Received 11 November 2015; accepted 13 February 2016; published 16 February 2016
Silica is air toxics when inhaled or ingested during product use, demolition work, building or home maintenance, repair, remodeling, etc.  . Exposure to silica containing respirable particulates (<0.5 to 5.0 µm in diameter) over long time causes silicosis, lung cancer and airways diseases  - . Some investigations of silica exposures were reported  - . In the present work, the ambient exposure of silica associated to coarse and fine particulates in the most industrialized area of the central India (Raipur city, capital, Chhattisgarh state) is described.
2. Methods and Materials
2.1. Study Area
The Raipur city (latitude: 21˚24'N and longitude: 81˚63'E), capital of Chhattisgarh state, India, was selected for the present study due to running of several industries in this region. The population of city including suburb area is »2 million, and being an important regional commercial and industrial destination for the coal, power, steel and aluminum industries. Several steel and ferro-alloys industries are running in this city.
2.2. Particulate Collection
The Partisol Model 2300 Sequential speciation air sampler (Thermo Scientific, USA) was used for collection of the PM10 and PM2.5 samples during period, June 2005 - May 2014. The PM10 and PM2.5 samples were collected simultaneously on 47-mm teflon quoted PTVC filter (Whatmann) housed in molded filter cassettes. One sample blank was used for collection of PM10 and PM2.5. The sampler was installed at the roof of the building, 3-m above from the ground level at residential site: Dagania (no. 2), Raipur, Figure 1. Similarly, PM collection was carried out in the commercial (Jaistambh, no. 3) and industrial area (Siltara, no. 1). The filter was prior heated up to 50˚C to reduce their blank value, and placed in clean polyethylene petri dish. The weighted filters were housed in the sampler and run for 24-hrs from 6.00 am - 6.00 am. The loaded filters were dismounted, brought to laboratory, transferred into the desicator, and finally weighted to record the particulate contents.
The elemental analysis of the collected aerosol samples has been carried out by PIXE (Particle Induced X-ray
Figure 1. Sampling locations in Raipur city, CG, India.
Emission) technique, using the external beam set-up of the 3 MV Tandetron accelerator of the LABEC laboratory (INFN) in Florence, Italy.
3. Results and Discussion
3.1. Particulate Concentration
The annual (June 2005 - May 2006) mass concentrations of PM10 and PM2.5 (n = 44) are in the range 37 - 501 µg/m3 and 19 - 259 µg/m3 with arithmetic mean value of 209 ± 38 µg/m3 and 95 ± 18 µg/m3, respectively, Figure 2. The annual mean ratio of [PM2.5]/[PM10] was found to be 0.46. The highest concentration of the PM10 and PM2.5 was observed in the winter season (December - January), may be due to temperature inversion and lowest air mass speed. The lowest seasonal average concentration of PM10 and PM2.5 observed was 50 and 20 µg/m3, respectively, could be considered as background levels in Raipur city.
3.2. Elemental Concentration
The elements i.e. Na, K, Cl, Al, Si, S, Ca and Fe are main constituents of the PM. The annual mean concentration of Na, K, Cl, Al, Si, S, Ca and Fe in the ambient air associated to the PM10 was 1.0 ± 0.1, 3.4 ± 0.6, 1.9 ± 0.5, 6.6 ± 1.1, 15.9 ± 2.8, 3.4 ± 0.3, 9.8 ± 1.9 and 12.7 ± 2.5 µg/m3, respectively, Figure 3. The concentration of Na, K, Cl, Al, Si, S, Ca and Fe in the air associated to the PM2.5 was 0.5 ± 0.1, 1.5 ± 0.3, 0.8 ± 0.4, 1.0 ± 0.2, 2.0 ± 0.4, 3.1 ± 0.4, 0.9 ± 0.2 and 1.6 ± 0.4 µg/m3, respectively. Silicon and sulfur showed the highest concentration in the PM10 and PM2.5 mode, respectively, Figure 3.
Figure 2. Distribution of PM10 and PM2.5 in the ambient air of Raipur city at residential area during year, June 2005 - May 2006.
Figure 3. Annual mean concentration of major elements in ambient air associated to the PM10 and PM2.5.
3.3. Silica Concentration
The existence of silicon in the particulates is assumed as silica, SiO2, and a factor of 2.14 was used to compute SiO2 concentration. The SiO2 concentration associated to the PM10 and PM2.5 was ranged from 6.6 - 102 and 0.3 - 15.1 µg/m3 with mean value of 40 ± 6 and 4.5 ± 1.0 µg/m3, respectively.
3.4. Seasonal, Spatial and Temporal Variations
The highest concentration of SiO2 in the particulate phase was observed during month of February, 2006 mainly due to lowest wind speed (<2 km/hr) and higher temperature inversion, Figure 4. In turn, the lowest concentration of SiO2 in the air was marked in the month of July due to the highest wind speed (>10 km/hr) and heavy rain fall (>50 cm/month). The SiO2 concentration in the commercial area (CA) and industrial area (IA) was increased enormously, Figure 5. The temporal variation of SiO2 was found to increase during year 2006 to 2009 due to vast industrialization, increase in vehicle numbers and unpaved nature of the road, Figure 6. Thereafter, the SiO2 concentration was found to decrease may be due to use of control devices for industrial effluents and improving the road quality.
3.5. Silica Exposure
Exposure of silica beyond the threshold limit value (TLV) of 25 µg/m3 in air causes respiratory diseases  . The annual mean value of silica exposed was 40 ± 6 µg/m3, higher than the TLV value of 25 µg/m3. The whole population is exposed with silica at elevated levels during period, October to April of a year.
Figure 4. Seasonal variation of oxide of Al, Si and Ca.
Figure 5. Spatial variation of SiO2 in residential (RA), commercial (CA) and industrial area (IA) of Raipur city during February, 2006.
Figure 6. Temporal variation of SiO2 in month of February at residential site of Raipur city.
The respirable aerosol concentration in the urban environment of India is found several folds higher than permissible limits during period, October - April of a year. They are associated with elevated levels of elements i.e. Si, Fe, Ca and Al. The concentration of silica in the ambient environment of Raipur city, central India was found to be higher than the permissible limit during period, October - April of a year. The mean silica concentration in the PM10 was observed to be 17.5% ± 1.7%.
We are thankful to the DST, New Delhi for granting project grant for aerosol studies in central India.
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