High Temperatures Combined With Pollution Caused 1.5 Times More Deaths In 10 Cities, Shows Study

Among 3.6 million deaths in these cities with daily mean PM2.5 levels between 20 to 100 μg/m3, researchers found that air pollution had a stronger impact on the death rate, especially during days with higher temperatures.

PM2.5 LEVELS WAY BEYOND SAFE LIMITS

At moderately high temperature, which could vary among the cities depending on their geographical location, a 10 µg/m³ increase in PM2.5 levels was linked to a 0.8 per cent increase in heat-related mortality. But association of PM2.5 on daily mortality increased, especially at extreme temperatures with the death rate surged up to 4.6 per cent.

The death risk increased steadily from 8.3 per cent at 20 µg/m³ of PM2.5 to 63.9 per cent at 100 µg/m³ of PM2.5 exposure level with consequent increase in extreme ambient temperature. “So, the risk of mortality is high on days when daily PM2.5 concentration and temperatures is extreme, and such trend was specifically noted in Ahmedabad, Chennai, Delhi and Kolkata," said co-author Tirthankar Banerjee from BHU, Varanasi.

The researchers also pointed out that air pollution’s impact is generally underplayed in summers due to good visibility, but the pollution persists. “In cities like Delhi, the PM2.5 level even during summer is way beyond the safe limits prescribed by the World Health Organization (WHO). The annual daily PM2.5 level has remained typically high, and now there is increasing heat stress too."

The study found a linear association of temperature with daily mortality as air pollution levels increased. The analysis is based on city-wide PM2.5 data available with Centre for Chronic Disease Control, New Delhi, fed into two models which estimated daily exposure levels and temperatures.

TOXIC AIR & HEAT STRESS: LETHAL MIX

It is not just the degree of air pollution, but also the composition and toxicity of PM2.5 which changes during the hottest months, sometimes favouring formation of other pollutants like Ozone and secondary particles through chemical reactions. While cities like Delhi may have more carcinogenic Black Carbon, others like Varanasi may have more dust.

Studies in the past have also shown that extreme heat stress can reduce the ability of the human body to detoxify air pollutants and increase vulnerability due to higher breathing rate, which leads to greater intake and distribution of air pollutants. Together, the exposures can increase oxidative stress, inflammatory response, cellular damage—all factors associated with high death risk.

The latest findings published in the peer-reviewed journal Environment International called upon policymakers to consider the potential combined impact of air pollution and heat stress while preparing climate change adaptation plans. “We urgently need to reduce air pollution as it will also help combat climate change by decreasing carbon emissions, which drive global warming, and lead to rising temperatures worldwide," the study concluded.