New Delhi: Most of the air pollution that is responsible for public health emergencies in Delhi every winter is caused by crop burning in neighbouring states, scientists at Harvard University say in a recent study.

The study conducted by Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), Harvard University puts to rest several questions on the extent to which stubble burning in north-western states including Punjab and Haryana contribute to the already severe pollution in Delhi.

Although Delhi witnesses a spike in particulate matter PM2.5 limits round the year, the ambient PM2.5 concentrations show spikes in October-November, which is also the peak season for paddy harvesting, when abundant crop residue is burnt by farmers to prepare for next crop.

“On certain days, during peak fire season, air pollution in Delhi which is located downwind of the fire is about 20 times higher than the threshold for safe air as defined by WHO," said Daniel H. Cusworth, from SEAS and first author of the paper.

Researchers used the surface measurements of PM2.5 from Central Pollution Control Board (CPCB), US Embassy in Delhi and IndiaSpend from 17 October - 30 November over a period of four years from 2012-2016.

Satellite data from Nasa was used to identify hotspots of active fires to model how much of the pollution is coming from crop fires. The data was fed in an algorithm which accounts for geography, wind patterns, and physics to predict how far and in what direction smoke particles travel.

Daily mean levels of PM2.5 pollution in Delhi often exceed the WHO threshold for unhealthy air (25 micrograms per cubic meter) as well as the daily mean threshold set by the Indian Central Pollution Control Board (CPCB) (60 micrograms per cubic meter).

“Extreme fires during the post-monsoon season can pump on average about 150 micrograms per cubic meter of fine particulate matter into the city, doubling the amount of pollution and increasing total levels 12 times higher than WHO recommendations," stated the paper.

Since the harvesting season coincides with the post-monsoon conditions which favours stagnation of wind in northern India, these conditions allow smoke to slowly permeate throughout the Indo-Gangetic region including Delhi. This smoke mixes with existing pollution from cars and factories creating a thick, deadly haze.

Despite a national ban and regulations by National Green Tribunal Act of 2010, the practice continues as farmers and government remain at loggerheads over a cost-effective alternative.

Pointing out that Delhi’s population is projected to grow to 36 million by 2050, researchers emphasized that the results were critical in developing strategies to reduce overall pollution exposure, especially in wake of the public health emergency every year.

“A relationship between pollution and mortality is well known," said Mickley, who co-authored the paper. Studies have shown that Delhi suffers from diseases related to air pollution at a rate which is 12 times higher than the national.

“Adverse effects of fire emissions need to continue to be seriously considered as population of Delhi continues to grow, leaving more people at risk," said the researchers, adding that the information would provide policymakers with a quantitative sense of the consequences of current agricultural burning practices in order to inform decision making.

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