NEW DELHI • As winter sets in, the air hanging over northern India thickens with smoke from burning fields, blocking out the sun and making it hard to breathe. This year, that could spell disaster for the country's battle against the coronavirus pandemic.
With some of the most polluted cities on the planet and more than eight million confirmed Covid-19 cases, India is battling a medical calamity.
Temperatures are falling and coronavirus cases and air pollution are spiking across major cities, including the capital New Delhi, one of the worst-affected areas for smog from rural burning.
Delhi's 28 million people have not had a single day of clean air since early September, based on the World Air Quality Index.
In the past two weeks, readings for particulate matter of 2.5 microns or smaller, or PM2.5, have risen to more than 400 in some parts of Delhi - eight times the safe level.
PM2.5 is closely watched because the particles are small enough to journey deep into the lungs and do the most damage.
"Covid-19 and air pollution are a dangerous combination," said Professor K. Srinath Reddy, president of the Public Health Foundation of India. Air pollution causes damage and inflammation in the lungs and other body tissues, reducing the body's ability to resist the virus, which may result in more severe forms of the infection.
With pollution and the virus both attacking lungs, India's creaking healthcare system may come under a double strain of patients competing for the same resources. A rise in respiratory illnesses due to air pollution will send more people to hospitals with symptoms similar to Covid-19, making identifying and isolating the virus cases difficult, said Delhi-based pulmonologist Pankaj Sayal.
While official data showed the country has seen a peak in infection numbers, a government panel of scientists attributed the recent slowdown to a harsh national lockdown earlier this year. They warned that the upcoming festival and winter seasons may raise the susceptibility to infection.
The World Health Organisation estimates that dirty air kills seven million people a year globally, with even short-term exposure causing or aggravating diseases including reduced lung function, asthma and, most importantly now, respiratory infections.
"Now if I get Covid-19, I have less of a margin," said Delhi-based advocate Meet Malhotra, 58, who developed asthma as a child. "If your oxygen level goes under 90, you need hospitalisation. What happens if you're already at 92-93 because of air pollution?"
Throughout the year, vehicle and industrial emissions contribute to India's poor air, combining with dust from road construction and millions of domestic fires lit by poor citizens for cooking.
But an extra wallop comes late in the year as farmers burn hundreds of thousands of hectares of crop stubble. The practice has been banned in some places but it is still the cheapest and fastest way to clear the ground for the new crop.
Bounded by mountains and hills, northern India's cold winter air traps the smoke for weeks.
That could make it easier for the coronavirus to be transmitted, said Professor Arun Sharma at the University College of Medical Sciences in Delhi. The risk of transmission of infection will be higher as more particles in the air give the virus more ways to travel from an infected person, he said.
An Environment Ministry spokesman said the government had introduced a new law to establish a panel to tackle poor air quality.
Officials from the Central Pollution Control Board had been divided into 50 teams to curb pollution, while a pilot project was under way to decompose rather than burn residue after the harvest.
Evidence has been mounting since March that air pollution worsens the effects of the coronavirus, including a Harvard study showing that Covid-19 death rates are higher in polluted areas, and studies linking mortality and pollutants in China and Europe.
Even for those who are trying to overcome the virus, the air quality can slow or even prevent the chance of a full recovery in a small number of cases.
Lung functions in some people who were in intensive care are not returning to normal, said Dr Arvind Kumar, who started the Delhi-based Lung Care Foundation. Some patients continue to need oxygen. Others develop problems such as fibrosis and breathlessness.