Public health experts are urging people across northern India to check air quality readings, wear well-fitting masks and run air purifiers as Diwali begins Sunday. Fireworks traditionally set off during the holiday, which an estimated 1 billion people celebrate worldwide, can make polluted air more dangerous, the experts said.

Diwali, a holiday of Hindus and Jains with ties to ancient epics such as the Ramayana, arrives each year as hazardous winter air returns to the region. The problem is especially severe in Delhi, where fine particle pollution averaged about 19 times the World Health Organization’s annual guideline last year. Monitors in the city of more than 20 million people have reached the maximum reading they can display multiple times in recent years. Authorities have responded with temporary restrictions on construction, industry and vehicles — measures public health experts said often fail to address the root causes — while carrying out a long-term plan to promote electric vehicles.

Fireworks add a sharp burst of pollution to a much bigger problem. Across India, Pakistan and other parts of South Asia, traffic, factories, construction dust and the burning of crop residues and waste foul the air, and weak winds and stagnant winter conditions allow the pollution to build up. “Poor air quality is our all-year-round problem in our country,” said Shweta Narayan, a public health expert at the Global Climate and Health Alliance. Narayan said firecrackers add to the problem.

The pattern extends beyond South Asia. Winter also brings severe smog to parts of Pakistan and China, where warmer air can act as a lid, trapping colder air and pollution near the ground. Burning coal and wood to heat homes adds to winter pollution in eastern Europe, while summer wildfires send smoke across vast areas of the United States and Canada.

India’s air quality index turns pollution measurements into health warnings. Readings of 201 to 300 mean “poor,” 301 to 400 “very poor” and 401 to 500 “severe” — levels that can harm even healthy people. Experts advised checking pollution levels on government or privately run websites before heading outdoors, looking for data from a nearby monitoring station and noting when it was last updated, since a citywide average can conceal a polluted neighborhood.

Dr. Arvind Kumar, a chest surgeon at Medanta hospital near Delhi and founder trustee of the Lung Care Foundation, recommended properly fitted N95 or N99 masks outdoors during periods of heavy pollution. Such masks are available at pharmacies, hardware stores and online retailers. N95 filters are designed to capture at least 95% of airborne particles, and N99 filters at least 99%, though the respirator masks do not filter harmful gases. Loose surgical masks, cloth masks and scarves offer unreliable protection against fine particles.

Kumar said a mask should cover the nose, mouth and chin, with the nose clip shaped and the seal checked every time; facial hair can break the seal. The aim, he said, is to ensure “all the breathing is through the mask and not around the mask.” Disposable masks should be replaced when they become dirty, damaged or hard to breathe through, masks should never be placed on children younger than 2, and people with heart or lung disease should seek medical advice about mask use.

Kumar said pollution can cause persistent coughing, inflame the airways and trigger asthma-like attacks. He cited increased risks of pneumonia, heart attacks and strokes while linking prolonged exposure to impaired lung development in children and lung cancer. He advised against strenuous outdoor exercise during heavy pollution, including early in the morning, when levels can be high.

Indoors, the experts pointed to air purifiers matched to the space they serve. The devices draw in air, filter out pollutants and recirculate the cleaner air. A purifier needs enough capacity for the room where it runs, should contain a high-efficiency particulate air, or HEPA, filter, and should be judged by its clean air delivery rate, the volume of filtered air it supplies. Experts advised placing the unit where people spend the most time, such as the bedroom, keeping its airflow clear and running it consistently.

HEPA filters trap particles, and substantial activated-carbon filters can remove some gases, but no purifier removes every pollutant. Machines that deliberately produce ozone should be avoided because ozone irritates the lungs. Cleaner indoor air reduces exposure, Kumar said, but a purifier cannot eliminate pollution’s health risks.

Kumar also advised avoiding incense, mosquito coils and candles, whose particles can linger indoors for hours, especially in poorly ventilated rooms. Fireworks pollution can persist overnight, depending on the weather, and when many households set off fireworks their combined emissions can worsen air quality across neighborhoods and entire cities, exposing people who are not taking part. Electric lights can be another way to celebrate the holiday, avoiding the toxic gases and smoke from firecrackers. “If one wants to celebrate and really enjoy the festival of Diwali, avoid bursting firecrackers,” Narayan said.

India’s health ministry recommends reducing smoky cooking and using a kitchen exhaust that vents outdoors. Damp-mopping floors and wiping dusty surfaces, rather than dry sweeping, can avoid stirring up particles that worsen air quality.

Narayan said the precautions have limits. People who earn their livelihoods outdoors cannot simply stay home, and many families cannot afford air purifiers. She called for sustained action on pollution sources and affordable health care for people already suffering. Masks and filters can reduce exposure, she said, but lasting protection requires governments to clean the air.

“It’s our responsibility to hold our governments accountable,” she said. “There’s no other way about it.”