As deadly fires burn in Northern and Southern California, destroying communities and lives, millions of people outside the burn zones are exposed to the dangerous smoke that’s billowing into their communities and settling like a dark shroud.

Wildfire smoke is dangerous because it contains fine particulates that can lodge deep in the lungs, which can cause or worsen respiratory problems, such as asthma. Some groups are especially vulnerable, including children and older people.

The particulate matter floating across much of Northern California in the past week has registered more than 18 times the levels recommended by the World Health Organization. Such levels can trigger acute symptoms like difficulty breathing and headaches — even in otherwise healthy people.

Health experts agree that the best defense is to stay indoors.

But many also suggest that if you must go outside for a prolonged period, it’s best to wear the right kind of mask — especially if you have a health condition such as asthma, heart problems or emphysema.

For more, read Samantha Young and Ana B. Ibarra’s coverage.