This study used the annual average data of PM2.5 between 2002–2016 and 2019 for the quantitative measures. The data for 2017 and 2018 are missing here. To understand the impact of PM2.5 on public health, a structured questionnaire survey was conducted, a qualitative analysis. Details of the methods are described here.
Retrieving PM2.5 data
The annual average data of PM2.5 were collected as raster-ASCII format with a 0.01 X 0.01 deg spatial resolution from Van Donkelaar et al. (2016), a study group at the Atmospheric Composition Analysis Group of Dalhousie University, Canada. They derived the PM2.5 from Aerosol Optical Depth (AOD) using the Moderate Resolution Imaging Spectroradiometer (MODIS), Multi-angle Imaging SpectroRadiometer (MISR), and SeaWiFS sensors. A robust Geographically Weighted Regression (GWR) method along with GEOS-Chem Models to simulate the spatiotemporal variations across the world, was applied (Van Donkelaar et al. 2016). The raster data were converted to point feature data within the study area using a district boundary (shapefile) as a mask applying open-source GIS software, QGIS 3.14 (QGIS 2016) where each pixel generated one point feature. The shapefile (mask) was collected from Bangladesh Local Government and Engineering Department (LGED 2020) with a coordinate reference system, World Geodetic System 1984 (WGS84). The converted point features were further used for geostatistical, hotspots, and risk zone analysis.
PM2.5 data validation
The PM2.5 derived from satellite images were validated using ground stations’ data during 2002–2019 from the Department of Environment, Government of Bangladesh (CASE 2019). There are only five ground stations (Fig. 1) available in the study area. The annual average MODIS and ground measured PM2.5 data were used in a statistical correlation (Ni et al. 2018). The Correlation Coefficient (R2) and the Adjusted Correlation Coefficient were estimated (Fig. 2). The extracted PM2.5 data from MODIS provided a good fit with the ground base measurement as R2 = 92.05% (Fig. 2). It revealed that the derived PM2.5 data were estimated with high accuracy.
Multivariable and Spatio-temporal analysis
Temporal analysis of basic statistics, e.g., minimum, maximum, and mean value of PM2.5 during the study period 2002–2019, was calculated. In this period, the mean annual rate of PM2.5 is increased by ⁓42% in the study area (Fig. 3a-c). The yearly trend of minimum values of PM2.5 is increased by 40%, while the maximum value is increased by 37% (Fig. 3). The concentration PM2.5 is almost stable to 60 ± 2 µg/m3 during 2003–2008 (Fig. 3b). The highest variation of PM2.5 is 8%, found from 2012 to 2016 (Fig. 3b). Besides, an upward trend of the mean values is observed from 2013 to 2019 (Fig. 3b), and the highest (Dhaka District) and lowest (Narsingdi District) increment happen with the gradient of 1.82 and 1.74, respectively (Fig. 3d, h). All these statistical values exceed the annual standard limit of the World Health Organization (WHO) that is 15 µg/m3 (Fig. 3a) (WHO 2016).
A time-series mapping was created using a specific year's average values of PM2.5 between 2002 and 2019 to visualize the spatiotemporal trend of PM2.5 (Fig. 4). However, to identify the most pollutant and affected zones in the study area, a general map was prepared using average value considering the entire study period (2002–2019), the average map in Fig. 4. In the Dhaka District, the average annual PM2.5 is 65–67 µg/m3 while it is 62–65 µg/m3 in Narayanganj, 60–66 µg/m3 in Gazipur, 61–64 µg/m3 Narshingdi, and 63–67 µg/m3 in Munshiganj Districts (Fig. 4). The Dhaka District, the central part of the study area, has more signatures of air pollution than other parts. Predominantly, all urban cities of the middle part have higher concentrations of PM2.5. On the other hand, the northern and southern parts of the study area have less pollution because of peri-urban and less industrial and brickfield activities (Fig. 4).
Hotspot analysis
The spatial process of the statistical clustering method was considered to identify the concentration of PM2.5 pollutants in the long-term spatiotemporal pattern of air pollution (e.g., Habibi et al. 2017). In ArcsGIS, the Hot Spot Analysis tool in Spatial Statistics was applied to find out the hottest and coldest areas. In the hotspot analysis, Getis–Ord Gi*cluster statistic method was selected as a local spatial statistic using average temporal vector data (point feature) of PM2.5. The Gi*cluster statistic works based on the weights and heterogeneity in each data point of PM2.5 (Songchitruksa and Zeng 2010). The Gi* statistic uses the following measures as mentioned by Environmental Systems Research Institute (ESRI) (ESRI 2019) to identify the hotspots areas:
$${G}_{i}^{*}=\frac{\sum _{j=1}^{n}{w}_{i,j}{x}_{j}-\stackrel{-}{X}\sum _{j=1}^{n}{w}_{i,j}}{\sqrt[s]{\frac{\left[n\sum _{j=i}^{n}{w}_{i,j}^{2}-{\left(\sum _{j=1}^{n}{w}_{i,j}\right)}^{2}\right]}{n-1}}}$$
where, xj is the value of j, wi,j is the spatial weight between feature i and j, n is equal to the number of features, \(\stackrel{-}{X}= \frac{\sum _{j=1}^{n}{x}_{j}}{n}\), and \(s= \sqrt{\frac{\sum _{j=1}^{n}{x}_{j}^{2}}{n}-{\left(\stackrel{-}{X}\right)}^{2}}\). A Getis–Ord Gi* produces z-scores and p-value. A higher z-score and a small p-value of a cluster signify the hottest spot while a negative z-score and a small p-value present the coldest area (Jana and Sar 2016). Readers are encouraged to read (ESRI 2019) more about z-score and p-value. The raster overlay was applied using the resultant hotspots areas and area-specific population and most critical public health data to find out the riskiest zones (e.g., Kumar et al. 2015). Further, the hotspot map was overlaid with upazlias’ total population (Fig. 5b), population 0–9 (Fig. 5c), population 60+(Fig. 5d), pneumonia patient (Fig. 5e), and pregnant women (Fig. 5f). The upazilas’ specific population data was collected from the Bangladesh Bureau of Statistics (BBS) (BBS 2020) to calculate area-specific, most vulnerable population groups whose ages between 0–9 and 60+ years. The upazilas’ specific pregnant and pneumonia patients were collected from Bangladesh Directorate General of Health Services (BDGHS) (DGHS 2019).
Near about one-third area is found as very high- and high-hotspot zones in the study (Fig. 5a). The spatial difference between the very high- and high-hotspots zones is almost negligible (Fig. 5a). Most of these very high-spot zones were found in all city areas of Dhaka, Gazipur sadar, Kaliganj, Rupganj, Sonargaon, Savar, and Dhamrai areas. Moreover, this research found 3640748 persons (16.5% out of total population) (Fig. 5b) of which 4.39% (969261 persons) are age group 0–9 (Fig. 5c), 0.95% (210999 persons) are age group 60+ (Fig. 5d), 5% (12,062,419 persons) are pregnant women (Fig. 5e), and 1% (24621 persons) are pneumonia patients (Fig. 5f) in very high- and high-hotspot zones.
Impact of PM2.5 on public health
The self-reported health impacts due to the PM2.5 in the very high-, high-hotspots, and low-spots zones, a primary survey was conducted considering 115 sample populations. For this health survey, a purposive sampling method (Palinkas et al. 2015) was followed to collect the individual specific in-depth information from each respondent using a mini-structured questionnaire from 26 December 2019 to 27 January 2020. The health impacts between very high- and low-spot zones are compared. A total of 85 samples were conducted to the population age 60+, of which 55 samples were from very high- and 30 samples were from low-spot zones. On the other hand, 30 samples were conducted to the pregnant women, of which 20 samples were from very high- and ten samples were from low-spot zones. For collecting data about pregnant women, the survey team went to government health facilities and practice chambers of the gynecologist.
After getting a consensus from a pregnant woman or her caregivers, data was collected. For the population age 60+, respondents were selected as one in a ⁓5 km radius to enhance the uniform distribution of the sample. After data collection and editing, significant sources of air PM2.5 and self-reported health impacts due to PM2.5 were analyzed using descriptive analysis. A non-parametric Mann-Whitney U (McKnight and Julius 2010) test to compare the health impacts between very high- and low-spot zones was conducted. This non-parametric test was selected because these two groups were not normally distributed, and the sample size was sufficiently small, what is one of the limitations of this study. However, this test tends to be more appropriate in this situation (McKnight and Julius 2010). Stata version 13 (StataCorp LLC n.d.) was used to conduct different statistical analyses.
Respondents’ knowledge about PM2.5
About 71% of the population age 60+ in the high-spot zone know well about PM2.5 (Fig. 6). Contrary, more than 73% of the same group (60+) do not know about PM2.5 in the low-spot zone (Fig. 6). Only 55% of pregnant women in the high-spot area know about PM2.5, while 70% of pregnant women have no idea about PM2.5 in the low-spot zone. In this analysis, pregnant women have less access to information related to PM2.5 than the population 60+ in both zones (Fig. 6).
Respondents’ knowledge of sources of PM2.5
Population age 60+ believe that road vehicle (64%), urbanization (84%), construction site (43%), brickfield (38%), and industrial emissions (44%) are responsible for PM2.5 in high-spot zone. Contrary, pregnant women in the high-spot zone suggest that dust (25%), construction site (57%), and brickfield (%) are the predominant controlling factors for PM2.5 (Table 1). In the low-spot zone, both pregnant women and population age 60+ mention that the dust (20%) and industrial emissions (56%) are the triggering factors for increasing PM2.5.
Table 1
Respondent’s perception about sources of PM2.5 in both high- and low-spot zones.
|
Pregnant women in low-spot
|
Pregnant women in high-spot
|
60 + pop in low-spot
|
60 + pop in high-spot
|
Road vehicle
|
9.1%
|
13.6%
|
13.6%
|
63.6%
|
Dust
|
20.0%
|
25.0%
|
35.0%
|
20.0%
|
Urbanization
|
16.7%
|
0.0%
|
0.0%
|
83.3%
|
Brickfield
|
3.1%
|
25.0%
|
34.4%
|
37.5%
|
Construction site
|
0.0%
|
57.1%
|
0.0%
|
42.9%
|
Industrial emission
|
0.0%
|
0.0%
|
56.3%
|
43.8%
|
Respondents’ responses about health risk due to PM2.5
Breathing problems, cold/cough, eye problems, skin problems, and asthma are identified as significant health problems in both high- and low-spot zones. The Mann-Whitney U test results suggest that the pregnant women group has a higher mean rank (16.43) in the high-spot than the low-spot zone (13.65) (Table 2). The Mann-Whitney U test is estimated to 81.5, and the p-value is to 0.422, which is higher than 0.05. So, the test is not statistically significant, but there the difference exists.
Table 2
Respondent’s self-reported health risk due to particulate matter
Groups
|
Low spot zone
|
High spot zone
|
Mann-Whitney U
|
p
|
n
|
Mean rank
|
n
|
Mean rank
|
Pregnant women
|
10
|
13.65
|
20
|
16.43
|
81.5
|
0.422
|
60+ population
|
30
|
33.35
|
55
|
48.26
|
535.5
|
0.006*
|
* Statistically significant at 0.05
However, a higher mean rank of 48.26 for the population 60+ is calculated in the high-spot, compared to that in the low-spot zone to 33.35. Besides, the Mann-Whitney U test is 535.5, and the p-value is 0.006., which is less than 0.05 (Table 2). It proves that there is a significant difference in the health impact of pregnant women and population 60+ between the high- and the low-spot zones. The health effects of pregnant women and population 60+ in high-spot regions are more significant than low-spot areas that satisfy the hypothesis.