The Union government on Tuesday ordered the withdrawal of restrictions imposed under Stage III of the Graded Response Action Plan in the National Capital Region after improvement in the overall air quality.

This would allow all Bharat Stage III petrol and Bharat Stage IV diesel four-wheeler vehicles in Delhi, Gurugram, Faridabad, Ghaziabad and Gautam Budh Nagar to operate again.

The response action plan is a set of incremental anti-pollution measures that are triggered to prevent further worsening of air quality once it reaches a certain threshold in the Delhi-NCR region.

Bharat Stage emission standards regulate the output of air pollutants from vehicle engines. The higher the Bharat Stage category, the less polluting the vehicle.

The restrictions under the air pollution mitigation mechanism were withdrawn by the Commission for Air Quality Management with immediate effect. The commission is a statutory body responsible for controlling air pollution in the national capital.

Restrictions on non-essential construction work, stone crushing and mining were also lifted.

The committee cited forecasts by the India Meteorological Department, stating that there is a likelihood of the air quality remaining in the “very poor” category in the next few days.

The average air quality index in Delhi at 4 pm on Tuesday was 312, down from 395 on Monday, it said. An air quality index between 301 and 400 “very poor” and 401 and 500 is “severe”.

At 7.30 am on Wednesday, the overall air quality index for Delhi was 318, according to data shared by the Ministry of Earth Sciences’ System of Air Quality and Weather Forecasting and Research.

Meanwhile, the PM10 index, which measures the concentration of particulate matter of 10 microns in diameter or less in the air, was 223 on Wednesday.

Levels of PM2.5 were 143 micrograms per cubic metre. Particulate matter smaller than 2.5 microns, or about a ten-thousandth of an inch, is particularly dangerous to human health as these particles are small enough to travel deep into the respiratory system, potentially impairing lung function.

In view of this, restrictions under Stage I and Stage II of the action plan will remain in place and “be implemented, intensified, monitored and reviewed by all agencies concerned in NCR” to ensure that the air quality index levels do not slip back into the “severe” category, the committee said.

Under Stage I and Stage II, construction and demolition project sites and industrial units that have been served specific closure orders on account of violations or non-compliance with various norms will remain closed.

The authority had imposed Stage IV restrictions under the Graded Response Action Plan on November 4, before rolling it back to Stage III on November 18.

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