The air quality of the national capital has improved a little this October, as compared to last year, say experts.
Central Pollution Control Board (CPCB) records for October 2015 show that the Air Quality Index (AQI) for nine days ranged between 300 and 400 -- categorised as "very poor" -- and at few stations even crossed 400, considered "severe".
In October 2016 so far, however, the air quality has remained "poor" and occasionally "moderate", with maximum AQI at 293 on October 4 and 285 on October 18.
The "poor" category leads to breathing discomfort and "very poor" to illness.
Experts attribute the marginally better air quality to a combination of factors such as a drop in the influx of trucks, dryness, rise in temperature, winds, low growth in automobile sales and a slight drop in open burning of agricultural residue in neighbouring states like Haryana and Punjab.
"We don't have exact numbers, but the number of trucks entering Delhi has dropped from 50,000-60,000 per day in October 2015 to currently about 30,000 per day after the Environment Communication Charge (ECC) was imposed in Delhi," Usman Nasim, a researcher with the Centre for Science & Environment (CSE), told a news agency.
The Supreme Court in November last year imposed an ECC of Rs 700 to Rs 1,300 on commercial diesel vehicles entering Delhi. A month later, the apex court doubled the amount.
The dryness in air and rise in temperature in October this year is also believed to have helped in reducing the air pollutant concentration in Delhi, making it convenient for winds to carry those away.
While the particles in air were the major reason behind poor air quality this year, Nitrogen Oxide and Ozone also affected air quality, especially on Wednesday, October 19 - the eighth day of consistently poor air quality.
AQI is calculated based on both particle and gaseous pollutants in the air, including PM10 and PM2.5 (particles with diameter less than 10 and 2.5 micro metre), NO2, SO2, NH3, lead, CO and O3.
PMs are the pollutants responsible for respiratory and cardiovascular diseases, while Nitrogen Dioxide (NO2) and others cause lung diseases, lung tissue damage, reduction of lung function and eye-related issues.
"Due to low humidity, concentration of the pollutants is low, as horizontal and vertical winds can easily carry them away," Vivek Chattopadhyay, air quality expert at CSE said.
Temperature records for October suggest that the temperature in Delhi has gradually increased. The maximum ranged from 36 to 33 degree Celsius in 2012, 39 to 33 degrees in 2014, 38.4 to 35 degrees in 2015 and 37.2 to 32 degrees so far in 2016.
Similarly, the minimum temperature recorded was 24 degree Celsius (upper limit) in 2012 and 2013, 26 degrees in 2014 and 2015, and 28 degrees in 2016 so far.
The open stubble burning, according to some experts, was a bit controlled this year, though Polash Mukherjee, a research associate at CSE, said that the "strictness is only on paper".
"Last year there were at least 15,000-odd fires detected in Punjab and Haryana from mid-October till mid-November. This year burning began from October start and is still going on at an alarming level," Mukherjee said.
According to him, fewer number of trucks entering the city and an overall slowdown in car sales growth due to speculation of a diesel vehicle ban are the causes for the slight improvement in air quality this year.
As winter approaches, the concentration of pollutants in air increases because atmospheric boundary layers come closer to land.
"Winters are the testing period for all the claims of pollution control. About 30 per cent respiratory problems increase. There are more stroke problems and related deaths," Chattopadhyay said.
A recent World Bank report said 1.5 million people died in India and 1.6 million in China due to air pollution in 2013.
"Many countries like China have developed an Energy Response System, under which all the emitting factories are shut, schemes like odd-even are imposed, bus services are upgraded and schools are shut when air quality becomes alarming," Chattopadhyay said, adding that Delhi too needs such measures.
In October 2015, two stations reported AQI to be "severe", threatening even the healthy people, while it remained "moderate" on two days and "satisfactory" on one day. "Satisfactory" and "moderate" indices were however short-spanned and based only on one out of seven monitoring stations. In October 2016 so far, AQI has remained "poor" on 15 days and "moderate" on four days.
(With IANS inputs)