On Monday morning, I opened the door to the smell of campfires — the scent of other Californians’ homes going up in smoke. The air stung my eyes and burned my lungs. It felt like I was breathing through a straw. I rounded up my half-empty inhalers, hoping they hadn’t expired. The next day, the sky turned brown and rained ash on me.

California is besieged by 22 wildfires that have incinerated more than 265 square miles and killed at least 21 people. Smoke and ash blanket the Bay Area in a layer of haze responsible for the worst air quality on record. The smoke has prompted local schools to close, and reduced visibility at Bay Area airports so much that flights were delayed, or canceled.

For people who have asthma, chronic obstructive pulmonary disease, or heart conditions, just breathing this noxious air can be dangerous. “If there’s smoke in the atmosphere, it affects the whole body,” says Sarah Henderson, professor of public health at the University of British Columbia. “The basic message is that forest fire smoke is not good for you.”

The data on the specific health effects of wildfire smoke is limited. “These smoke events are unpredictable and episodic,” says public health professor Michael Brauer, also at UBC. “So it’s hard to gather a study population that you know is going to be exposed, for example.”

But there are a few things we know for certain, like that wildfire smoke kills people, for example. It’s also harmful to lungs: people wind up filling more prescriptions for asthma medication and visiting the ER for asthma attacks during wildfires, according to an analysis of 350 individual studies. The review also showed that wildfire smoke generally worsens chronic obstructive pulmonary disorder, bronchitis, and pneumonia.

Even kids who don’t have asthma find it harder to breathe smoke-tainted air, and it can make otherwise healthy people cough up phlegm, a report from the Environmental Protection Agency says. A growing body of evidence suggests the smoke is also dangerous for fetuses: two studies showed that babies have lower birth weights when their mothers are exposed to wildfire smoke or agricultural burning during pregnancy.

It’s harder to pin down the effects of wildfire smoke on heart disease. But studies of an especially nasty ingredient in smoke called particulate matter show that inhaling the stuff is bad for the circulatory system, too. Particulate matter is catch-all term for the a mix of microscopic liquid droplets and solid particles pumped into the air by fires, trash incinerators, car exhaust, and power plants, for example.

The smaller these particles are, the further they can penetrate into the body, and the more damage they can do. Particulate matter less than 2.5 micrometers across — a small fraction of the diameter of a human hair — is called “fine” particulate matter or PM2.5. For vulnerable people like the elderly and people with existing heart disease, just hours of exposure to elevated levels of it can increase risk for heart attack, stroke, and irregular heart beats, the American Heart Association says. All told, particulate matter kills more than 3 million people every year. That accounts for roughly 3 percent of all deaths from heart and lung disease, and 5 percent of deaths from lung cancer, the World Health Organization estimates.

Scientists are still figuring out exactly why particulate matter is so dangerous, Henderson says. One reason might be that the body sees these tiny particles as dangerous invaders, and tries to fight them off. Since the immune system can’t kill smoke, it keeps attacking, increasing inflammation that can worsen chronic heart and lung diseases. It’s also possible that the very smallest particles may cross from the lungs into the bloodstream, although Henderson says that what the particles do once they’re there isn’t well understood.

To be clear, short-term, one-off exposures to wildfire smoke probably won’t make an otherwise healthy person sick, Brauer says. “These shorter-lived events, we don’t think they’re as harmful as the longer-term exposure that you may get from just being exposed day in and day out to things like vehicle exhaust,” Brauer says. “What becomes more of a concern is if you happen to live in a community where you’re getting weeks of exposure every single year.”

That’s already the case in parts of Southeast Asia, Africa, and Latin America, and it could soon be true for the West Coast. “It’s upon us. This is the new normal, and we just need to learn how to respond to it,” Brauer says. “It’s not a happy story.”

Henderson says that the best thing to do is stay indoors if possible, ideally in a place with air filtered through a special HEPA or electrostatic filter. (Ozone air filters can make the air even more irritating, she says.) And if you do go outdoors, those little fabric masks that stretch over your mouth won’t cut it. For real protection, you’d need something called an N95 respirator, but they have to be specially fitted, and aren’t designed for children. Also, don’t exercise outside where you’d be breathing big lungfuls of smoky air, Henderson says. “Smokey days are a great time to take it easy.”

Of course, not everyone has a place to shelter from the smoke, and Brauer says that smoke protection will also need to happen at the community level to keep people safe. “Running air filters indoors, or even at a community level making sure there are clean air shelters, those kinds of things do seem to work,” Brauer says. “It’s really at the point where we just need to accept this, and make sure we’re well prepared for it.”