The last time Delhi’s air quality was recorded in the ‘poor’ category was on December 10, after which it remained in the ‘very poor’ and ‘severe’ zones for several days before improving again on Wednesday.

The conditions, however, are forecast to worsen again. According to Indian Institute of Tropical Meteorology (IITM), Delhi’s AQI is likely to remain in the ‘Very Poor’ category up to December 27.

The improvement witnessed on Wednesday was largely driven by stronger surface winds and better atmospheric dispersion conditions through the day.

According to the India Meteorological Department (IMD), west to south-westerly winds with speeds reaching up to 22 kmph prevailed over Delhi during the past 24 hours, helping dilute and disperse accumulated pollutants that had built up under calm conditions earlier in the week.

The shift was driven by a rise in daytime temperatures and increased mixing of air near the surface. The IITM’s Air Quality Early Warning System indicated that Delhi’s maximum mixing height – the vertical layer within which pollutants can disperse – rose to around 1,200 m on Wednesday. This, coupled with a ventilation index of about 9,000 m²/s, created more favourable conditions for pollutant dispersion. A ventilation index above 6,000 m²/s is considered conducive for clearing pollution.

However, wind speeds are forecast to weaken over the next few days, with the ventilation index expected to drop sharply to 3,500 m²/s on December 25 and further to 1,500 m²/s on December 26 — levels considered unfavourable for dispersion. Calm conditions, lower mixing heights and falling minimum temperatures are likely to allow pollutants to accumulate again.

The IMD has also issued a yellow alert for moderate to dense fog from December 26 to 27, with shallow to moderate fog expected on several mornings through the end of the month. Dense fog, low wind speeds and colder nights typically trap pollutants close to the ground, worsening air quality.

The pollutants largely have remained the same. Data from IITM’s Decision Support System showed that the transport sector remained the single largest contributor to PM2.5 levels within Delhi, accounting for 17.25% of the pollution load. Industrial sources contributed around 8.6%, followed by residential emissions at 4.25%. Among neighbouring regions, Jhajjar in Haryana accounted for a significant 15.56% of the PM2.5 contribution, followed by Rohtak (6.58%), Sonipat (3.98%) and Gurugram (around 2%).

Delhi on Wednesday recorded a maximum temperature of 22.7 degrees Celsius, two notches above normal, and a minimum of 10.2 degrees Celsius, 2.7 degrees above normal, the India Meteorological Department (IMD) said.

The IMD has forecast a minimum temperature of 6 degrees Celsius and a maximum of 22 degrees Celsius for Christmas Day on Thursday, with mainly clear sky after shallow to moderate fog at many places during morning hours.

GRAP-IV revoked, Stage III remains in effect: How air purifiers work

# Air purifiers reduce indoor air pollution by physically trapping particles suspended in the air, especially fine particulate matter such as PM2.5. As polluted air is pulled into the device, larger particles are trapped by the filter fibres, while smaller particles are caught as they brush past through the filter material. Some particles also settle down due to gravity or are attracted to electrically charged surfaces, depending on the type of purifier. These processes together lower the amount of harmful particles circulating indoors.

# This is especially important in cities where opening windows may allow polluted outdoor air to enter homes, schools and offices, making natural ventilation less effective. In such cases, air can be cleaned either before it enters a building through ventilation systems or repeatedly filtered within the room using air purifiers, according to a 2019 study by researchers in University of Surrey and Queensland University of Technology.

# Studies show that HEPA-based air purifiers are among the most effective tools for reducing indoor particulate pollution. When used continuously, they have been found to lower PM2.5 levels indoors by about one-third to nearly three-quarters, depending on the room size, airflow and pollution levels outside. Even short-term use can make a measurable difference: experiments show that portable air purifiers can cut PM2.5 levels by around 45% within 90 minutes, with higher fan speeds removing more pollution.

# However, these devices use electricity, can increase energy costs, and do not remove pollution at its source. Experts, therefore, stress that air purifiers are best used as a supporting measure, especially in highly-polluted urban areas.