On Wednesday, the pollution levels were the highest after Diwali-October 23 and 24. System of Air Quality Weather Forecasting and Research (SAFAR), under Indian Institute of Tropical Meteorology (IITM), has linked the peaking of PM2.5 (fine, respirable particles) to burning of agricultural waste in Punjab and Haryana. In fact, monitoring by SAFAR revealed that Patiala in Punjab recorded the highest PM2.5 and PM10 (coarse particles) levels over the past few days.

On the bright side, the temperature hasn't fallen too much as it would have aggravated the smog-like conditions. However, PM2.5 levels have gone up to almost 200 microgram per cubic metre compared to 137 on October 29. Noida was the worst placed with PM2.5 hovering around 280 micrograms per cubic metre. IITM's other project, Modelling Air Pollution and Networking, which monitors air quality in 12 other cities, has found alarming pollutant levels in Patiala with PM2.5 touching 309 microgram per cubic metre and PM10 settling at 488.

"Our assessment shows that air pollution levels have gone up in Delhi because of biomass burning in Punjab. It stands to reason as Patiala has been recording very high PM2.5 and PM10 levels. The winds blowing towards Delhi from the northwest are passing by the area where a lot of agricultural waste is being burned. If it gets colder in the coming days, the smog will intensify," said Gufran Beig, chief project scientist, SAFAR. Recent images from Nasa suggested a thick blanket of aerosols lying over northern Indian states, including Delhi. It also showed dots of fire in the Punjab-Haryana region.

The combination of pollution from neighbouring states and local emissions is already taking its toll. "Every year after Diwali, complaints from people with asthma and bronchitis go up. Their aggravated condition is clearly linked to high air pollution which also leads to more hospitalization. Nasal and sinus problems also increase. Those who are healthy often experience with cough or throat problems. If you don't consider the infections during monsoon, the jump in cases of aggravated bronchitis or asthma is 30% to 35% compared to those in summer. We have seen an overall jump in asthma and chronic bronchitis in Delhi over the years," says Dr Neeraj Jain, senior consultant of chest medicine at Sir Ganga Ram Hospital.

Experts say biomass burning is only adding to the already high local emissions. "We have been tracking air pollution since July. In July and September there were some days with low pollution; in October, pollution levels have crossed the limit on all days. We may see some spike due to external events like Diwali or biomass burning but that's only because the existing levels are alarming. Even after biomass burning, every winter PM2.5 and PM10 levels are extremely high," said Anumita Roy Chowdhury, head of the clean air programme of Centre for Science and Environment.

Even rural areas are not spared. A research paper, 'Recent climate and air pollution impacts on Indian agriculture', published recently in the Proceedings of National Academy of Sciences, analysed the impacts of pollution over 30 years and found that short-lived climate pollutants (SLCPs) like smog, ground-level ozone and black carbon may have reduced India's potential wheat yields by about 50%. IITM scientists said their own studies have also found severe impacts on agricultural output from ground-level ozone.

According to US Environment Protection Agency, ground-level ozone is not emitted directly into the air but is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOCs). Ozone is likely to reach unhealthy levels on hot sunny days in urban environments. Ozone can also be transported long distances by wind.