India has among the highest levels of air pollution in the world, with most people living in areas with fine particulate matter levels (PM2.5) well above the safe limit by the World Health Organization (WHO).

The findings of the Million Death Study (MDS), published in the journal Environmental Health Perspectives, linked concentrations of PM2.5 derived from satellite-based measurements and deaths in over 7,400 small areas covering a total population of 6.8 million.

The researchers quantified the relationship between PM2.5 exposures over several years to the subsequent mortality risk, focusing on the leading killers of adults: respiratory disease, heart disease, stroke and overall mortality.

Their analyses adjusted for risk factors that make these diseases more common such as smoking, urban or rural residency, and education. Because these diseases and PM2.5 tend to cluster in some areas but not others, they also adjusted for spatial clustering.

Analysing over 200,000 deaths at ages 15–69 years, the researchers found a nine per cent excess risk in stroke deaths for every 10 microgrammes per cubic metre increase in PM2.5 exposure. However, there were no excess risks for respiratory or heart disease and for total mortality (after excluding stroke), they said.

Study results were broadly similar in different age-groups and in areas with higher or lower use of household solid fuel -- which itself is the major contributor to PM2.5 exposure as measured by satellites. In contrast to these direct observations, earlier estimates of air pollution deaths in India used complex models that assumed a much higher mortality risks from PM2.5 exposure, the researchers said.

"Measuring the health effects of air pollution is complicated,' said study lead author Professor Patrick Brown, from the University of Toronto in Canada.?"There are so many things to tease out to establish if a relationship with deaths is true, or just noise in the data. Even the modest excess risks we found for stroke might reflect the uncertainty that is inherent in these types of epidemiological studies,” Brown said.

The researchers noted that direct studies like the Million Death Study (MDS) are far preferable to extrapolating from models, many of which take reports from non-Indian settings and apply them to India. Earlier models have probably overstated the extent to which air pollution kills adults in India, they said.

"Most existing studies on air quality and health are done in high-income countries, where PM2.5 exposures are far lower and disease patterns are substantially different than in low and middle-income countries such as India," said study ?co-author Professor George D'Souza, Dean of St. John's Medical College, Bengaluru, India.

"That needs to change,” D'Souza said.?The results are one of many insights being generated by MDS – one of the largest studies of premature mortality in the world. The study, covering deaths from 2001 to 2014, is a long-running effort to better understand the causes of mortality in countries such as India, where the vast majority of people die at home without a death certificate.

Launched by Professor Prabhat Jha at Dalla Lana School of Public Health, The University of Toronto, it has helped governments and health providers to better allocate scarce public health funding.?Despite the good news, the researchers said there are many reasons to act on air pollution in India.

“PM2.5 may not be the big killer in India as has been touted, but action to reduce PM2.5 exposure is still necessary,” said Jha. "Air pollution worsens child lung health and lowers quality of life, and that is sufficient justification to act," he added.

(With PTI inputs)