Delhi and other Indian cities should replace the one-size-fits-all air quality index (AQI) with a city-specific health index focused on premature death risks, a new study recommends.

NEW DELHI – Almost three-quarters of the world’s top 100 most-polluted cities are in India, but does its widely used air quality index (AQI) adequately warn people of short-term health risks, especially death?

It is an established scientific fact that air pollution can lead to premature death, particularly amongst vulnerable groups, like the elderly and ill, in the short and long term; it is the second highest risk factor for non-communicable diseases (NCDs). 

But the AQI, which is meant to effectively communicate the short-term health risks, doesn’t reflect this. Instead, the current warnings are relatively benign, ranging from “minor breathing discomfort to sensitive people” when pollution is low, to “affects healthy people and seriously impacts those with existing diseases” for severe pollution. 

A new study proposes adding the risk of death, which could make the AQI a lot more compelling, and calling it the Air Quality Health Index (AQHI). 

Source: Dr Santu Ghosh, St Johns Medical College, Bangalore.

When air quality is categorised as ‘moderate’, the AQHI has a risk of 2% excess mortality in a day due to air pollution. This moves to 6% for ‘poor’, 9% for ‘very poor’ and jumps to 16% excess mortality per day when the air pollution is poor. 

“That is, in a day if the average mortality in Delhi is 250, in a ‘severe’ AQHI day, 40 additional deaths could be added on that day attributed to air pollution,” Dr Santu Ghosh, one of the authors and a biostatistics professor at Bangalore’s St John’s Medical College, explained to Health Policy Watch. 

The other authors are Franciosalgeo George from St John’s National Academy of Health Sciences, Pallavi Joshi and Sagnik Dey from the Indian Institute of Technology in Delhi, and RK Mall from Banaras Hindu University in Varanasi. 

The study, A Framework For City-Specific Air Quality Health Index, builds on earlier research, which shows how every small increase in pollution, 10 micrograms in fine particulate matter (PM2.5), increases the risk of death by a fraction of a percentage point.

As the table shows, these fractions can add up devastatingly fast, especially in places in northern India with a chronic air pollution crisis. 

Air Quality Health Index explained

The table is for Delhi so it uses an AQHI classification exclusively for Delhi. This is a key difference between AQHI and AQI. To be effective, AQHI should be tailored for each locality to “accurately communicate” health risks posed by air pollution, the researchers argue. 

In contrast, the national AQI, launched in 2015, has a single index for the entire country. But India is demographically and epidemologically diverse, and has different climate zones so there cannot be a one-size-fits-all AQI, according to the authors.

The AQHI is based on a different framework for each city or locality. For the study, mortality and air pollution data from two cities were analysed – Delhi, a megacity, and Varanasi, a small city. 

Both are politically relevant. Delhi is India’s capital and often ranked as the world’s most polluted capital. Varanasi is Prime Minister Narendra Modi’s constituency. 

From a communication point of view, the major change is that the AQI uses numbers from 0-500, while the AQHI uses a range of 0-100. 

The AQHI authors say that the AQI’s thresholds are not supported by any health studies, and a 2015 government document on AQI acknowledges this, asserting that “in view of no specific studies in India”, the US Environmntal Protection Agency’s (EPA) health advisories, or breakpoints, can be adopted. 

Indices can vary greatly, unlike measurements of pollutants. An AQI transforms complex air quality data of various pollutants into a single number, that is, the index value. 

Using a stock market analogy, a pollutant is like a stock; it is measured by its concentration, which largely remains the same if similar instruments are used to measure it in the same area, like one stock’s price is roughly the same in different stock markets. 

But an air quality index depends on the formula it uses, like different stock market indices for the same country. 

For those who track AQI regularly, the table below shows how the older one compares to the proposed health index. If the concentration of PM2.5, a fine, toxic particulate matter, is 120, then the AQI would be 300, but AQHI would be 46 in Delhi and 64 in Varanasi. 

India’s air pollution: AQHI vs AQI 

Source: Report authors

As the authors only focused on mortality due to exposure to high pollution, the report does not list health warnings or breakpoints for asthma, cardio-respiratory illness and other ailments for each category. 

However, despite the differences, the authors say they don’t want the AQI to be abandoned, but updated.

 “Policy makers need to understand that air pollution is a risk factor. It is not a disease,” says Dey. If a person dies of high blood pressure, a stroke, heart failure or something else, especially when air pollution is high, then investigate the link with pollutants; look at existing data and gather more data at a country-wide level. 

Air pollution deaths: Government pushback

The Indian government has consistently maintained in Parliament that there’s “no conclusive data available in the country” to establish that death or disease is exclusively due to air pollution. 

Dr Dey, one of the authors, told Health Policy Watch that he can understand why they are hesitant, using the analogy of smoking: “[If] I can put the onus on the individual: ‘why did you smoke?’, so my responsibility goes down. But for air pollution… we can reduce our own footprint, but that may be a tiny fraction. Ultimately, the government has to step in.”

Developing an AQHI for hundreds of Indian cities should not take more than a few months, Dey says, and there is “hardly any cost”. All that is required is city-specific data, which is then put through the framework code that already exists to calculate the city-specific AQHI cut-offs. 

But the challenge is data, which is very difficult to access, according to Dey.  The mortality data is available with the health ministry and the National Centre for Disease Control (NCDC). 

The 11-year-old AQI has in recent years made it to common parlance, even as fodder for stand-up comedians. But there is little evidence that people follow the few, brief warnings it provides. With the risk of death added, the air quality health index may be taken more seriously by the public and government. 

Image Credits: Raunaq Chopra/ Climate Outreach.