Air Quality Index, or AQI, figures are released daily to show the state of air pollution in cities. But do you know how it is decided whether the air is clean or toxic? AQI converts the levels of different pollutants present in the air into a single number. This helps people understand how safe the outdoor air is for their health.
AQI is decided by eight pollutants
In India, the national AQI is calculated on the basis of eight major pollutants. These include PM 2.5, PM 10, nitrogen dioxide, sulphur dioxide, carbon monoxide, ozone, ammonia and lead. Pollution monitoring centres measure the levels of these elements. A separate sub-index is prepared for each pollutant on the basis of the prescribed standards. The highest of these sub-indices is considered the overall AQI for that location. AQI cannot be issued if sufficient data is not available.
Six categories of AQI
The national AQI is divided into six categories. Air from 0 to 50 is considered good, 51 to 100 satisfactory, 101 to 200 moderate, 201 to 300 poor, 301 to 400 very poor and 401 to 500 severe. The higher the AQI, the greater the health risk.
What GRAP is and its four stages
The Graded Response Action Plan, or GRAP, is implemented to control pollution in Delhi-NCR. It has four stages. In Stage 1, when AQI is 201 to 300, or in the poor category, steps such as dust control, road cleaning and monitoring of construction sites are taken. In Stage 2, when AQI is 301 to 400, or very poor, measures such as increasing public transport, raising parking charges and controlling diesel generators are taken. In Stage 3, when AQI is 401 to 450, or at a severe level, non-essential construction activities may be stopped and restrictions may be imposed on some vehicles. In Stage 4, when AQI rises above 450, the strictest steps are taken.
Action under GRAP does not depend only on the current AQI figure. Restrictions can also be imposed in advance after considering weather and pollution forecasts. Its objective is to prevent pollution from becoming more severe.
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