor chemical pollutants were CO2 and HCHO, with the highest concentration of 5647 mg/m3 and 204 μg/m3 documented in Europe and Asia, respectively (Table 3). Portuguese nurseries from the European continent recorded the highest concentration of CO (4956 μg/m3) and TVOCs (3899 μg/m3), respectively. In addition, the highest concentration of O3 (123 μg/m3) was recorded in Asia while Europe had the highest levels of NO2 (189 μg/m3), C6H6 (9.4 μg/m3) and Rn (84 Bq/m3).

However, the intercontinental comparison indicates that Iran in Asia recorded indoor mean PM10 concentrations of 1920.71 μg/m3 in day-care centres which exceeded the acceptable Korean PM10 IAQ standard limits of 75 μg/m3 with a 24-h average (Harbizadeh et al. 2019). In South Korea, indoor PM10 levels of range 0–754.7 μg/m3, 21–216 μg/m3 and 20.9–147.5 μg/m3 by Kim et al. (2021), Yang et al. (2009) and Guak et al. (2021) in the day-care centres, nursery and kindergartens, respectively exceeded the Korean PM10 IAQ standard of 75 μg/m3 with a 24-h average. Also, Thailand’s results showed that three out of eleven DCCs had mean PM10 levels of 70 μg/m3 higher than the set limit in all the seasons (Siwarom et al. 2017). Similarly in Africa (Nigeria), some of the PM10 levels monitored in the morning and afternoon (536.8 μg/m3 and 677 μg/m3) exceeded the WHO PM10 guideline limit of 50 μg/m3 with a 24-h average (Ana and Umar 2013). European children educative facilities in Portugal equally indicated elevated PM10 levels across different years as follows: 421 μg/m3 (Mendes et al. 2014), 88.0 μg/m3 (Branco et al. 2020), 58.28 μg/m3 (Nunes et al. 2016) and 56.77 μg/m3 (Branco et al. 2014) showing consistently the exceeded threshold of 50 μg/m3 established by national regulations for air quality of public buildings in Portugal. Also, in Poland, Mainka and Zajusz-Zubek (2015) and Mainka et al. (2015) reported PM10 indoor concentration range of 68.26–104.90 μg/m3 and 117.57–149.81 μg/m3 in the day-care centres and the urban and rural nurseries, respectively. This finding revealed indoor PM10 contributions above the 24-h mean concentration of 50 μg/m3 limit set by WHO and the Polish legislation.

Furthermore, for PM2.5, South Korea and Singapore in Asia recorded the highest concentration of 260.74 and 279.2 μg/m3 by Harbizadeh et al. (2019) and Kim et al. (2021) respectively, above the Korean PM2.5 IAQ threshold of 35 μg/m3 with a 24-h average. In the European continent, reports from Poland by Mainka and Zajusz-Zubek (2015) (range: 41.17–80.94 μg/m3) and Mainka et al. (2015) (range: 70.53–106.06 μg/m3) clearly indicated elevated concentrations of some indoor fine particulate in the day-care centres for the urban and rural nurseries, respectively. Also, some indoor PM2.5 concentration from Greece and Portugal exceeded the threshold of a 24-h average standard of 25 μg/m3 established by national regulations for air quality of public buildings in Portugal as shown: 72–83 μg/m3 (Kalimeri et al. 2016), 49.0–54.7 μg/m3 (Branco et al. 2020) and 18.17–48.94 μg/m3 (Branco et al. 2014). Overall, the levels of PMs in most studies exceeded the various regulatory limits set by both local and international bodies. It is worth noting that regulatory bodies such as the USEPA and WHO have identified PM as a priority pollutant with a high probability of causing pulmonary diseases, shortness of breath, asthma, allergic reactions and several other respiratory-related disorders such as coughing, sneezing and wheezing. To lower the concentration of particulate matter, an adequate ventilation strategy is critical. This was verified in a study by Lee et al. (2020), where it was noted that day-care facilities practising good hygiene procedures experienced a considerable decrease in particulate matter concentration. It is worth noting that the indoor air pollutants above the established threshold were usually observed during the occupancy period implying that control ventilation and indoor activity can assist in obtaining better indoor air quality. Continents and countries with evidenced exceedance in the concentrations of CO2 above the regulatory limits set by both local and international bodies include Asia, North America and Europe. In Asia, South Korea indoor CO2 levels of range; 381–3623 ppm, 895–2257 ppm, 555.00–1675.00 ppm and 502.7–1261.40 ppm by Kim et al. (2021), Oh and Song (2021), Yang et al. (2009) and Hwang et al. (2017) in some of the day-care centres and kindergartens exceeded the Korean IAQ recommended levels of 1000 ppm. Also, Malaysia with highest CO2 levels of 1005.9 ppm (Kamaruzzaman and Razak 2011) exceeded the Malaysia Department of Safety and Health set limit of 1000 ppm given by the American Society for Heating, Refrigerating and Air Conditioning Engineers (ASHRAE). Furthermore, in North America, St-Jean et al. (2012)’s report indicated that about 85% of the DCCs had CO2 concentrations (range: 723–2252 ppm and mean 1333) higher than Canada’s Residential IAQ Guideline of 1000 ppm. Similarly, European nurseries confirmed non-compliance to set regulatory limits indoor CO2 concentration range of 420–4207 ppm (Telejkoa and Zender-Świercza 2016) which were far above the < 1000–3700 ppm CO2 concentration range recorded by Mainka et al. (2015) in Poland. Both CO2 values were well above the recommended levels of 1000 ppm. From Greece, Theodosiou and Ordoumpozanis (2008) observed that some of the monitored values (range: 480–2500 ppm) exceeded the maximum recommended CO2 concentration in a classroom of 800 ppm above that of outdoors. Also, in Slovenia, Lovec et al. (2020) with CO2 levels of 410–2452 ppm exceeded the national required value of 1667 ppm in some locations (Dovjak et al. 2020) while Portuguese highest CO2 levels above the stipulated regulatory limits were observed by Cano et al. (2012) (5647 mg/m3), Mendes et al. (2014) (3087 ppm), Manuel et al. (2021) (2518 ppm), Branco et al. (2020) (2335 mg/m3), (Araújo-Martins et al. (2014) (2137 ± 368 ppm) and Carreiro-Martins et al. (2014) (1440 ppm). Similarly, Roda et al. (2011) and Canha et al. (2016) with CO2 concentration of 2037 ppm and 1200±400 mg/m3, respectively, exceeded the 1000 ppm regulatory limit in France. From the foregoing, CO2 level exceeding the specified threshold was commonly reported in most DCCs across the globe, a pointer to inadequate ventilation systems. However, many of the studies reviewed suggest that the presence of an effective mechanical ventilation system and a large surface of play area per child was significantly associated with lower CO2 level.

Exceedance in the concentration of VOCs above the standard value set by both local and international bodies was equally reported in Asia and Europe. From Asia, Kamaruzzaman and Razak (2011) in Malaysia noted that from the indoor VOC concentration range of 0.08–0.54 mg/m3, only the VOC rate of 0.54 mg/m3 was above the Malaysian Department of Safety and Health set limit of 0.1 mg/m3, given by ASHRAE. The concentrations of TVOCs above the standard value set by both local and international bodies were equally reported in Asia and Europe. From Asia, Yang et al. (2009) reported TVOC concentration range of 264.00–1024.00 μg/m3 in four (4) DCCs which was far above Hwang et al. (2017)’s TVOC concentration range of 133.0–512.9 μg/m3 in South Korea. However, both TVOCs were higher than the Korean recommended level of 400 μg/m3. Also in Europe, Oliveira et al. (2016) reported the highest TVOC levels of 3.91 mg/m3 (3910 μg/m3) which was slightly above Mendes et al. (2014)’s TVOC concentration of 3899 μg/m3 and far above Branco et al. (2015)’s TVOC levels of 2330 μg/m3. All the authors confirm TVOC levels to be well above the Portuguese legislation (Portaria n° 353-A/2013) indoor concentration limit value of 600 μg/m3. Recently, Sa et al. (2021) reported indoor TVOC concentration range of 348–1570 μg/m3 above the reference Portuguese legislated limit value of (1,200 μg/m3) in a nursery classroom during the COVID-19 pandemic. Furthermore, in Asia, a single measured SO2 level exceeded the WHO standard level of 20 μg/m3 for 24 h of indoor exposure to SO2 in three DCCs while O3 levels of 123 μg/m3 exceeded the WHO standards level of 100 μg/m3 (8 h) for short-term indoor exposure in 80% of DCCs during the winter season (Siwarom et al. 2017). Based on our findings from the studies included in this review, only the levels of NO2, Rn and CO were consistent with the existing national and international reference values for IAQ and health protection across the continents.

Review on indoor air quality in Asia, Europe and North America day-care centres

As shown in Fig. 1, thirteen (13) papers within the scope of this review have been published from Asian countries. Eleven (11) pollutants were observed in the following decreasing trend: PM > CO2 > CO > VOCs = TVOCs = HCHO = O3 > NO2 > C6H6 = SO2 = Rn. Iran had the highest concentration of PMs, while South Korea had the highest concentration of VOCs, TVOCs and CO2. Underlying factors, such as rapid economic development and urbanization, accounted for the decline in IAQ in most Asian indoor microenvironments. Despite significant improvement according to the Clean Air Initiatives for Asian Cities (CIA-Asia, 2010), PMs and VOCs still exceeded the WHO threshold limits. Furthermore, European cities with twenty papers recorded the highest number of reported studies within the context of this review. The distribution according to countries was Portugal (11 studies), Poland (3 studies), Greece (2 studies) France (2 studies), Spain (1 study) and Slovenia (1 study). The investigated pollutants are as shown: PM > CO2 > HCHO > TVOCs > CO> NO2 > VOC = O3 > C6H6 > Rn. However, in most European studies, very high concentrations of CO2 were recorded in children daycare centres, with Portugal (5647 mg/m3) taking the lead. Portugal also had the highest concentration of PM10 (421 μg/m3), TVOCs (3899 μg/m3) and HCHO (204 μg/m3) in Europe (Table 3). Only two (2) papers within the scope of this review were published in North America. The pollutants investigated in North America include VOCs, CO2, HCHO and C6H6. Only VOC was present in both studies, with the highest concentration of 163.2 μg/m3 recorded in Canada. Although not considered among the common indoor chemical pollutants in this review, it is worth noting that 2-(2-methoxyethoxy) ethanol which has never been observed and documented in any research, was detected in one of the classrooms in the USA study (Vu et al. 2019).

Review on indoor air quality in Africa day-care centres

According to a United Nations Children’s Fund (UNICEF) report, indoor air pollution in Africa is said to be the highest in the world due to inadequate modern energy access in rural areas (UNICEF 2019). The continent of Africa consists of 54 countries, with Nigeria being the most populous, yet there is no sufficient information on this sensitive research focus. Figure 3 shows the scarcity of work on IAQ in DCCs in Africa. At the time of this review, only two (2) published studies, Ana and Umar (2013) and Nkwocha and Egejuru (2008), both in Nigeria, had been recorded, as shown in Table 2. However, minimal parameters such as particulate matter (PM), nitrogen dioxide (NO2), sulphur dioxide (SO2) and carbon monoxide (CO) were investigated in these studies. The findings showed that the concentration of particulate matter exceeded the WHO set limit. The results obtained from the Nigeria studies are in agreement with the research works carried out in Malaysia (Sara et al. 2020), Portugal (Branco et al. 2020), Iran (Harbizadeh et al. 2019), Thailand (Siwarom et al. 2017), Portugal (Nunes et al. 2016), Greece (Oliveira et al. 2016), Poland (Mainka et al. 2015) and a review study conducted by Zhang et al. (2021).

Fig. 3
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Map of Africa depicting the paucity of data on indoor air quality in DCCs

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Another vital reason for unhealthy IAQ in Africa is the unavailability of real-time air monitoring stations in most parts of the continent. Although knowledge about air pollution on the African continent is growing, the severe health effects and epidemiological studies are still unknown (Agbo et al. 2021). Compared to other continents, such as North America and Europe, only about 6% of children on the African continent live within a 50-km radius of online real-time air monitoring stations. Approximately seven African countries, including Zambia, Zimbabwe, Madagascar, Ethiopia, Ghana, Botswana and Tanzania, have adequate and dependable real-time air pollution monitors. These shortcomings and differences necessitate a serious and timely intervention (UNICEF 2019; WHO 2021a, 2021b; Jafta et al. 2017; Kouao et al. 2019; Manisalidis et al. 2020; da Rocha Silva et al. 2018; Makoni 2020; Anake et al. 2020; Nicholl 2019).

Comparing the results of the indoor chemical pollutant (PM10, NO2, CO and SO2) studies carried out in African nurseries with those obtained in other global areas, as given in Tables 2 and 3 and outlined in section A, our findings show that the highest PM10 concentration range of 677 μg/m3 from African DCCs was lower than those reported in Asian countries except South Korea and Thailand (Guak et al. 2021; Yang et al. 2009; Siwarom et al. 2017), but significantly higher than those in Europe (Mendes et al. 2014; Branco et al. 2020): 58.28 μg/m3 (Nunes et al. 2016). Table 3 indicates that NO2 levels were reported only in Europe (Portugal: 6–136 and 51.2–54.2 μg/m3; France: 9.5–53.5 μg/m3; Greece: 4.6–43 μg/m3; Spain: 8.1-25.2 μg/m3) and Africa (Nigeria > 88 μg/m3). The highest mean level of NO2 in European preschool (136 μg/m3) was significantly higher than that in Africa (88 μg/m3). However, both values were within the WHO 1-h indoor nitrogen dioxide guideline of 200 μg/m3. Previous preschool studies have shown that indoor NO2 levels are usually indicators of outdoor levels, in the absence of an indoor source (Villanueva et al. 2018; Sadrizadeh et al. 2022). From Table 3, it is shown that the indoor CO concentration range of 1.83 μg/m3 recorded in Africa was significantly lower than the 4956 μg/m3 levels reported in Europe but higher than those reported in Asia (1.4 μg/m3). From the reports, indoor studies with observed CO were attributed mainly to traffic-related pollutants from outdoor-related sources (Nunes et al. 2016; Zhang et al. 2021). Only a fewer studies monitored indoor SO2 concentration in DCCs. The mean level of indoor SO2 recorded in Africa (> 50 μg/m3) was higher than that in Asia (16 μg/m3). However, as shown in Table 3, North American studies did not monitor PM10, NO2, CO and SO2 and as such not included in the comparison.

It is worth mentioning that several studies in other environments, such as residential homes, bakeries and school buildings, have been conducted in Africa (Jafta et al. 2017). Highlights of the studies are documented in Table S1, including Uganda (villages) (Nakora et al. 2020), Malawi (households) (Rylance et al. 2019), Côte d’Ivoire (homes) (Kouao et al. 2019), Ethiopia (homes) (Downward et al. 2018), South Africa (homes) (Jafta et al. 2017), Botswana (national review) (Wiston 2017), Kenya (homes) (Yip et al. 2017), Nigeria (homes) (Mbanya and Sridhar 2017), Nepal, Kenya and Sudan (homes) (Bikram et al. 2011). In comparison with Nigerian studies, a study carried out in six villages in Uganda showed that PM exceeded the WHO limit (Nakora et al. 2020), and CO was above the threshold in forty-five households in Kenya (Yip et al. 2017) and Uganda (Nakora et al. 2020). Furthermore, NO2 exceeded the set limit in a Bostwana national study while SO2 was within the WHO limit in Bostwana but higher than the threshold in South Africa.

Sources of indoor chemical pollutants and health risk effect

Indoor air can be contaminated by two major types of indoor air pollutants namely biological and chemical pollutants (Abaje et al. 2020). Chemical pollutants occur naturally or are caused by human activities. Poor aeration also encourages indoor chemical pollutants (Kim et al. 2021). Therefore, indoor CO2 concentration is always used as an indoor air quality evaluator and not necessarily considered a pollutant (Telejkoa and Zender-Świercza 2016; Salthammer et al. 2016; Schibuola and Tambani 2020; Branco et al. 2020; Zhang et al. 2021). Table 4 outlines some significant indoor chemical pollutants, their sources and their health effects. Exposure to these chemicals has been associated with several health issues such as minor to acute respiratory-related illnesses, including cough, cold, bronchitis, cardiovascular diseases, headaches, eye irritation, dizziness, fatigue, delayed child development and lifetime illnesses like chronic asthma, even in families without a history of the condition (Simwela et al. 2018; Siwarom et al. 2017; Guak et al. 2021; Manuel et al. 2021; Persson et al. 2019; Rees et al. 2019; Stamatelopoulou et al. 2019). When children breathe in high amounts of chemical contaminants from the environment, it can hinder their growth and have negative effects on their immunological and respiratory systems (WHO 2018). Excessive indoor air pollution can impair lung growth and function and increase the likelihood of metabolic disorders in human physiology. It also inhibits brain maturation and the development of cognitive function in schoolchildren. Lee et al. (2020) and Jafta et al. (2017) reported that the IAQ has significant effects on the intelligence quotient (IQ) of a child. Another study discovered that infants exposed to polluted air in the womb could have a four-point drop in their intelligence quotient (IQ) at age 5 (Perera et al. 2019).

Table 4 Sources and health risk effect of the most probable indoor chemical pollutants in DCCs
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Similarities and differences have been observed in the sources of indoor chemical pollutants affecting DCCs across different countries and global areas. Similar sources have been associated with vehicular activities, infiltration from the outdoor environment, improper ventilation, inadequate floor spacing and proximity to busy roads and industrial activities (Guak et al. 2021; Harbizadeh et al. 2019; Kim et al. 2021; Othman et al. 2019; St-Jean et al. 2012; Vu et al. 2019). Underlying reasons for variation in the sources of indoor chemical pollutants in different countries and continents have been attributed to geographical, climatic and seasonal differences; indoor activities and the nature of the building and interior decoration materials (Vardoulakis et al. 2020; Yoon et al. 2011; Roda et al. 2011; Vu et al. 2019). For example, low- and medium-income continents like Africa and Asia, which rely heavily on solid fuel, have a higher concentration of particulate matter compared to European and North American continents. Also, a very high concentration of VOCs and TVOCs attributed to the innovation of new chemical substances was observed in Europe.

Indoor air quality remediation methods in day-care centres

  • Addressing indoor air quality in a day-care environment through source identification and eradication is a cost-effective and time-efficient method (Stamatelopoulou et al. 2019; Siwarom et al. 2017; Rylance et al. 2019; Gola et al. 2019).

  • Appropriate ventilation and adequate floor spacing should be employed. Proper aeration regulates room temperature and dilutes indoor airborne pollutants (Yip et al. 2017; Bukina 2018; Namvar et al. 2020; Langer et al. 2020; Agarwal et al. 2021; Wolkoff 2018; Kedare et al. 2020).

  • Heating, ventilation and air conditioning systems (HVACs) should be serviced and inspected regularly to avoid the accumulation of indoor pollutants (Canha et al. 2016; Chen et al. 2020; Lucattini et al. 2018)

  • Frequent cleaning of classrooms using microfiber mops and a vacuum with a clean high-efficiency particulate air (HEPA) filter (Rosário Filho et al. 2021; Zainudin et al. 2019).

  • Low-emission materials should be used rather than general building finishing materials in childcare facilities (Arar and Jung 2021).

  • Educating preschool administrators about indoor air quality management (Sadrizadeh et al. 2022).

  • Growing an indoor air pollution tolerance plant is critical because it absorbs toxic substances from the air and purifies the air in that environment (Anake et al., 2018; Brilli et al., 2018).

  • Air purifiers can deactivate suspended particles in the atmosphere by trapping a high proportion of airborne dust particles, allergens and odours, thereby improving indoor air quality (IAQ) in a room. However, it must be maintained optimally to prevent ozone emissions (Agarwal et al. 2021; Wolkoff 2018; Kedare et al. 2020; Chen et al. 2020; Lucattini et al. 2018. Nicholl 2019; Yoda et al. 2020).

Important findings from the study

  • A limited number of chemical pollutants were investigated by different researchers across different study locations. This makes it difficult to provide an in-depth assessment of the IAQ in the studied areas.

  • The top 5 predominant pollutants examined in most studies were particulate matter, carbon dioxide, formaldehyde, carbon monoxide and total volatile organic compounds while benzene, sulphur dioxide and radon were the least monitored indoor chemical pollutants.

  • Poor IAQ characterizes most of the DCCs, as evident in the high concentrations of the investigated pollutants exceeding the WHO and available regulations.

  • Only the levels of nitrogen dioxide, radon and carbon monoxide were consistent with the existing national and international reference values for IAQ and health protection across the continents.

  • African studies were evaluated in this global review, which had never been reported in any global review on IAQ in DCCs.

  • The studies with good indoor air quality were due to adequate floor ventilation, consistent cleaning habits and an appropriate heating and ventilation system.

Research priorities

The global state and health consequences of IAQ in DCCs were examined in this review. PRISMA approach was adopted for identifying eligible studies suitable for inclusion in the review. The day-care location, toxic building materials and external air contaminant infiltration all play a significant role in the decline of indoor air quality in the majority of day-care centres. Consequently, this has led to various respiratory symptoms and diseases in children, including acute lower respiratory infection, asthma and impairment of cognitive conditions. Particulate matter was projected as the most investigated pollutant among other predominant indoor pollutants due to its unique characteristic and severe health challenges. The following are additional important information gaps, recommendations and suggestions for future studies.

  • A comprehensive evaluation of chemical pollutants in a single research study across the continents is required and should be done on a periodic basis.

  • Integrating processes such as the cultivation of indoor air tolerance plants, adequate HVACs systems, continuous equipping and training of school administrators and staff are highly recommended to improve IAQ in these sensitive environments.

  • The design of affordable and mobile analytical equipment will aid in affordable ground-level data accessibility, especially in low- and medium-income countries.

  • Concentration on the relationship between respiratory-related diseases in children and indoor chemical pollutants should be prioritized especially in continents with paucity of data.

  • Finally, there is a dire need to increase research focus in day-care centres for both criteria and emerging indoor chemical pollutants.