A new health advisory by the Delhi government on Monday urged people to follow similar measures as they did for several days of severe air pollution levels earlier this month: stay indoors, use carpooling, do not go for morning and late evening walks and avoid morning assemblies at schools.

Air quality in the city is set to decline further, after it stayed in the “very poor” zone for four straight days, PTI reported. High moisture content and lack of winds are the reason again, the Delhi government said, while the Centre said it would take “harsh measures” if needed.

The concentration of particulate matter of diameter less than 2.5 microns – called PM2.5 – was 344.8 microgram per cubic metre at 9 am, according to the Central Pollution Control Board. A 24-hour average of over 250 is classified as “severe”.

The air quality index on Monday, as measured by the CPCB, was 362, higher than on Sunday by 10 units. A level between 301 and 400 is “very poor”.

An emergency is declared if the PM2.5 and PM10 levels are recorded over 300 and 500 microgram per cubic metre, respectively, for 48 hours. The levels are expected to rise further over the next three days, according to SAFAR, which runs under the Centre’s Ministry of Earth Science.

Pollutants are not getting dispersed because of slower surface winds and fluctuating wind directions, CPCB’s air lab head Dipankar Saha told PTI. However, he said pollution will not reach levels it did earlier this month.

Delhi and several parts of North India woke up to a toxic smog on November 7, leaving the governments of the National Capital Region scrambling to bring in measures in the eleventh hour to bring down pollution levels. This had happened in 2016 as well.