Explained: Why Delhi's AQI Was 494 Today But International Monitor Said 1,600

Delhi's air quality remained severely toxic on Tuesday, with the overall air quality index (AQI), which measures pollutants, spiked to 494. According to the Central Pollution of Control Board (CPCB) data, most of the monitoring stations in the national capital recorded an AQI of 500, forcing all schools across Delhi-NCR to go online. However, at the same time, the international monitoring app, IQAir, showed Delhi's AQI at 1,600.

Why Delhi's AQI Was 494 but IQAir showed 1,600?

All countries have different standards for measuring the air quality index. It varies from country to country, depending on pollutants and their measuring scale. In India, the scale of PM 2.5 is 60, while in some countries, that have adopted the World Health Organisation (WHO) standard, is five or ten.

Similarly, India's AQI has been capped at 500 - which means that an air quality index bigger than that is a big warning that the level of pollution is extremely severe.

According to the CPCB, an AQI between 0 and 50 is considered good, 51 and 100 satisfactory, 101 and 200 moderate, 201 and 300 poor, 301 and 400 very poor, 401 and 450 severe, and above 450 to 500 is severe-plus.

On the other hand, IQAir - which is based on the US model developed by the country's Environment Protection Agency, has 'hazardous' as the highest level of scale (over 500). 

How do international agencies monitor India's AQI?

International agencies such as IQAir have installed sensors at some places across Delhi-NCR. 

However, there is no clear information on whether the sensors have been installed at the right place with the standard equipment.

Which monitor's AQI should we follow?

While there is no particular right or wrong monitor, in India, citizens should consider the AQI released by the CPCB - which has 40 stations across Delhi-NCR for assessing pollution. 

Delhi's air remains toxic, GRAP 4 imposed 

As the air quality in Delhi-NCR entered the 'severe plus' category on Monday, the Delhi government imposed Stage 4 of the Graded Response Action Plan (GRAP), an anti-pollution plan - in addition to the first three stages.

Under this, a ban has been imposed on Delhi-registered BS-IV or older diesel medium and heavy goods vehicles. All trucks and light commercial vehicles registered outside Delhi will not be allowed to enter the national capital, except for those carrying essential commodities or providing essential services.

Additionally, public and private offices have been asked to work at 50 per cent capacity and the rest have been recommended to work from home. All schools have been either closed or gone online.