# Air Quality and Human Health Risk Assessment in the Residential Areas at the Proximity of the Nkolfoulou Landfill in Yaoundé Metropolis, Cameroon

Authors: Gilbert Feuyit; Serge Nzali; John Ngolui Lambi; Samuel Laminsi
Journal: Journal of Chemistry
 (2019)

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## Abstract

Landfill operations generate particulate matters (PM) and toxic gases that can jeopardize human health. This study was conducted in February 2016 to assess the air quality in the residential areas around the Nkolfoulou landfill in Yaoundé. The concentrations of PM2.5 and PM10 were determined with Dust Sentry while those of CO, O3, NO2, CH4, CO2, CH2O, H2S, and SO2 were measured using gas sensors. At the landfill neighborhood, 30% of the daily mean concentrations of PM2.5 and PM10 crossed the daily safe limits. The concentrations of CO, O3, NO2, SO2, and H2S recorded at the propinquity of the landfill complied with the emission standards. Near the landfill, hourly mean concentrations of CH2O and H2S higher than their odour thresholds were recorded at each sampling site. The concentrations of CH4 were less than its lower explosive limit while those of CO2 were far below the safe limit for occupational health. The values of cancer risk (CR) due to the inhalation of CH2O were >10−6 while those of hazard index (HI) due to the inhalation of CH2O, H2S, and SO2 were <1. Thus, there might be increased cancer risks at the Nkolfoulou landfill neighborhood, whereas the increased non-cancer risks were low. 96.76% of the daily average levels of air pollutants registered near the landfill surpassed those recorded at the remote control site. Hence, the landfill operations might be supplying air pollutants to the neighbouring residential areas.


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## Body

## 1. Introduction

Landfilling is the most widely used method of solid waste disposal across the world [1–3]. Landfill operations generate air pollutants such as particulate matters (PM) and gases [4]. The landfill gases (LFG) emitted into the environment may originate from the waste or may be generated during its decomposition [2]. Pristine air is a prerequisite for good health [5, 6]. Outdoor air pollutants are carcinogen Group 1 to humans; they induce lung cancer [7]. Air pollutants may conduce to the pathogenesis of upper airway diseases, viz., sinusitis, rhinitis, mild otitis, sinonasal cancer, and olfactory impairment [8]. Breathing polluted air during pregnancy may cause foetus growth retardation and abortion [6, 9, 10].A link between short- or long-term exposure to airborne PM and human mortality and morbidity has been substantiated by several epidemiological studies [11–14]. Chronic exposure to PM2.5 and PM10 damages the respiratory and cardiovascular systems, while exposure to high concentrations of ozone (O3) is a major factor in asthma morbidity and mortality [6]. High levels of sulfur dioxide (SO2) reduce lung function and may provoke the irritation of the nose and the throat [15]. Hydrogen sulfide (H2S) is the predominant landfill odour gas [16, 17]. Subjection to low and high concentrations of H2S may induce the irritation of the throat and respiratory distress, respectively [18]. Formaldehyde (CH2O) is not only a human carcinogen Group 1, causing cancer of the nasopharynx [19], but is also an irritant gas [20]. Many studies have been carried out elsewhere on the impact of landfill on the ambient air quality [4, 11, 21–25]. But, in Cameroon, data related to this issue are scanty. Therefore, this study focuses on the influence of the Nkolfoulou landfill activities on the ambient air quality.


## 2. Materials and Methods

### 2.1. Study Site Description

The study area has tropical climate and is located at the apex of a hill called Nkolfoulou. The Nkolfoulou landfill is situated at about 16 km away from the Yaoundé center. It was established in 1989 and was still in operation during this study. It covers a total land area of about 45 ha [26] and receives about 1300 tons of waste generated daily in the town of Yaoundé [27]. Employing a geographical positioning system (GPS) Magellan Triton-300, the geographical coordinates of the selected study stations were recorded. ArcGIS 10 software was used to draw the map of the study area and to gauge the distances between the sampling sites and the landfill boundary. Table 1 represents the locations of the monitoring sites, while Figure 1 displays the map of the study area.Table 1
Specifications of the monitoring sites.
Site code
GPS coordinates
DLB (m)
E
N
SL (m)
RA0
11°34′17.043″
3°56′25.866″
694
1000.00
RA1
11°34′15.801″
3°55′27.96″
701
161.37
RA2
11°34′36.123″
3°55′29.889″
720
34.40
RA3
11°34′43.421″
3°55′29.084″
734
118.58
RA4
11°34′26.945″
3°55′49.69″
730
239.06
RA5
11°34′24.22″
3°56′0.289″
708
305.02
RA6
11°34′12.123″
3°55′58.821″
661
58.25
RA7
11°34′4.757″
3°55′54.325″
667
165.74
RA8
11°34′0.786″
3°55′52.929″
693
255.33
RA9
11°33′59.289″
3°55′46.615″
669
189.27
RA10
11°33′51.403″
3°55′37.557″
673
279.40
SL = sea level; DLB = distance from the landfill boundary.Figure 1
Map of the study area.


### 2.2. Data Collection and Health Risk Evaluation

In February 2016, towards the end of the long dry season, the measurements of air pollutants were performed first at ten sites coded RA1, RA2, RA3, RA4, RA5, RA6, RA7, RA8, RA9, and RA10, selected in the residential areas around the landfill, and finally at a background site RA0 carefully chosen for control. The concentration of gases was measured using a handheld Aeroqual Gas Sensor model S-500L, battery-operated, possessing an interchangeable sensor head. For each site, the concentrations of gases were recorded continuously for every 1 hour at intervals of 30 minutes, each making 16 hours of measurement daily (24 hours). For each hour, gas concentrations were measured after every 5 minutes giving 12 readings per hour for each gas. Thus, 192 readings were recoded for each gaseous pollutant per site during a day (24 hours). The airborne particulates (PM10 and PM2.5) measurements were carried out using a digital Aeroqual Dust Sentry (made by Aeroqual Limited, New Zealand) equipped with a laser. During measurements, the instrument was placed on a tripod of 1.5 m height. The measuring device was configured to record average concentrations of PM hourly at a flow rate of 2.0 L/min. Before measurements, all the instruments were calibrated according to the manufacturer’s instructions.The non-cancer risks induced by the inhalation of CH2O, H2S, and SO2 were evaluated by calculating the hazard quotient (HQ) using equation (3) deduced from equation (1), whereas the cancer risk (CR) due to the inhalation of CH2O was computed from equation (4) deduced from equation (3) [28]:(1)HQ=ECMRL,where EC = exposure concentration (μg/m3) and MRL = minimal risk level (μg/m3). (2)CR=IUR×EC,where IUR = inhalation unit risk (μg/m3)−1. HQ and CR are unitless.For acute exposures (exposure lasting 24 hours or less), EC = CA [28], where CA = contaminant concentration in air (μg/m3). Hence, equations (1) and (2) become(3)HQ=CAMRL,(4)CR=IUR×CA.For exposure to multiple non-carcinogenic substances, the resulting hazard index (HI) was calculated from the following equation [29, 30].(5)HI=∑inHQi.The MRLs of CH2O, H2S, and SO2 are 0.04 ppm (49.2 μg/m3) [20], 0.07 ppm (98 μg/m3) [18], and 0.01 ppm (26.2 μg/m3) [15], respectively, for acute exposures while the IUR of CH2O is 1.3 × 10−5 (μg/m3)−1 [31].


## 2.1. Study Site Description

The study area has tropical climate and is located at the apex of a hill called Nkolfoulou. The Nkolfoulou landfill is situated at about 16 km away from the Yaoundé center. It was established in 1989 and was still in operation during this study. It covers a total land area of about 45 ha [26] and receives about 1300 tons of waste generated daily in the town of Yaoundé [27]. Employing a geographical positioning system (GPS) Magellan Triton-300, the geographical coordinates of the selected study stations were recorded. ArcGIS 10 software was used to draw the map of the study area and to gauge the distances between the sampling sites and the landfill boundary. Table 1 represents the locations of the monitoring sites, while Figure 1 displays the map of the study area.Table 1
Specifications of the monitoring sites.
Site code
GPS coordinates
DLB (m)
E
N
SL (m)
RA0
11°34′17.043″
3°56′25.866″
694
1000.00
RA1
11°34′15.801″
3°55′27.96″
701
161.37
RA2
11°34′36.123″
3°55′29.889″
720
34.40
RA3
11°34′43.421″
3°55′29.084″
734
118.58
RA4
11°34′26.945″
3°55′49.69″
730
239.06
RA5
11°34′24.22″
3°56′0.289″
708
305.02
RA6
11°34′12.123″
3°55′58.821″
661
58.25
RA7
11°34′4.757″
3°55′54.325″
667
165.74
RA8
11°34′0.786″
3°55′52.929″
693
255.33
RA9
11°33′59.289″
3°55′46.615″
669
189.27
RA10
11°33′51.403″
3°55′37.557″
673
279.40
SL = sea level; DLB = distance from the landfill boundary.Figure 1
Map of the study area.


## 2.2. Data Collection and Health Risk Evaluation

In February 2016, towards the end of the long dry season, the measurements of air pollutants were performed first at ten sites coded RA1, RA2, RA3, RA4, RA5, RA6, RA7, RA8, RA9, and RA10, selected in the residential areas around the landfill, and finally at a background site RA0 carefully chosen for control. The concentration of gases was measured using a handheld Aeroqual Gas Sensor model S-500L, battery-operated, possessing an interchangeable sensor head. For each site, the concentrations of gases were recorded continuously for every 1 hour at intervals of 30 minutes, each making 16 hours of measurement daily (24 hours). For each hour, gas concentrations were measured after every 5 minutes giving 12 readings per hour for each gas. Thus, 192 readings were recoded for each gaseous pollutant per site during a day (24 hours). The airborne particulates (PM10 and PM2.5) measurements were carried out using a digital Aeroqual Dust Sentry (made by Aeroqual Limited, New Zealand) equipped with a laser. During measurements, the instrument was placed on a tripod of 1.5 m height. The measuring device was configured to record average concentrations of PM hourly at a flow rate of 2.0 L/min. Before measurements, all the instruments were calibrated according to the manufacturer’s instructions.The non-cancer risks induced by the inhalation of CH2O, H2S, and SO2 were evaluated by calculating the hazard quotient (HQ) using equation (3) deduced from equation (1), whereas the cancer risk (CR) due to the inhalation of CH2O was computed from equation (4) deduced from equation (3) [28]:(1)HQ=ECMRL,where EC = exposure concentration (μg/m3) and MRL = minimal risk level (μg/m3). (2)CR=IUR×EC,where IUR = inhalation unit risk (μg/m3)−1. HQ and CR are unitless.For acute exposures (exposure lasting 24 hours or less), EC = CA [28], where CA = contaminant concentration in air (μg/m3). Hence, equations (1) and (2) become(3)HQ=CAMRL,(4)CR=IUR×CA.For exposure to multiple non-carcinogenic substances, the resulting hazard index (HI) was calculated from the following equation [29, 30].(5)HI=∑inHQi.The MRLs of CH2O, H2S, and SO2 are 0.04 ppm (49.2 μg/m3) [20], 0.07 ppm (98 μg/m3) [18], and 0.01 ppm (26.2 μg/m3) [15], respectively, for acute exposures while the IUR of CH2O is 1.3 × 10−5 (μg/m3)−1 [31].


## 3. Results and Discussion

### 3.1. Particulate Matter

The concentrations of each air pollutant were averaged for each hour and then for 24 hours. The levels of outdoor PM2.5 and PM10 measured at the monitoring sites are encapsulated in Table 2. The lowest hourly mean level of PM2.5 was recorded at RA10 (9.53 μg/m3), while the highest was registered at RA3 (44.02 μg/m3). The hourly mean levels of PM10 varied from 18.86 (RA10) to 114.45 μg/m3 (RA3). The hourly high level of PM2.5 and PM10 in the study area could be owing to landfill operations since they generate dust by a variety of mechanical and chemical processes [22].Table 2
Concentration of particulate matter at the monitoring sites (n = 24).
Site code
PM2.5 (μg/m3)
PM10 (μg/m3)
Range of hourly mean
Daily mean
Range of hourly mean
Daily mean
Min
Max
Min
Max
RA0
10.50
17.50
12.85
18.90
31.50
23.12
RA1
12.09
22.34
19.19
25.39
46.91
40.29
RA2
12.50
39.50
32.75
35.00
110.60
91.34
RA3
18.20
44.02
37.57
47.32
114.45
97.69
RA4
12.57
21.75
19.23
18.86
42.63
35.71
RA5
13.40
22.58
20.49
28.14
47.42
43.04
RA6
14.20
36.80
31.89
36.92
95.68
82.91
RA7
13.91
21.56
19.56
29.21
45.28
41.07
RA8
12.20
23.19
20.36
24.16
45.92
40.32
RA9
9.53
21.35
18.59
20.34
40.65
37.84
RA10
11.64
24.82
21.63
26.70
42.95
28.84
RA = residential area; ND = not detected;n = number of measurements per day (24 hours).The daily mean concentrations of PM2.5 and PM10 varied from 18.59 μg/m3 (RA9) to 37.57 μg/m3 (RA3) and 28.84 μg/m3 (RA10) to 97.69 μg/m3 (RA3), respectively. The daily mean levels of PM2.5 of 32.75 (RA2), 37.57 (RA3), and 31.39 μg/m3 (RA6) were higher than the daily safe limit of 25 μg/m3 set by the WHO [6]. Likewise, the daily mean levels of PM10 of 91.34 (RA2), 97.69 (RA3), and 82.91 μg/m3 (RA6) surpassed the daily safe limit of 50 μg/m3 laid down by the WHO [6]. Several studies have provided strong evidence that subjection to high concentration of PM may induce cardiopulmonary disease (CPD) and ischemic heart disease (IHD) mortality [32]. The hourly and daily average levels of PM2.5 and PM10 recorded at the proximity of the landfill were lower than those registered at the background site RA0, implying that the landfill operations might be contributing to PM2.5 and PM10 to the ambient air. The movement of vehicles and motorbikes on the unpaved and poorly maintained roads in the study area as well as the ongoing construction works may have constituted additional sources of PM.


### 3.2. Odourless Gases

Although O3 has a shocking smell, humans get rapidly acclimated to it. Moreover, the frequently associated presence of nitrogen oxides suppresses its perception [33]. For these reasons, it was classified among odourless gases in this study. Table 3 lists the concentrations of odourless gases in the study area.Table 3
Concentration of odourless gases at the monitoring sites (n1 = 12; n2 = 192).
Site code
CO (mg/m3)
O3 (μg/m3)
NO2 (μg/m3)
Range of hourly mean
Daily mean
Range of hourly mean
Daily mean
Range of hourly mean
Daily mean
Min
Max
Min
Max
Min
Max
RA0
ND
0.30
0.05
ND
10.51
1.32
ND
ND
NC
RA1
ND
1.00
0.27
ND
40.01
5.73
ND
ND
NC
RA2
ND
4.58
0.92
ND
58.90
10.63
ND
75.26
35.92
RA3
ND
8.02
1.48
ND
39.41
6.54
ND
94.07
49.60
RA4
ND
1.55
0.42
ND
58.90
6.54
ND
ND
NC
RA5
ND
4.01
0.46
ND
137.42
26.18
ND
ND
NC
RA6
ND
7.44
1.07
ND
38.94
5.73
ND
ND
NC
RA7
ND
1.86
0.36
ND
39.26
8.18
ND
ND
NC
RA8
ND
0.64
0.13
ND
78.53
6.54
ND
ND
NC
RA9
ND
1.37
0.05
ND
19.63
5.89
ND
ND
NC
RA10
ND
2.40
0.04
ND
58.90
19.63
ND
ND
NC
RA = residential area; ND = not detected; NC = not calculated because not detected;n1 = number of measurements per hour; n2 = number of measurements per day (24 hours).The hourly mean concentrations of CO and O3 ranged from ND (not detected) to 6.44 mg/m3 (RA3) and ND to 137.42 μg/m3 (RA5), respectively, while their daily average levels varied from 0.04 (RA10) to 1.48 mg/m3 (RA3) and 5.73 (RA9) to 26.18 μg/m3 (RA5) in the same order. NO2 was detected only at RA2 and RA3. Its hourly and daily mean levels ranged from ND to 94.07 μg/m3 (RA3) and 35.92 (RA2) to 49.60 μg/m3 (RA3). During this study, none of the CO value exceeded the safe limit of 100, 60, 30, and 10 mg/m3 for the averaging duration of 15 mn, 30 mn, 1 hr, and 8 hr, respectively, set by the WHO [5]. So also, all the concentrations of O3 and NO2 were far below their maximum emission limits laid down by the WHO in [5, 6], respectively. Relatively high levels of CO and NO2 recorded at RA2 and RA3 compared with other sites may be attributable to their proximity to the highway.The hourly mean value of 6.44 mg/m3 for CO registered in this work was lower than the 8-hour mean level of 7.79 mg/m3 recorded in a residential area around On-Nooch solid waste disposal site in Bangkok (Thailand) [21]. It was also less than 4 ppm (4.64 mg/m3) obtained in a residential area at the vicinity of Eneka landfill in Port Harcourt (Nigeria) [25]. But, the higher hourly mean value of 94.07 μg/m3 (0.947 mg/m3) for NO2 recorded in this work was greater than the hourly mean figure of 0.034 mg/m3 found around On-Nooch dumpsite (Thailand) [21].


### 3.3. Odorous Gases

H2S, CH2O, and SO2 are colorless and malodorous gases. H2S has the characteristic odour of rotten eggs [18] while CH2O has a pungent smell [20] as well as SO2 [15]. Their concentrations are depicted in Table 4. In the residential areas adjacent to the landfill, the hourly mean levels of CH2O, H2S, and SO2 ranged from ND to 206.76 μmg/m3 (RA6), ND to 236.40 μg/m3 (RA6), and ND to 28.56 μg/m3 (RA3), respectively, while their daily average varied from 14.49 (RA5) to 32.25 μg/m3 (RA1), 8.74 (RA5) to 28.06 μg/m3 (RA6), and 1.05 (RA9) to 4.18 μg/m3 (RA3) in the same order. The maximum 30-minute mean limit of 100 μg/m3 for CH2O [5] (Table 5) was crossed at all the sampling points near the landfill, whereas the maximum daily mean safe limit of 20 μg/m3 for SO2 [6] (Table 5) was not violated at any site. Comparatively, all the daily mean values of SO2 were much lower than the daily mean value of 8.91 mg/m3 recorded at the vicinity of On-Nooch dumpsite [21]. High concentrations of CH2O irritate the nose, the throat, and the eyes [5, 20]. Subjection to a high level of SO2 exacerbates asthma and can cause lung dysfunction [6, 15, 34].Table 4
Concentration of odorous gases at the monitoring sites (n1 = 12; n2 = 192).
Site code
CH2O (μg/m3)
H2S (μg/m3)
SO2 (μg/m3)
Range of hourly mean
Daily mean
Range of hourly mean
Daily mean
Range of hourly mean
Daily mean
Min
Max
Min
Max
Min
Max
RA0
ND
ND
NC
ND
ND
NC
ND
1.74
0.82
RA1
ND
185.56
32.25
ND
152.97
20.45
ND
14.28
2.14
RA2
ND
133.95
24.88
ND
139.06
19.60
ND
26.18
3.56
RA3
ND
130.32
22.02
ND
122.49
11.02
ND
28.56
4.18
RA4
ND
172.20
25.71
ND
166.87
22.45
ND
11.90
2.28
RA5
ND
138.62
14.49
ND
62.97
8.74
ND
6.80
1.30
RA6
ND
206.76
30.83
ND
236.40
28.06
ND
16.66
1.70
RA7
ND
147.15
28.13
ND
194.68
25.13
ND
12.14
1.90
RA8
ND
181.10
27.79
ND
180.78
24.02
ND
11.05
1.45
RA9
ND
149.08
21.65
ND
139.06
20.25
ND
4.92
1.05
RA10
ND
142.03
17.59
ND
125.15
17.53
ND
5.35
1.18
RA = residential area; ND = not detected; NC = not calculated because not detected;n1 = number of measurements per hour; n2 = number of measurements per day (24 hours).Table 5
WHO ambient air quality standards.
Pollutant
Averaging duration
Maximum allowable limit
Reference
PM2.5
24 hours
25μg/m3
[6]
Annual
10μg/m3
[6]
PM10
24 hours
50μg/m3
[6]
Annual
20μg/m3
[6]
CO
15 minutes
100 mg/m3
[5]
30 minutes
60 mg/m3
[5]
1 hour
30 mg/m3
[5]
8 hours
10 mg/m3
[5]
SO2
10 minutes
500µg/m3
[6]
24 hours
20μg/m3
[6]
CH2O
30 minutes
0.1 mg/m3
[5]
H2S
24 hours
150μg/m3
[5]
O3
8 hours
100μg/m3
[6]
NO2
1 hour
200μg/m3
[5]
Annual
40μg/m3
[5]At the proximity of the landfill, all the maximum hourly and daily mean values of H2S were higher than its odour threshold contained in the approximate range of 0.5–8 ppb (0.7–11.2 μg/m3) [35, 36]. So also, all the maximum hourly and daily mean values of CH2O at RA1 and RA6 exceeded its odour threshold which is in the range 30–600 μg/m3 [5]. Besides, all the daily mean concentrations of H2S crossed the safe limit of 7 μg/m3, while all the maximum hourly mean concentrations of CH2O violated the safe limit of 100 μg/m3. These safe limits are prescribed by the WHO [5] for an averaging time of 30 min to prevent annoyance and sensory effects. Subjection to low levels of H2S may induce headaches and breathing difficulties in some asthmatic patients [18]. These gases may worsen the poor health conditions of patients in the healthcare center or bring about discomfort and annoyance to pupils in the primary school since both areas are situated close to RA3.At the background site RA0, CH2O and H2S were not detected while the values of SO2 were less than those recorded at the vicinity of the landfill, suggesting that the landfill may be the main contributor of CH2O and H2S to its surroundings. CH2O and H2S may have originated, respectively, from the decomposition of carbohydrate and protein [37] in the landfill. Meanwhile, CH2O could have another source since aldehydes can be generated either from photochemical oxidation of hydrocarbons (HC) in the atmosphere [38] or through the incomplete combustion of fuel [39]. High hourly and daily mean concentrations of SO2 registered at RA3 cause one to think that the traffic was also contributing to SO2 by the combustion of sulfur-containing fuels. The nearness of RA6 to the landfill, the closeness of RA2 to the entrance of the landfill and to the highway, and the proximity of RA3 to the highway and the motorbike park may explain the high levels of CH2O, H2S, and SO2 recorded at these sites.


### 3.4. Potential Greenhouse Gases

CH4 and CO2 are the main constituents of landfill gases (LFG) [40]. They are generated during the putrefaction of waste. The CH4 and CO2 concentrations in the study area are depicted in Table 6. The hourly mean levels of CH4 and CO2 were found, respectively, between ND and 2.30 ppm (RA6) and 401.60 (RA9) and 649.27 ppm (RA3) while their daily average ranged from 0.01 (RA10) to 1.76 ppm (RA6) and 459. 85 (RA8) to 573.02 ppm (RA3) in the same order. The higher hourly and daily mean concentrations of CH4 recorded at RA6 could be due to its proximity to the landfill, whereas the higher hourly and daily mean concentrations of CO2 recorded at RA3 could be attributable to its location very close to both the gate of the landfill and the highway. So, it is reasonable to think that some CO2 at these stations may have originated from the combustion of fuel in motor vehicles.Table 6
Level of potential greenhouse gases at the monitoring sites (n1 = 12; n2 = 192).
Site code
CH4 (ppm)
CO2 (ppm)
Range of hourly mean
Daily mean
Range of hourly mean
Daily mean
Min
Max
Min
Max
RA0
ND
ND
NC
421.00
498.04
461.82
RA1
ND
0.06
0.03
450.25
503.12
481.64
RA2
ND
0.05
0.02
478.61
602.41
550.16
RA3
ND
0.03
0.01
490.25
649.27
573.02
RA4
ND
0.07
0.04
420.31
510.00
464.39
RA5
ND
0.04
0.01
425.50
503.74
466.41
RA6
ND
2.30
1.76
451.63
612.10
541.37
RA7
ND
0.18
0.08
428.50
505.31
467.32
RA8
ND
0.09
0.05
421.75
510.80
459.85
RA9
ND
0.07
0.04
401.60
507.28
469.63
RA10
ND
0.06
0.01
425.00
501.37
460.11
RA = residential area, ND = not detected, NC = not calculated because ND,n1 = number of measurements per hour, n2 = number of measurements per day (24 hours).All the concentrations of CH4 were less than its lower explosive limit (LEL) which is 5% [40] while all the levels of CO2 were far below 5000 ppm as the maximum concentration level for occupational health [41]. Therefore, CH4 and CO2 are not a threat in the area under study for now.Near the landfill, as far as the daily mean concentrations of gaseous pollutants were concerned, their abundance was in the following order: CO2 > CO > CH4 > CH2O > H2S > O3 > NO2 > SO2.


### 3.5. Correlation Matrix

The correlation matrices for 9 measured air pollutants at the vicinity of the landfill are illustrated in Table7. The significant positive correlation observed between PM2.5 and CO (r = 0.65, p≤0.05), PM10 and CO2 (r = 0.69, p≤0.05), and PM10 and CO (r = 0.89, p≤0.01) signifies that CO and CO2 are the major contributors of PM in the study area. At the 0.05 P level, a significant positive correlation was observed between CO and CO2 (r = 0.70) and between CO and SO2 (r = 0.70) implying that these pair variables have almost the same sources that could be either the combustion of fuel, fire wood, kerosene, or cooking gas in the study area. A significant high positive correlation was observed between CH4 and H2S (r = 0.93, p≤0.01), CH4 and CH2O (r = 0.71, p≤0.05) and between CH2O and H2S (r = 0.89, p≤0.01) indicating that these pair variables have the same source which could be the landfill through the degradation of refuse. The negative significant correlation observed between O3 and CH2O (r = −0.69, p≤0.05) signifies that when one of the variable rises, the other decreases. This is because O3 is formed from CH2O by photochemical reactions.Table 7
Correlation matrix.
PM2.5
PM10
CH4
H2S
CH2O
CO2
SO2
CO
O3
PM2.5
1
PM10
0.564
1
CH4
−0.320
0.042
1
H2S
−0.309
−0.139
0.927
∗∗
1
CH2O
−0.212
−0.091
0.709
∗
0.818
∗∗
1
CO2
0.420
0.685
∗
0.018
−0.127
0.188
1
SO2
0.410
0.600
−0.030
0.018
0.382
0.576
1
CO
0.648
∗
0.891
∗∗
0.042
−0.067
0.127
0.697
∗
0.697
∗
1
O3
0.350
−0.043
−0.615
−0.572
−0.689∗
−0.332
−0.117
−0.049
1
∗Correlation is significant at the 0.05 level; ∗∗Correlation is significant at the 0.01 level; bold values are statistically significant.


### 3.6. Non‐cancer and Cancer Risk Assessment

The noncarcinogenic risks associated with the exposure to CH2O, H2S, and SO2 via inhalation were evaluated by calculating the hazard quotient (HQ) and the hazard index (HI), whereas the carcinogenic risks due to CH2O through inhalation was estimated by computing the cancer risk (CR). HQ or HI values below 1.0 indicate that the pollutant under investigation is not likely to cause health impairment, whereas HQ or HI values above 1.0 indicate risk levels that are likely to damage health [42, 43]. The CR values > 10−6 indicate that potential carcinogenic effects may occur, whereas CR values ≤ 10−6 represent an admissible level [43]. The data for HQ and HI are depicted in Figure 2 while those for CR are displayed in Figure 3. In the residential areas bordering the landfill, the values of HQCH2O,HQH2S, and HQSO2 varied from 2.95E − 01 (RA5) to 6.55E − 01 (RA1) (mean 4.99E − 01), 8.92E − 02 (RA5) to 2.86E − 01 (RA3 and RA6) (mean 2.24E − 01), and 4.01E − 02 (RA9) to 1.36E − 01 (RA2 and RA3) (mean 7.66E − 02), respectively. In this same area, the HI values ranged from 4.33E − 01 (RA5) to 9.76E − 01 (RA6) (mean 8.00E − 01), while those of CR due to CH2O was found between 1.88E − 04 (RA5) and 4.19E − 04 (RA1) (mean 3.19E − 04). None of the HQ and HI values exceeded the threshold value, set at the unity, implying that CH2O, H2S, and SO2 are not likely to induce adverse health effects in the area under study for now. All the CR values were higher than 10−6 indicating that the nearby residents to the landfill are at risk of developing cancer in future owing to the inhalation of CH2O. Comparatively, all the CR values due to CH2O registered in this study were higher than 2.9 × 10−5 recorded near a plant treating organic waste in Catalonia (Spain) [44].Figure 2
Non-cancer risks at the monitoring sites (the non-cancer risks of CH2O and H2S at RA0 were not calculated because they were not detected at that site; the horizontal line represents the admissible level of non-cancer risk [42, 43]).Figure 3
Cancer risks (CR) at the monitoring sites (CR at RA0 was not calculated because CH2O was not detected at that site).The risk levels in this study might have been overestimated as the chemical concentrations were measured solely for 24 hours instead of one year. Contrastingly, risks might have been underestimated because only the concentrations of CH2O, H2S, and SO2 among a multitude of volatile toxic compounds that might be present were considered for the assessment of health risk. Furthermore, only exposure via inhalation was considered although exposure through ingestion and skin absorption may occur even if it is most often much lower [43].


## 3.1. Particulate Matter

The concentrations of each air pollutant were averaged for each hour and then for 24 hours. The levels of outdoor PM2.5 and PM10 measured at the monitoring sites are encapsulated in Table 2. The lowest hourly mean level of PM2.5 was recorded at RA10 (9.53 μg/m3), while the highest was registered at RA3 (44.02 μg/m3). The hourly mean levels of PM10 varied from 18.86 (RA10) to 114.45 μg/m3 (RA3). The hourly high level of PM2.5 and PM10 in the study area could be owing to landfill operations since they generate dust by a variety of mechanical and chemical processes [22].Table 2
Concentration of particulate matter at the monitoring sites (n = 24).
Site code
PM2.5 (μg/m3)
PM10 (μg/m3)
Range of hourly mean
Daily mean
Range of hourly mean
Daily mean
Min
Max
Min
Max
RA0
10.50
17.50
12.85
18.90
31.50
23.12
RA1
12.09
22.34
19.19
25.39
46.91
40.29
RA2
12.50
39.50
32.75
35.00
110.60
91.34
RA3
18.20
44.02
37.57
47.32
114.45
97.69
RA4
12.57
21.75
19.23
18.86
42.63
35.71
RA5
13.40
22.58
20.49
28.14
47.42
43.04
RA6
14.20
36.80
31.89
36.92
95.68
82.91
RA7
13.91
21.56
19.56
29.21
45.28
41.07
RA8
12.20
23.19
20.36
24.16
45.92
40.32
RA9
9.53
21.35
18.59
20.34
40.65
37.84
RA10
11.64
24.82
21.63
26.70
42.95
28.84
RA = residential area; ND = not detected;n = number of measurements per day (24 hours).The daily mean concentrations of PM2.5 and PM10 varied from 18.59 μg/m3 (RA9) to 37.57 μg/m3 (RA3) and 28.84 μg/m3 (RA10) to 97.69 μg/m3 (RA3), respectively. The daily mean levels of PM2.5 of 32.75 (RA2), 37.57 (RA3), and 31.39 μg/m3 (RA6) were higher than the daily safe limit of 25 μg/m3 set by the WHO [6]. Likewise, the daily mean levels of PM10 of 91.34 (RA2), 97.69 (RA3), and 82.91 μg/m3 (RA6) surpassed the daily safe limit of 50 μg/m3 laid down by the WHO [6]. Several studies have provided strong evidence that subjection to high concentration of PM may induce cardiopulmonary disease (CPD) and ischemic heart disease (IHD) mortality [32]. The hourly and daily average levels of PM2.5 and PM10 recorded at the proximity of the landfill were lower than those registered at the background site RA0, implying that the landfill operations might be contributing to PM2.5 and PM10 to the ambient air. The movement of vehicles and motorbikes on the unpaved and poorly maintained roads in the study area as well as the ongoing construction works may have constituted additional sources of PM.


## 3.2. Odourless Gases

Although O3 has a shocking smell, humans get rapidly acclimated to it. Moreover, the frequently associated presence of nitrogen oxides suppresses its perception [33]. For these reasons, it was classified among odourless gases in this study. Table 3 lists the concentrations of odourless gases in the study area.Table 3
Concentration of odourless gases at the monitoring sites (n1 = 12; n2 = 192).
Site code
CO (mg/m3)
O3 (μg/m3)
NO2 (μg/m3)
Range of hourly mean
Daily mean
Range of hourly mean
Daily mean
Range of hourly mean
Daily mean
Min
Max
Min
Max
Min
Max
RA0
ND
0.30
0.05
ND
10.51
1.32
ND
ND
NC
RA1
ND
1.00
0.27
ND
40.01
5.73
ND
ND
NC
RA2
ND
4.58
0.92
ND
58.90
10.63
ND
75.26
35.92
RA3
ND
8.02
1.48
ND
39.41
6.54
ND
94.07
49.60
RA4
ND
1.55
0.42
ND
58.90
6.54
ND
ND
NC
RA5
ND
4.01
0.46
ND
137.42
26.18
ND
ND
NC
RA6
ND
7.44
1.07
ND
38.94
5.73
ND
ND
NC
RA7
ND
1.86
0.36
ND
39.26
8.18
ND
ND
NC
RA8
ND
0.64
0.13
ND
78.53
6.54
ND
ND
NC
RA9
ND
1.37
0.05
ND
19.63
5.89
ND
ND
NC
RA10
ND
2.40
0.04
ND
58.90
19.63
ND
ND
NC
RA = residential area; ND = not detected; NC = not calculated because not detected;n1 = number of measurements per hour; n2 = number of measurements per day (24 hours).The hourly mean concentrations of CO and O3 ranged from ND (not detected) to 6.44 mg/m3 (RA3) and ND to 137.42 μg/m3 (RA5), respectively, while their daily average levels varied from 0.04 (RA10) to 1.48 mg/m3 (RA3) and 5.73 (RA9) to 26.18 μg/m3 (RA5) in the same order. NO2 was detected only at RA2 and RA3. Its hourly and daily mean levels ranged from ND to 94.07 μg/m3 (RA3) and 35.92 (RA2) to 49.60 μg/m3 (RA3). During this study, none of the CO value exceeded the safe limit of 100, 60, 30, and 10 mg/m3 for the averaging duration of 15 mn, 30 mn, 1 hr, and 8 hr, respectively, set by the WHO [5]. So also, all the concentrations of O3 and NO2 were far below their maximum emission limits laid down by the WHO in [5, 6], respectively. Relatively high levels of CO and NO2 recorded at RA2 and RA3 compared with other sites may be attributable to their proximity to the highway.The hourly mean value of 6.44 mg/m3 for CO registered in this work was lower than the 8-hour mean level of 7.79 mg/m3 recorded in a residential area around On-Nooch solid waste disposal site in Bangkok (Thailand) [21]. It was also less than 4 ppm (4.64 mg/m3) obtained in a residential area at the vicinity of Eneka landfill in Port Harcourt (Nigeria) [25]. But, the higher hourly mean value of 94.07 μg/m3 (0.947 mg/m3) for NO2 recorded in this work was greater than the hourly mean figure of 0.034 mg/m3 found around On-Nooch dumpsite (Thailand) [21].


## 3.3. Odorous Gases

H2S, CH2O, and SO2 are colorless and malodorous gases. H2S has the characteristic odour of rotten eggs [18] while CH2O has a pungent smell [20] as well as SO2 [15]. Their concentrations are depicted in Table 4. In the residential areas adjacent to the landfill, the hourly mean levels of CH2O, H2S, and SO2 ranged from ND to 206.76 μmg/m3 (RA6), ND to 236.40 μg/m3 (RA6), and ND to 28.56 μg/m3 (RA3), respectively, while their daily average varied from 14.49 (RA5) to 32.25 μg/m3 (RA1), 8.74 (RA5) to 28.06 μg/m3 (RA6), and 1.05 (RA9) to 4.18 μg/m3 (RA3) in the same order. The maximum 30-minute mean limit of 100 μg/m3 for CH2O [5] (Table 5) was crossed at all the sampling points near the landfill, whereas the maximum daily mean safe limit of 20 μg/m3 for SO2 [6] (Table 5) was not violated at any site. Comparatively, all the daily mean values of SO2 were much lower than the daily mean value of 8.91 mg/m3 recorded at the vicinity of On-Nooch dumpsite [21]. High concentrations of CH2O irritate the nose, the throat, and the eyes [5, 20]. Subjection to a high level of SO2 exacerbates asthma and can cause lung dysfunction [6, 15, 34].Table 4
Concentration of odorous gases at the monitoring sites (n1 = 12; n2 = 192).
Site code
CH2O (μg/m3)
H2S (μg/m3)
SO2 (μg/m3)
Range of hourly mean
Daily mean
Range of hourly mean
Daily mean
Range of hourly mean
Daily mean
Min
Max
Min
Max
Min
Max
RA0
ND
ND
NC
ND
ND
NC
ND
1.74
0.82
RA1
ND
185.56
32.25
ND
152.97
20.45
ND
14.28
2.14
RA2
ND
133.95
24.88
ND
139.06
19.60
ND
26.18
3.56
RA3
ND
130.32
22.02
ND
122.49
11.02
ND
28.56
4.18
RA4
ND
172.20
25.71
ND
166.87
22.45
ND
11.90
2.28
RA5
ND
138.62
14.49
ND
62.97
8.74
ND
6.80
1.30
RA6
ND
206.76
30.83
ND
236.40
28.06
ND
16.66
1.70
RA7
ND
147.15
28.13
ND
194.68
25.13
ND
12.14
1.90
RA8
ND
181.10
27.79
ND
180.78
24.02
ND
11.05
1.45
RA9
ND
149.08
21.65
ND
139.06
20.25
ND
4.92
1.05
RA10
ND
142.03
17.59
ND
125.15
17.53
ND
5.35
1.18
RA = residential area; ND = not detected; NC = not calculated because not detected;n1 = number of measurements per hour; n2 = number of measurements per day (24 hours).Table 5
WHO ambient air quality standards.
Pollutant
Averaging duration
Maximum allowable limit
Reference
PM2.5
24 hours
25μg/m3
[6]
Annual
10μg/m3
[6]
PM10
24 hours
50μg/m3
[6]
Annual
20μg/m3
[6]
CO
15 minutes
100 mg/m3
[5]
30 minutes
60 mg/m3
[5]
1 hour
30 mg/m3
[5]
8 hours
10 mg/m3
[5]
SO2
10 minutes
500µg/m3
[6]
24 hours
20μg/m3
[6]
CH2O
30 minutes
0.1 mg/m3
[5]
H2S
24 hours
150μg/m3
[5]
O3
8 hours
100μg/m3
[6]
NO2
1 hour
200μg/m3
[5]
Annual
40μg/m3
[5]At the proximity of the landfill, all the maximum hourly and daily mean values of H2S were higher than its odour threshold contained in the approximate range of 0.5–8 ppb (0.7–11.2 μg/m3) [35, 36]. So also, all the maximum hourly and daily mean values of CH2O at RA1 and RA6 exceeded its odour threshold which is in the range 30–600 μg/m3 [5]. Besides, all the daily mean concentrations of H2S crossed the safe limit of 7 μg/m3, while all the maximum hourly mean concentrations of CH2O violated the safe limit of 100 μg/m3. These safe limits are prescribed by the WHO [5] for an averaging time of 30 min to prevent annoyance and sensory effects. Subjection to low levels of H2S may induce headaches and breathing difficulties in some asthmatic patients [18]. These gases may worsen the poor health conditions of patients in the healthcare center or bring about discomfort and annoyance to pupils in the primary school since both areas are situated close to RA3.At the background site RA0, CH2O and H2S were not detected while the values of SO2 were less than those recorded at the vicinity of the landfill, suggesting that the landfill may be the main contributor of CH2O and H2S to its surroundings. CH2O and H2S may have originated, respectively, from the decomposition of carbohydrate and protein [37] in the landfill. Meanwhile, CH2O could have another source since aldehydes can be generated either from photochemical oxidation of hydrocarbons (HC) in the atmosphere [38] or through the incomplete combustion of fuel [39]. High hourly and daily mean concentrations of SO2 registered at RA3 cause one to think that the traffic was also contributing to SO2 by the combustion of sulfur-containing fuels. The nearness of RA6 to the landfill, the closeness of RA2 to the entrance of the landfill and to the highway, and the proximity of RA3 to the highway and the motorbike park may explain the high levels of CH2O, H2S, and SO2 recorded at these sites.


## 3.4. Potential Greenhouse Gases

CH4 and CO2 are the main constituents of landfill gases (LFG) [40]. They are generated during the putrefaction of waste. The CH4 and CO2 concentrations in the study area are depicted in Table 6. The hourly mean levels of CH4 and CO2 were found, respectively, between ND and 2.30 ppm (RA6) and 401.60 (RA9) and 649.27 ppm (RA3) while their daily average ranged from 0.01 (RA10) to 1.76 ppm (RA6) and 459. 85 (RA8) to 573.02 ppm (RA3) in the same order. The higher hourly and daily mean concentrations of CH4 recorded at RA6 could be due to its proximity to the landfill, whereas the higher hourly and daily mean concentrations of CO2 recorded at RA3 could be attributable to its location very close to both the gate of the landfill and the highway. So, it is reasonable to think that some CO2 at these stations may have originated from the combustion of fuel in motor vehicles.Table 6
Level of potential greenhouse gases at the monitoring sites (n1 = 12; n2 = 192).
Site code
CH4 (ppm)
CO2 (ppm)
Range of hourly mean
Daily mean
Range of hourly mean
Daily mean
Min
Max
Min
Max
RA0
ND
ND
NC
421.00
498.04
461.82
RA1
ND
0.06
0.03
450.25
503.12
481.64
RA2
ND
0.05
0.02
478.61
602.41
550.16
RA3
ND
0.03
0.01
490.25
649.27
573.02
RA4
ND
0.07
0.04
420.31
510.00
464.39
RA5
ND
0.04
0.01
425.50
503.74
466.41
RA6
ND
2.30
1.76
451.63
612.10
541.37
RA7
ND
0.18
0.08
428.50
505.31
467.32
RA8
ND
0.09
0.05
421.75
510.80
459.85
RA9
ND
0.07
0.04
401.60
507.28
469.63
RA10
ND
0.06
0.01
425.00
501.37
460.11
RA = residential area, ND = not detected, NC = not calculated because ND,n1 = number of measurements per hour, n2 = number of measurements per day (24 hours).All the concentrations of CH4 were less than its lower explosive limit (LEL) which is 5% [40] while all the levels of CO2 were far below 5000 ppm as the maximum concentration level for occupational health [41]. Therefore, CH4 and CO2 are not a threat in the area under study for now.Near the landfill, as far as the daily mean concentrations of gaseous pollutants were concerned, their abundance was in the following order: CO2 > CO > CH4 > CH2O > H2S > O3 > NO2 > SO2.


## 3.5. Correlation Matrix

The correlation matrices for 9 measured air pollutants at the vicinity of the landfill are illustrated in Table7. The significant positive correlation observed between PM2.5 and CO (r = 0.65, p≤0.05), PM10 and CO2 (r = 0.69, p≤0.05), and PM10 and CO (r = 0.89, p≤0.01) signifies that CO and CO2 are the major contributors of PM in the study area. At the 0.05 P level, a significant positive correlation was observed between CO and CO2 (r = 0.70) and between CO and SO2 (r = 0.70) implying that these pair variables have almost the same sources that could be either the combustion of fuel, fire wood, kerosene, or cooking gas in the study area. A significant high positive correlation was observed between CH4 and H2S (r = 0.93, p≤0.01), CH4 and CH2O (r = 0.71, p≤0.05) and between CH2O and H2S (r = 0.89, p≤0.01) indicating that these pair variables have the same source which could be the landfill through the degradation of refuse. The negative significant correlation observed between O3 and CH2O (r = −0.69, p≤0.05) signifies that when one of the variable rises, the other decreases. This is because O3 is formed from CH2O by photochemical reactions.Table 7
Correlation matrix.
PM2.5
PM10
CH4
H2S
CH2O
CO2
SO2
CO
O3
PM2.5
1
PM10
0.564
1
CH4
−0.320
0.042
1
H2S
−0.309
−0.139
0.927
∗∗
1
CH2O
−0.212
−0.091
0.709
∗
0.818
∗∗
1
CO2
0.420
0.685
∗
0.018
−0.127
0.188
1
SO2
0.410
0.600
−0.030
0.018
0.382
0.576
1
CO
0.648
∗
0.891
∗∗
0.042
−0.067
0.127
0.697
∗
0.697
∗
1
O3
0.350
−0.043
−0.615
−0.572
−0.689∗
−0.332
−0.117
−0.049
1
∗Correlation is significant at the 0.05 level; ∗∗Correlation is significant at the 0.01 level; bold values are statistically significant.


## 3.6. Non‐cancer and Cancer Risk Assessment

The noncarcinogenic risks associated with the exposure to CH2O, H2S, and SO2 via inhalation were evaluated by calculating the hazard quotient (HQ) and the hazard index (HI), whereas the carcinogenic risks due to CH2O through inhalation was estimated by computing the cancer risk (CR). HQ or HI values below 1.0 indicate that the pollutant under investigation is not likely to cause health impairment, whereas HQ or HI values above 1.0 indicate risk levels that are likely to damage health [42, 43]. The CR values > 10−6 indicate that potential carcinogenic effects may occur, whereas CR values ≤ 10−6 represent an admissible level [43]. The data for HQ and HI are depicted in Figure 2 while those for CR are displayed in Figure 3. In the residential areas bordering the landfill, the values of HQCH2O,HQH2S, and HQSO2 varied from 2.95E − 01 (RA5) to 6.55E − 01 (RA1) (mean 4.99E − 01), 8.92E − 02 (RA5) to 2.86E − 01 (RA3 and RA6) (mean 2.24E − 01), and 4.01E − 02 (RA9) to 1.36E − 01 (RA2 and RA3) (mean 7.66E − 02), respectively. In this same area, the HI values ranged from 4.33E − 01 (RA5) to 9.76E − 01 (RA6) (mean 8.00E − 01), while those of CR due to CH2O was found between 1.88E − 04 (RA5) and 4.19E − 04 (RA1) (mean 3.19E − 04). None of the HQ and HI values exceeded the threshold value, set at the unity, implying that CH2O, H2S, and SO2 are not likely to induce adverse health effects in the area under study for now. All the CR values were higher than 10−6 indicating that the nearby residents to the landfill are at risk of developing cancer in future owing to the inhalation of CH2O. Comparatively, all the CR values due to CH2O registered in this study were higher than 2.9 × 10−5 recorded near a plant treating organic waste in Catalonia (Spain) [44].Figure 2
Non-cancer risks at the monitoring sites (the non-cancer risks of CH2O and H2S at RA0 were not calculated because they were not detected at that site; the horizontal line represents the admissible level of non-cancer risk [42, 43]).Figure 3
Cancer risks (CR) at the monitoring sites (CR at RA0 was not calculated because CH2O was not detected at that site).The risk levels in this study might have been overestimated as the chemical concentrations were measured solely for 24 hours instead of one year. Contrastingly, risks might have been underestimated because only the concentrations of CH2O, H2S, and SO2 among a multitude of volatile toxic compounds that might be present were considered for the assessment of health risk. Furthermore, only exposure via inhalation was considered although exposure through ingestion and skin absorption may occur even if it is most often much lower [43].


## 4. Conclusion and Recommendations

According to the results of the present study, at the vicinity of the landfill, 30% of the daily mean concentrations of PM2.5 and PM10 and all the detected levels of CH2O crossed the daily maximum safe limit, while the concentrations of CO, O3, NO2, SO2, and H2S were within the emission standards. However, noxious gases, viz., CH2O and H2S, were detected at the concentrations higher than their odour thresholds. Continuous dispatch of these gases into the ambient air may significantly reduce air quality and imperil public health and welfare. The values of cancer risk (CR) and hazard index (HI), respectively, were higher than 10−6 and less than the unity. Thus, the nearby residents to the Nkolfoulou landfill may experience an increase in risks of developing cancer while there was no significant increase of non-cancer risks. 96.76% of the daily average levels of air pollutants recorded in the neighborhood of the Nkolfoulou landfill exceeded those found at the remote control site, implying that the landfill operations might be contributing to air pollutants to the ambient air.By this study, the following mitigation strategies can be recommended:(a)
Daily cover of odorous wastes or odour treatment at the landfill site.(b)
The road linking the highway to the landfill should be paved or thoroughly watered daily to keep the concentrations of PM at bay.(c)
Planting trees around the landfill to absorb air pollutants.



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*Source: 3021894-2019-07-04.xml*