This study was conducted in Ewekoro and surrounding settlements nearby the Ewekoro Lafarge Cement Factory of Ogun State, Nigeria. The cement facility is situated along the Lagos-Abeokuta Express Road at latitudes 6˚54'N and 6˚55'N and longitudes of 3˚12'E and 3˚13'E ( Figure 1 ). The town is largely rural and known for its rich natural resources. The town has a total land area of 594 km² and a population of 108,944 inhabitants.

The Ewekoro Lafarge Cement Factory is Nigeria’s oldest cement factory. Since being established in 1959 in Ewekoro, the cement factory has contributed greatly to the state’s economy. The limestone rock in this region provided the cement material ( Agbede et al., 2022 ). About 1.8 million tonnes of cement are produced annually from the Lafarge Cement Factory. The production technology evolved from a semi-wet system in 1960 to a wet system in 1978 and a fully dry system in 2002. Each of these systems produced different levels of pollution ( Agbede et al., 2022 ).

The Lafarge Cement Factory in Ewekoro has been a major source of environmental problems since its opening ( Afolabi et al., 2012 ). Environmental pollution caused by the Lafarge Cement Factory is a major problem with serious health consequences for Ewekoro residents and nearby settlements. Cement production releases toxic emissions into the air, including dust particles, carbon dioxide, nitrogen oxide, suspended particulate matter, and heavy metals ( Oluseyi et al., 2011 ). In addition, effluent discharge into the Akinbo River is also of major concern ( Agbede et al., 2022 ).

Air monitoring for PM_2.5 and PM₁₀ was carried out in the study area eight hours daily for 12 days (96 hours). This sampling was done during the dry season for each sampling site. The neighbourhoods were inspected to choose appropriate sampling sites based on their proximity to the cement factory.

An ARA N-FRM air sampler, an alternative to the Federal US EPA Federal Reference Method (FRM) (also called near FRM, or n-FRM), was used to monitor the air quality for PM₁₀ and PM_2.5 concentrations at locations near the cement factory. The ARA N-FRM air sampler is a battery-operated, portable air sampler fitted with a PEET anemometer. The sampler has a 47 mm filter cassette that traps air pollutants, which can be further analyzed in the laboratory ( ARA Instruments, 2024 ).

The particulate sampler chosen is a new class of air sampler. Althoµgh not a certified US EPA Federal Reference Method (FRM), the particulate sampler is “near FRM” and can be validated ( ARA Instruments, 2024 ). The ARA N-FRM sampler delivers FRM accuracy at a tiny fraction of the cost (ARA Instruments, 2024). The ARA N-FRM air sampler has an automated weather station device that measures the locations’ wind direction, speed, and pressure. The ARA N-FRM ( Figure 2 ) is an affordable, readily accessible particulate monitoring system that boosts air quality surveillance networks for spatial and local variations in PM concentrations ( Solademi & Thompson, 2023 ).

The sampler was calibrated according to the instruction manual before each sampling event to reduce the sampling error. The sampler was set at nose height, 5.5 feet above the ground. Table 1 lists the sampling locations near the Ewekoro Lafarge Cement factory with their geographical coordinates and characteristics.

The air was monitored for eight continuous hours daily, from approximately 10:00 am to 6:00 pm. The air sampler logs the PM_2.5 and PM₁₀ μg/m³ values every 15-minute interval for eight hours. Air quality at the cement factory area was monitored for four days (32 hours), i.e. eight continuous hours daily. The blasting/production area (BPA) was monitored for (16 hours) and the Packaging area (PA) (16 hours) in the cement factory area, respectively, totaling 32 hours of monitoring in the cement factory area. Air quality at Papalantoro was monitored for three days (24 hours), Lapeleko for three days (24 hours) and Itori for two days (16 hours).

Figure 3 shows the map sampling points near the cement factory (the blasting/production area and the packaging area) and the impacted neighbourhoods, Papalantoro, Lapeleko, and Itori. The cement factory is very close to residential settlements and communities, and its activities harm neighbouring communities.

Krµg et al. (2021) validated the data recorded by the ARA N-FRM sampler through collocation with designated EPA PM_2.5 FRM and Tisch Environmental (Cleves, OH) Model TE-WILBUR filter-based FRM samplers. The ARA N-FRM was the only small filter-based sampler to meet accuracy standards for EPA PM_2.5 FRM mass measurement performance criteria. The ARA Instruments model N-FRM sampler satisfies the performance specifications of the FRM standards and guidelines for PM_2.5 mass measurement accuracy in both simulated wild fire circumstances (98.2% ± 1.4%) and average ambient conditions (97.3% ± 1.9%). According to Krµg et al. (2021) , the validity of ARA N-FRM with Tish FRM PM2.5 air monitoring showed r² = 0.99 and r² = 0.99 for ambient and chamber testing.

The 47 mm filter cassette in the ARA N-FRM tapped air pollutants Which were then analyzed in the laboratory. The exposed filter paper sample was cut into pieces and then digested. The cut filter was transferred into a glass tube, with 5 ml of mixed acid extractant, 65 ml concentrated (conc.) nitric acid and 185 ml conc. hydrochloric acid dissolved in 1L of deionized distilled water. The acid sample was heated at 1000 C for 30 minutes in a water bath and 500 degrees C for 30 minutes in an ultrasonicator. The cycle was repeated, and the resulting digested sample was transferred into a graduated vial bottle and dilute with 10 ml distilled water. A reverse agua regia procedure was adopted for the digestion of Teflon filters. The digested sample was analyzed for heavy metals of interest using an atomic absorption spectrophotometer.

Ingestion, inhalation, and dermal contact are the three principal ways that individuals are exposed to heavy metals in the air ( Wang et al., 2018 ). This exposure assessment to heavy metals was computed and calculated for adults and children according to the Human Health Evaluation Manual (Part A), Supplemental Guidance for Dermal Risk Assessment (Part E), and Supplemental Guidance for Inhalation Risk Assessment (Part F) ( Wang et al., 2018 ). The exposure assessment was calculated individually for ingestion (CDI), inhalation (EC), and dermal contact (DAD).

The formulas for calculation are shown in Equations (1), (2) and (3):

$\begin{array}{l}\text{Chemical daily ingestion}\left(\text{mg}/\text{kg}\right)\left(\text{CDI}\right)\\ =\left(\text{C}\times \text{IR}\times \text{EF}\times \text{ED}\times \text{CF}\right)/\left(\text{BW}\times \text{AT}\right)\end{array}$ (1)

$\begin{array}{l}\text{Exposure concentration inhalation}\left(\text{g}/{\text{m}}^3\right)\left(\text{EC}\right)\\ =\left(\text{C}\times \text{ET}\times \text{EF}\times \text{ED}\right)/\text{ATn}\end{array}$ (2)

$\begin{array}{l}\text{Dermally absorbed dose}\left(\text{mg}/\text{kg}\right)\left(\text{DAD}\right)\\ =\left(\text{C}\times \text{SA}\times \text{AF}\times \text{ABS}\times \text{EF}\times \text{ED}\times \text{CF}\right)/\left(\text{BW}\times \text{AT}\right)\end{array}$ (3)

LEGEND: Parameters for exposure assessment for adults and children defined for notation and units (source: Revised from Wang et al., 2018 ).

A risk characterization assessment was undertaken to calculate the non-carcinogenic risk. The hazard quotient (HQ) assesses the non-carcinogenic risk of a particular pollutant in an exposure route ( Wang et al., 2018 ). The non-carcinogenic hazard quotient (HQ) is computed as the quotient between the atmospheric exposure and the reference dose (RfD) ( Koki et al., 2015 ). An HQ below one indicates a lower probability of non-carcinogenic side effects is less. A higher HQ indicates a higher probability of non-carcinogenic effects ( Wang et al., 2018 ). The HQ was calculated using the method by Wang et al. (2018) in Equations (4), (5) and (6).

Non-carcinogenic Hazard Quotient (HQ) = CDI/RfDo (4)

HQ =DAD/(RfDo _ GIABS) (5)

HQ = EC/(RfCi _ 1000) (6)

Legend (Source: Wang et al., 2018 ):

CDI - Chemical daily intake throµgh ingestion (mg/kg) (CDI),

EC - Exposure concentration throµgh inhalation (g/m³) (EC),

DAD - Dermally absorbed dose (mg/kg) (DAD),

RfDo - oral reference dose (mg/kg/day),

RfCi - inhalation reference concentration (mg/m³) *,

GIABS - gastrointestinal absorption factor* SFo - oral slope factor ((mg/kg/day) *.

The data were analyzed using descriptive statistically significant and variance analysis (ANOVA). The ANOVA statistical difference was defined at P < 0.05 ( Oguntoke et al., 2012 ). The correlation coefficient (r), 0.1 < /r/ < 0.3, signifies low correlation, while 0.3 < /r/ < 0.5 and /r/ > 0.5 signifies a moderate and strong correlation ( Lala et al., 2023 ).

The sampling data at Ewekoro were validated with daily mean values of PM_2.5 from a 24-hour monitoring station at the University of Ibadan, Centre for Atmospheric Research, National Space Research and Development Agency (CAR-NASRDA). The pollutant levels are compared with Nigeria’s National Environmental Standard and Regulation Enforcement Agency guideline ( National Environmental Air Quality Control and Regulation, 2021 ), Canadian Ambient Air Quality Standards (CAAQS) ( Canadian Council Ministers of the Environment, 2014 ) and World Health Organization (WHO).

An area map was created using Arc-GIS v10.3 to show the relationships between polluters and polluted areas. Maps or time series information were undertaken to represent the spatial and temporal variations in the concentration and dispersion of particulate matter and metal air pollutants. The method undertaken by Solademi & Thompson (2023) was undertaken using ArcGIS ( Solademi & Thompson, 2023 ). The map was interpolated with geostatistical kriging analysis, and the green and yellow colour representations were used to show the level of the air quality zone as compared to the CAAQS. The interpolated maps used Green and red to show the WHO standards limit for heavy metals (Cd, Cr, Cu, Zn, Ni).

The mean PM concentrations in the cement factory (blasting/production area and packaging area), Papalantoro, Lapeleko and Itori are shown in Table 2 . The daily concentration of PM_2.5 were highest at Itori 1 (18.48 μg/m³), followed by the packaging area (PA2) (18.04 μg/m³), and Papalantoro 1(17.24 μg/m³). The cement factory pollution is near to neighbourhoods, with Itori being 4 km away, Palantoro 1km, Lapeleko 2 - 3 km and the Ewekoro sampling site 500 m - 2 km from the cement factory. For PM₁₀, the packaging areas had similar concentrations at 19.5 to 19.4 µg/m³. Papalantoro has the highest daily concentration of (19.53 μg/m³), followed by Itori 1 (19.47 (μg/m³) and Blasting/production area (BPA1) (19.41 μg/m³).

The pollutant averages for each sample were averaged for each site to compare their pollutant levels. The daily mean PM_2.5 concentration was highest in the packaging area (PA2) (17.22 ± 1.17 μg/m³), which was closest to the cement factory (less than 500 meters). The lowest daily mean PM_2.5 concentration (9.20 ± 3.85 μg/m³) was recorded at Lapeleko, about 3 km from the factory, which, according to the windrose, was upwind of both the cement blasting and manufacturing operations that sampling day.

The minimum hourly concentration of PM_2.5 for all the monitoring sites ranges between 4 - 8 μg/m³, being very similar across the different sites. However, the maximum hourly concentration of PM_2.5 varies greatly, ranging from 7 - 62 μg/m³. Furthermore, the minimum hourly concentration of PM₁₀ was between 4 - 72 μg/m³, while the maximum hourly concentration of PM₁₀ ranged from 21 - 75 μg/m³. This study revealed that none of the locations exceeded the particulates 24-hr NESREA guideline of 40 μg/m³ for PM_2.5 and 150 μg/m³ for PM₁₀. These levels are below the levels reported by Bada et al. (2013) for this Ewekoro region, finding a PM_2.5 concentration of 76 μg/m³ and PM₁₀ at 93 μg/m³ near this Lafarge factory. The levels we found at Ewekoro are also below the PM₁₀ levels near cement factories in Indian cities of 100 - 400 μg/m³ ( Sharma et al., 2003 ). The lower rates at this cement factory are explained by pollution prevention measures and limitations of sampling - sampling for eight hours despite a 24-hour operation at the cement factory.

Figure 4 shows the average 10-minute values of PM_2.5 at the CAR—NASRDA and the University of Ibadan monitoring stations for March 18th, 19th, and 20th, 2024, over the full 24-hour day. This time series data plot of PM_2.5 provides colour coding by the Air Quality Index values. Figure 4 shows the colour codes of acceptable levels in green, occurring mostly in the early morning and late afternoon, with worsening air quality from yellow to orange during the late night, peaking around midnight, and a second peak around noon. These reference sites never reached the red level.

Figure 5 shows the respirable particulate levels at the Ewekoro Lafarge Cement Factory. The monitoring points are northwest to northeast for blasting/production area (BPA) measurements and southeast to southwest for packaging area (PA) measurements. The PM_2.5 levels in the packaging area (PA) appears similar to the blasting/production area (BPA), except for certain episodes, which could be during material delivery, batch operation, cleaning or other shorter-duration activity. On the first day of sampling, packaging area (PA1) had a peak at 9:00 a.m. of 40 µg/m³ and remained well below 20 µg/m³ until 12:30, reaching 30 µg/m³ at 2:30, 40 µg/m³ at 3:15 and almost 70 µg/m³ at 4:15. The second day of sampling packaging area (PA2) showed a large peak at 9:00 am above 90 µg/m³, with the pollution going up and down between 70 and 13 µg/m³ for the remainder of the day Blasting/production area values are much more constant, without sudden peaks, mainly hovering around 8 µg/m³ and 15 µg/m³.

The level at the packaging area sampling points surpasses the 25 µg/m³, and the level at the blasting/production area sampling points never surpassed the threshold level. The average value for PM_2.5 in the cement factory BPA and PA is 11.8 μg/m³ (variance of 9.4 μg/m³, standard deviation of 3.1 μg/m³) and 15.4 μg/m³ (variance of 28.9 μg/m³, standard deviation of 5.4 μg/m³) respectively. No statistically significant difference occurred between the blasting/production area (BPA) and packaging area (PA) means of PM_2.5, where (F = 3.156) and P-value = 0.097) using one-way ANOVA.

Exposure to respirable particulate matter aggravates lung disease, causes asthma and acute bronchitis and increases susceptibility to respiratory infections. The daily particulate means are below the regulatory levels. Figure 6 shows the daily PM_2.5 and PM₁₀ mean values compared to regulatory standards. The PM₁₀ μg/m³ levels were similar at the cement factory area and other neighbourhoods monitored and well below Nigeria’s NESREA standard. The respirable particulate matter (PM_2.5 μg/m³) mean concentration at the blasting/production cement factory area are lower than in the packaging area. The PM_2.5 and PM₁₀ mean concentrations are highest in the packaging area of the cement factory and lowest in the blasting/production area, which includes Lapeleko. The average value for PM_2.5 in the neighbourhood varies from 9.2 μg/m³ (variance of 14.8 μg/m³, standard deviation of 3.8 μg/m³) at Lapeleko to 16.1 μg/m³ (variance of 10.5, standard deviation of 3.2 μg/m³) downwind at Itoro as shown in Table 2 in the appendix. Trucks loaded with cement at the packaging area monitoring locations appeared to contribute to the higher values of PM_2.5 at this location. The Lapeleko PM_2.5 mean was slightly below the blasting /production area, as Lapeleko was directly upwind during monitoring.

The direction in which the pollutant (particulate matter) travels is the result of the wind ( George et al., 2008 ) and shifts with the wind. Wind rose plots were investigated for the different sampling locations to examine the effects of meteorological parameters on the dispersion of the particulate matter. Figure 7 shows wind speed and direction measured in the study area using a 16 radial arm wind rose (22.5˚) for data ( Jurado et al., 2021 ) monitored from wind sensors placed 10 meters from the ground. The monitoring locations considered in this study give distinct but complementary information.

Figure 7 shows that the wind was low-speed during most sampling near the cement factory. Higher velocities occurred during the sampling of the packaging area (PA1). However, most monitoring days had velocities hovering near Itori, the closest neighbour to the cement factory on the northeast side, the lack of wind on both monitoring days allowed pollution to build up around the plant and in Itori. Itori was not downwind of the cement factory or the blast areas, but when the weather is calm, the pollution tends to linger in the nearby vicinity and put people at risk.

The Air Quality Index (AQI) provides important information on the health of the air based on pollutant levels. ( Lala et al., 2023 ). The AQI details the potential health dangers of breathing in polluted air and any side effects that can appear hours or days later. A lower AQI value is considered better-quality air.

The daily mean ± SD in the study area was applied to the AQI. These levels were Blasting area/production area (49.85 ± 1.48 μg/m³), Cement packaging area (60.73 ± 1.43 μg/m³), Itori (59.60 ± 6.79 μg/m³), Papalantoro (51.60 ± 10.85 μg/m³) and Lapeleko (37.53 ± 14.63 μg/m³) as shown in Table 1 in the appendix. The AQI levels were worse closer to the cement factory when downwind. The daily mean AQI was highest at Itori 1 (64.4 μg/m³).

The risk assessment of PM_2.5 in the cement factory area and impacted neighbourhoods was evaluated using the Canada Ambient Air Quality Standards ( Canadian Council Ministers of the Environment, 2014 ). This study applied the 27 μg/m³ 24-hour CAAQS guideline for PM_2.5, as shown in Table 3 in the appendix. The highest daily mean value of PM_2.5 in the cement factory area was 18.04 μg/m³ which was below the 24-hour CAAQS guideline. However, for air quality management, this value falls in the yellow colour category of the CAAQS guideline of 11 μg/m³ to 19 μg/m³, which implies continuous actions are needed to improve air quality in the cement factory area. The highest daily mean values of PM_2.5 at Papalantoro, Lapeleko and Itori are 17.24 μg/m³, 13.58 μg/m³, and 18.89 μg/m³ respectively. These values are below the CAAQS of 27 μg/m³ but also fall in the yellow colour category of CAAQS guideline PM_2.5 values between 11 μg/m³ to 19 μg/m³ in the air quality management level.

Figure 8 shows the spatial distribution of PM_2.5. All areas are below the WHO standards, and so this diagram shows the CAAQ standards. Figure 8 shows that the level of PM_2.5 for Itori 1 and all samples for Papalantoro is satisfactory for the CAAQs standard, shown in green. The PM_2.5 concentration for Lapeleko, the Ewekoro Lafarge Cement Factory blast and packing areas, falls above 10, signified by yellow on the map. Thus, these areas need continuous action to improve air quality, according to CAAQS.

Cement production elevates the concentration of heavy metals in the surrounding environment ( Abimbola et al., 2007 ). Table 3 shows that the different heavy metal concentrations in the particulate air samples vary across the cement factory area, Papalantoro, Lapeleko, and Itori. However, a noticeably higher level of most heavy metals was found in the cement factory’s blasting area, packaging area, and nearby neighbourhoods of Papalantoro and Lapeleko.

Heavy metals are dangerous pollutants that can build up in human tissues ( Leili et al., 2008 ). Cadmium (Cd) was not detected in most locations. However, higher concentrations of Cd (0.03 mg/m³) were observed in PA1. The harmful effects of Cd include carcinogenicity, mutagenicity, disruption of the endocrine system, fragile bones (osteoporosis), and impairment of lungs and kidneys, among other biological disorders ( Bandow & Simon, 2016 ; Oguntade et al., 2020 ).

Legend: nd = not detected, Red numbers indicate levels higher than the WHO limit.

All samples contained Chromium (Cr). Some Cr comes from the limestone and clay materials used to manufacture cement. During the clinkerisation process chromate (Cr(III)) can partly oxidize to turn to Cr(VI), which is allergenic. The linings for the rotaries in cement factories contain Cr. The rotary friction causes wear and tear on the linings, which release into the air ( Banat et al., 2005 ; Okoro et al., 2017 ). Malignancies of the lung, kidney failure, heart attack, bronchial cancer, prostatic proliferative lesions, and fractures in the bones are among the long-term consequences of cadmium exposure ( Ayua et al., 2020 ). Chromium ulcers, acidic reactions on the septum of the nasal cavity, allergic eczematous skin irritation, and acute irritative dermatitis are associated with substances containing chromium(VI) ( Leili et al., 2008 ).

Nickel(Ni) is present at high levels in cement in a non-water soluble form. Overexposure to Ni may result in skin irritation, such as irritating hands and other areas ( Ayua et al., 2020 ). Elevated Ni levels are linked to asphyxia, convulsions, and digestive issues. Chronic exposure can cause headaches, light-headedness, weakness, memory loss, hair loss, chest tightness, sleeplessness, and reduced libido. All samples had Ni at levels that exceeded WHO standards. Some samples exceeded the levels by almost 30 times, reaching 1.5 compared to 0.05 mg/m³. The highest concentration of Ni was observed in Itori 2 at 1.5 mg/m³, respectively. Nickel exposure causes lung and nasal cancers ( Ayua et al., 2020 ).

Most heavy metal levels near the Ewekoro Lafarge Cement Factory exceeded the WHO standard. Chromium (Cr) had the highest concentration of 2.71 mg/m³ at Papalantoro 1, followed closely by Papalanto 3 with 2.67 mg/m³. The heavy metals’ mean concentration (mg/m³) and standard deviation were calculated. The mean levels were Cd (0.01 ± 0.02 mg/m³), Cr (1.80 ± 0.522 mg/m³), Cu ((0.02 ± 0.02 mg/m³), Ni (0.66 ± 0.21 mg/m³), and Zn (0.10 ± 0.12 mg/m³) for the cement factory area (BPA and PA). Several chemicals did not exceed the WHO levels for any sample. These chemicals were Cu, Pb and Zn. Lead (Pb) was not detected in any sample. The levels of Zn were highest at packaging area sampling points of the cement factory at 0.25 mg/m³, followed by Papalantoro 1 at 0.20 mg/m³.

Figure 9 compares the heavy metal concentrations (mg/m³) in the air at the Ewekoro Lafarge Cement Factory and nearby neighbourhoods. The cement factory area, Palantoro, Lapeleko, and Itori, has visibly higher chromium and nickel concentration levels. The concentrations of Cd in all the areas monitored were lower.

Figures 10(a)-(e) show the levels of heavy metal concentrations in all the areas monitored compared with the WHO/EU permissible limit. Cadmium, nickel, and chromium levels are higher than the WHO/EU permissible limit, while zinc and copper levels are lower.

Figure 11 shows the relationship between the hourly values of PM_2.5 in the blasting area and the packing area at Ewekoro’s Lafarge Cement factory. The moderate correlation (r = 0.458) between them indicates similar pollutants from similar sources ( Lala et al., 2023 ). A linear relationship between the hourly values of PM_2.5 between the blasting area and the packing area at Ewekoro’s Lafarge Cement factory.

Figures 12(a)-(d) show the exposure and non-carcinogenic risks assessment (HQ) of the heavy metals at the Ewekoro Lafarge factory and the impact on the neighbourhoods—Papalantoro, Lapeleko, and Itori. This HQ considered children and adults throµgh ingestion, inhalation, and dermal contact. A HQ higher than the safe level (=1) signifies the potential non-carcinogenic effects on the community members. A HQ lower than the safe level signifies no significant non-carcinogenic effects to residents ( Wang et al., 2018 ).

Figures 13(a)-(e) show the spatial distribution of heavy metals (Cadmium Chromium, Copper, Nickel and Zinc) at Ewekoro Lafarge cement factory and the and impact on nearby neighbourhoods. Figure 13(b) and Figure 13(c) show Zn and Cu concentrations in all locations monitored. The concentration of Zn and Cu falls in the low-risk category below the WHO/EU threshold level. The concentration of Cr in the air was higher than the WHO threshold level in most locations sampled, as indicated in Figure 13(d) . The concentration of Cd was higher than the WHO threshold level at some sampling locations near the cement factory area and impacted neighbourhoods, and it was low in other locations monitored. The concentration of Cr was high in most of the locations monitored.