Quick Take: Why is Air Pollution Worse in the Winter?

This winter, thick and toxic smog blanketed the Indian capital of New Delhi for days at a time, obliterating sunlight and making it nearly impossible to breathe. Such episodes are nothing new in the city, which has some of the worst air pollution in the world. But the most extreme events come when temperatures fall.

That’s a global phenomenon, explains Jim Zhang, Ph.D., a professor of global and environmental health at the Duke Global Health Institute and the Nicholas School of the Environment. In this Quick Take video. Zhang says in winter, air near the ground can be much colder than the air above, a condition known as temperature inversion, which can keep harmful pollutants from dispersing into higher levels of the atmosphere. 

“With that roof lower, you have less volume to dilute pollutants and less space for them to disperse so they’re condense,” he says.

Zhang also notes that people tend to use more energy in colder months, leading to increased emissions from the burning of fossil fuels.

Air pollution contributes to noncommunicable diseases such as lung cancer and chronic pulmonary disease. “It’s a lot of things coming together and reacting that can affect the respiratory and cardiovascular system,” says Zhang, whose research focuses on the health and climate benefits of interventions that improve air quality. “It can make blood vessels less flexible and blood more likely to coagulate, and those are risk factors for stroke and heart attack.”

Zhang says one step people can take to protect themselves from harmful pollution is using air purifiers when they are indoors. He is currently overssing studies in Los Angeles to evaluate the effectiveness of placing air purifiers in the homes of elderly residents and people with heart disease who live in areas with poor air quality. 

“Because people spend more time in buildings, offices and at home, I’ve wanted to better understand how indoor air quality affects people’s health and whether using air purifiers can improve people’s health,” he says. 

Zhang visited India in 2017 as part of a workshop to address the country’s air pollution problems. He says some of the participants developed a cough and fever after breathing in smog over several days. “The sky was black,” he recalls. “You couldn’t see very far, not even 30 feet away.”

But Zhang says India is far from alone in struggling to deal with air pollution. Governments at all levels need to enact stronger policies to protect air quality and ensure people can breate healthy air, he says. 

“We know a lot more now about repeated, long-term exposure to air pollution,” he says. “We know how bad it is, we know what to do, but where is the action? Will people change their behaviors to make a difference?”