People in Delhi inhaled “very poor” air on Wednesday morning, a dangerous marker in the air quality standard that could make even healthy adults sick.

The level of ultrafine particulate matter or PM 2.5 clocked 321 microgram per cubic metre on the Air Quality Index (AQI), according to the data collated by the monitoring stations of System of Air Quality and Weather Forecasting and Research (SAFAR).

Data from Hindustan Times air quality map showed that particulate matter levels touched 350 ug/m3 in Shahdara, Shantipath and Anand Vihar in the morning when people commute and their exposure to air is the highest.

Dip in air quality has been primarily caused by an increase in vehicular traffic in the festival season and poor dispersal of pollutants due to cloudy morning conditions.

This makes the air “very poor” and people are advised to avoid outdoor activity, especially the elderly and young children, and use anti-pollution masks, health experts said.

The permissible level of PM 2.5 is 60 micrograms per cubic meter. Air quality is classified as “very poor” if the AQI ranges from 301 to 400. The pollution level becomes severe if AQI is higher up to 500, which calls for desperate measures like shutting down schools and suspending work in factories in countries such as China.

According to the forecast, situation will worsen in next few days as more vehicles are expected to be on road for Diwali, the festival of light in which pollution levels jump by almost five times as revelers burst millions of firecrackers.

On Wednesday morning, SAFAR recorded 353 microgram of PM2.5 for a cubic metre of air at its Pusa monitoring station. Other stations registered similar figures, way exceeding the safe upper limit of 60 microgram.

At Lodhi Road it was 301, at Ayanagar 310, Dhirpur 313, Delhi University 321, 325 at the Airport (T3) while the Ali was 341 in Pitampura in terms of PM2.5 dust, which can cause grave respiratory disorders.

The city of about 20 million, which ranks among the world’s top cities with foul air on a WHO list, has been struggling to clean up its air that contains a toxic cocktail of dust, smoke and gases from vehicle, factory exhausts and coal-fired power stations.

The condition worsens every autumn and winter as the city, buffeted by farmers burning crop stalks in neighbouring states and atmospheric changes, records higher levels of air pollution.

“Rising pollution levels a regular trend during winters. Things will get worse around Diwali,” an environment department official said.