Dr. Praveen Buddiga knew he would find a packed waiting room when he arrived at his office that warm September day in California’s Central Valley. White flakes drifted from the sky, as if he were inside a snow globe.

The Rough Fire, a 152,000-acre blaze sparked by lightning in the Sequoia National Forest, was lofting thick smoke, soot, and ash into the air—and into the lungs of Buddiga’s patients 35 miles away, in Fresno.

As an allergist, Buddiga knows that wildfires pose a serious, sometimes lethal, threat to people’s health, particularly for those with asthma or heart disease.

“Older [patients] made the universal choking sign—you know, hands around the throat,” Buddiga says. “Younger ones just pointed to their chests. The Rough Fire was devastating for us.”

Around the world, billions of people are finding that the air carries a dangerous dose of smoke as wildfires become bigger and more intense.

“We see these trends, and the emissions from wildfire smoke have tremendous public health implications,” says Dr. Wayne Cascio, head of environmental public health at the Environmental Protection Agency’s National Health and Environmental Effects Research Laboratory in North Carolina.

View Images The city of Palangkaraya in Indonesia is blanketed with yellow, acrid smoke from peat fires. The government is expected to call a national emergency as the fires and smoke spread. Photograph by Bay Ismoyo, AFP, Getty Images

Worldwide, wildfire smoke kills 339,000 people a year, mostly in Asia and sub-Saharan Africa, researchers estimate. In addition, other studies document up to a tenfold increase in asthma attacks, emergency-room visits, and hospital admissions when smoke blankets places where people live.

This week, much of Southeast Asia is blanketed with thick smoke from peat fires in Indonesia that are expected to smolder for months. An estimated 40 million Indonesians in five provinces are breathing the soot, and the government is expected to declare a national emergency.

When peat, a marshy material, burns, it spews “far more smoke and air pollution than most other types of fires,” according to NASA, which warns that the Asian fires are likely to worsen and spread vast distances.

Satellite images confirm that smoke can cross mountain ranges, continents, and oceans. Fires 250 miles away have triggered 911 calls in Albuquerque. Quebec blazes have sent plumes 800 miles to make New Yorkers wheeze.

The eyes of Texans have been reddened by Honduran farmers’ burning 1,500 miles to the south. And North American smoke has traveled 5,000 miles to twitch nostrils in Eastern Europe. Some, such as Indonesia’s fires, have plumes of smoke so huge they obliterate entire countries in photographs shot from outer space.

Smoke’s Hidden Dangers

Fires can smolder for months, and layers of stagnant air known as inversions, common in the western U.S., can hold smoke down where people breathe. On some days, the health threat can far exceed the air pollutant levels that federal standards allow.

View Images A NASA satellite image shows smoke from Indonesia’s fires. Peat fires burn there frequently because farmers engage in “slash and burn agriculture,” which involves burning of rainforest to clear land for crops and livestock. Photograph by NASA Earth Observatory

Smoke’s biggest threat comes from the airborne, microscopic particles that slip past the body’s defenses and reach the alveoli, the farthest ends of the respiratory system. Some of these particles enter the blood and form a thick goo. Smoke also contains carbon monoxide, which can cause long-lasting damage to the heart, and chemicals such as benzene and formaldehyde, known to cause cancer in humans.

“Air pollution generically, and particles, specifically, are carcinogens,” says Dr. Jonathan Samet, a pulmonary physician and researcher at the University of Southern California. “That would also be the case with smoke from wildfires, from the burning of vegetative matter.”

New evidence is emerging that the heart is vulnerable to damage inflicted by smoke.

EPA researchers found that emergency-room visits for heart failure jumped 37 percent following the smokiest days of a big 2008 peat fire in eastern North Carolina. ER trips for breathing problems rose by 66 percent. Poor people faced the most risk, even with access to medical care, EPA statistician Ana Rappold says.

And in 2015, Dr. Anjali Haikerwal of Australia’s Monash University reported a 7 percent increase in out-of-hospital cases of cardiac arrest when bushfire smoke blanketed greater Melbourne in 2006-07. “Now we have very, very strong evidence,” she says.

Babies in the womb also are at risk. During Southern California’s 2003 fires, babies weighed on average 0.2 ounces less at birth than those born just before the fires, a 2012 study found. Those exposed in the second trimester had the biggest deficit, just over one-third ounce. The differences might not matter much in the long run, but they add to evidence that smoke affects health.

View Images A cyclist rides beneath a smoke-filled sky in Middletown, Calif., in September. Health experts advice people not to exercise on smoky days because the soot and gases can trigger asthma attacks and heart problems. Photograph by Stephen Lam, Getty

Coping With Bigger Fires

Reasons for the trend toward bigger and more damaging fires vary, but the shifting relationship between people and the planet is a primary culprit. People are moving into cities’ wilder, fire-prone edges, particularly in California, Texas, and Florida. Especially in the West, decades-long suppression of blazes has set the conditions for today’s raging fires. Globally, while fire is an age-old farming tool, burning to clear cropland has reached an industrial scale; in East Asia, forests are being burned down to transform them into palm-oil plantations.

Climate change also is worsening exposure to wildfire smoke. In 2011, the National Research Council estimated that for each 1.8 degree F. (1 degree C) rise in global temperature, the number of acres burned in the western U.S. could increase by 200 to 400 percent.

One-fourth of the Earth’s vegetated surface is seeing much longer fire seasons, according to U.S. Forest Service scientists. “If these fire weather changes are coupled with ignition sources and available fuel,” the researchers wrote, “they could markedly impact global ecosystems, societies, economies and climate.”

So what can people and society do? Unfortunately, choices are limited -- “you cannot block smoke with some enormous filter in the sky,” says Klaus Moeltner, a Virginia Tech economist who has studied wildfire costs. Nevertheless, limiting smoke exposure could become a goal of firefighting strategies. “Upwind of a big populated area might be a good place to start,” Moeltner says.

Also, setting small, controlled blazes would eliminate the vegetation that fuels huge fires.

“The challenge is in front of us,” says Pete Lahm, smoke manager at the U.S. Forest Service. “We know that these ecosystems will burn.”

But that approach presents problems. Many state and local air-quality rules limit when prescribed fires are allowed, and fire guidelines generally forbid burning when smoke might reach neighborhoods.

For those in the smoke, experts advise people to stay inside with windows shut and air conditioners and air filters on. And don’t work out when the world seems like a giant campfire.