NEW DELHI: Delhi was shrouded in dense smog as pollution levels spiked massively in different parts of the city on Thursday morning, making it the most polluted day of the season so far.

Anand Vihar recorded PM 2.5 (fine, respirable pollution particles) levels of over 500 micrograms per cubic metre between 5am and 11.30am, more than eight ti mes the safe limit. Both R K Puram and Mandir Marg recorded PM 2.5 levels at more than 250 micrograms during the same hours.

Delhi's air quality index, released by the Central Pollution Control Board (CPCB), stood at 355, which is in the "very poor" range and inching closer to the poorest "severe" category (400 and above). The AQI at different hours of the day touched the danger mark of 500 or 'severe' at several locations such as Anand Vihar, R K Puram and Punjabi Bagh.

The current 'very poor' levels of air quality in the city could cause respiratory illness on prolonged exposure while `severe' levels could affect healthy people and seriously impact those with existing health conditions, says the health advisory associated with these categories.

One of the reasons for the spike in pollution was the calm wind condition as the horizontal and vertical wind speed at most locations ranged from 2 metres per second to 0.2 metres per second. Air quality experts said local sources such as heavy Diwali traffic and waste burning are also contributing to poor air quality in a big way.

Meanwhile, an analysis of crop fires in north India -especially Punjab and Haryana -showed massive fires all through last week. DPCC's analysis of air pollution levels (24hour average) between October 17 and October 25 showed PM 10 (coarse pollution particles) to be over 250 micrograms per cubic metres on most days which is more than twice the safe standard. PM 2.5 levels also remained well over 120 micrograms per cubic metres.

This severe build-up of air pollution will only mean very toxic air quality during Diwali and later. "It's not that only Delhi is affected. Look at other cities in the Indo-Gangetic plains. The entire region is suffering. The conditions in Delhi are becoming calm which means we are going to have a heavily polluted Diwali," said a senior official from the Delhi Pollution Control Committee.

On Thursday, Agra and Faridabad also reported `very poor' air quality while Kanpur reported 'severe' levels, according to CPCB. TOI had reported on October 25 that the wind pattern is likely to change just before Diwali making it very calm and conducive for accumulation of pollution particles closer to the surface. Emission sources within the city are also to blame."This traffic intensity is absolutely insane, it's visible to everyone. CPCB is beginning to see a spike in nitrogen dioxide levels which is clearly linked to traffic emissions. Most complaints coming to the hawa badlo app are related to waste burning. What is the government doing about it?" said Anumita Roy Chowdhury, head of Centre for Science and Environment (CSE's) clean air campaign. "There are also industrial sources and the Badarpur power plant which are both pollution sources within the city. The personal exposure of Delhiites is huge because we are close to several emission sources," Chowdhury said.

In such conditions, several global cities issue emergency directions to shut down certain sources of pollution.

Neither Delhi or the Centre has a policy in place to issue health warnings despite deliberations on the subject for a couple of years now.