According to an ongoing study being conducted by Bihar State Pollution Control Board (BSPCB), the extent of ground-level ozone — a colourless and highly irritating gas that forms just above the earth’s surface — has been found lower than national permissible limit at all 12 observation points in the city.

The Central Pollution Control Board (CPCB) states that motor vehicle exhausts and industrial emissions are two major sources of nitrogen oxide (NOx) and volatile organic compounds, which upon reaction in sunlight produce ground-level ozone (O3). As per the measurement of ground-level ozone at 12 locations in Patna by the BSPCB during a month-long exercise in May, it has been found that the highest level of O3 was at highly trafficked locations.

For instance, the highest level of ground-level ozone was measured at Dakbungalow roundabout (84.5 µg/ metres cube) and Patna Junction roundabout (75.2 µg/ metres cube).

On the other hand, places like Indira Gandhi Institute of Medical Sciences and Sanjay Gandhi Biological Park, which are, to an extent bit away from vehicular traffic, have shown the lowest level of O3 — 33.8 µg/ metres cube and 36.3 µg/ metres cube, respectively.

Also, the level of O3 at Patliputra Industrial Area, the lone industrial area in the city, was recorded at 50.7 µg/metres cube, much lower than permissible, which is 180µg/metres cube. Sources claimed that lack of heavy industries and very few active units can be few of the reasons behind such lower level of O3 in the industrial area in the city.

The study is being conducted by the BSPCB after five years. “The last study on ground-level ozone in Patna was conducted in the 2009 winter. As the formation of O3 depends heavily on sunlight, the intensity of which is comparatively less in the winters, thus this time it was decided to do the study in the summer season. Thankfully, the level of O3 at all locations has been found to be well under the permissible level,” said Naveen Kumar, assistant scientific officer, BSPCB.

According to the CPCB, strong sunlight and hot weather cause ground-level ozone to form harmful concentrations in the air. Many urban areas are subject to high ozone levels, as winds carry NOx emissions hundreds of miles away from their original sources. Few of the factors that contribute to ozone formation are changing weather patterns, especially the number of hot, sunny days and periods of air stagnation among others.

The O3 level being low in the city, environmentalists have appealed to abide by environment-friendly practices to maintain it. “Since there is minimal industry in the city, thus whatever amount of ground-level ozone has been measured must have been mostly due to vehicular emission. Authorities should remove old vehicles from the city streets and also allow only fuel-efficient vehicles,” said R.C. Sinha, chief executive, Centre for Environment and Nature Conservation, department of zoology, Patna University.

Though the intensity of ground-level ozone in the city is much lower even during the hot weather of May but that does not mean that the air people breathe in the city is free from pollutants. The level of respiratory suspended particulate matter (PM-10) — fine particulate matter that invades the lungs and triggers respiratory diseases and cancer —are still more than double of the respective standards. As per the annual air pollution report of the BSPCB for 2013-14 released in May, the actual level of PM-10 was found to be 152 µg/ metres cube against its respective standard of 60 µg/ metres cube.

By and far, the suspended road dust particles and vehicular emission have been linked to higher level of PM-10 in Patna. The volume of sales of vehicles — both two-wheelers and four-wheelers — in the state has been on the rise over the past couple of years.

World Health Organisation (WHO), through its 2014 version of the ambient air pollution database released on May 7, declared Patna as the second most air-polluted city in the country, only after the national capital New Delhi. However, officials in the BSPCB claimed that the CPCB as well as ministry of environment and forests have contradicted the report.

“We have air pollution data of the city for nearly three decades, which is sent to the CPCB as well. However, no such trend has come to our notice ever. Thus, the WHO report has been refuted,” said a scientist at the BSPCB.