500: What a maxed out AQI means

What these numbers don’t indicate is that concentrations of the ultra-fine and hazardous particulate matter (PM 2.5 or PM 10) in Delhi could be much higher than indicated by the maximum index value of 500.

On Tuesday morning, around 9.30 am, at least 18 stations in Delhi reported an air quality index (AQI) of 500, the highest value on the pollution measurement metric. A few private air quality monitoring platforms even reported an AQI of 999, indicating that the pollution measuring values had maxed out.

A view of Qutub Minar on a smoggy morning in New Delhi on Tuesday, November 10.(Sanchit Khanna/HT PHOTO)

What these numbers don’t indicate is that concentrations of the ultra-fine and hazardous particulate matter (PM 2.5 or PM 10) in Delhi could be much higher than indicated by the maximum index value of 500.

However, experts say the index is not underestimating the crisis by capping the value at 500.

The PM concentrations may be far higher than the criteria for meeting an AQI of 500. But ‘500’ indicates worst air quality with worst health impacts, the experts said.

For example, an AQI of 401 to 500 represents PM 2.5 concentrations of over 250 micrograms per cubic metres and PM 10 concentration of over 430 micrograms per cubic metres on an average.. Since Monday, PM 2.5 and PM 10 concentrations in Delhi have been over 350 micrograms per cubic metres and 550 micrograms per cubic metres, respectively, but the AQI values have remained within or up to 500.

The daily safe limit of PM 2.5 is 60ug/m3 while that of PM 10 is 100 ug/m3.

A Central Pollution Control Board scientist explained, “In our estimate, the AQI cannot deteriorate infinitely. Exceeding the 500 scale indicates the situation is terrible. At 500, maximum damage is done so there is no point going beyond that. It’s a situation similar to the US Environment Protection Agency’s (EPA) hazardous level. Its recommended that people remain indoors but indoor air quality is also impacted.”

“Not much has been studied as to what happens after we reach an AQI of 500. Health impacts plateau after a certain point. There is not much point in monitoring beyond that,” the scientist added.

“An AQI of 500 is exceptionally high. One can always conflate numbers. But it only means that air quality is in a very severe condition,” said Mukesh Sharma, scientist, department of civil engineering, IIT Kanpur, who helped develop the AQI system for India.

The United States EPA considers the AQI as a yardstick that runs from 0 to 500. The higher the AQI value, the greater the level of air pollution and the greater the health concern.

“The kind of air quality you are breathing depends on the concentrations of pollutants such as PM 2.5 or PM 10. The AQI is basically taking pollution levels from different pollutants and grading them in sub-indices and then calculating an index value based on the pollutant, which is of highest concentration at the time…,” said Anumita Roy Chowdhury, executive director, Centre for Science and Environment (CSE).

“The maximum AQI values have been calculated based on the kind of pollution experienced during the London smog of 1952. There is no logic in extending AQI values. We need systemic changes to control air pollution levels in the region,” Roy Chowdhury said.

Severe category air could have respiratory effects even on healthy people and serious health impacts on people with lung/heart diseases. The health impacts may be experienced even during light physical activity, according to Central Pollution Control Board’s AQI report released in 2015.

RoyChowdhury said all action to curb severe air quality should kick well in advance and not when AQI is 500.