“It is completely unsafe to be here at this moment,” said Jennifer Franco, a resident of Fairfield, California, on Wednesday afternoon, as massive wildfires ripped through Santa Rosa and Napa a few miles west. But she wasn’t talking about the flames—she was talking about the smoke. Accelerated by high-speed seasonal winds, ash-laden air was blowing eastward, directly into her neighborhood. “Since Tuesday morning, air quality is beyond terrible,” she said. “I’ve been having chest pain, and now I’m using a respirator.”

In Sebastopol, just west of Santa Rosa, winds were blowing in a more favorable direction. But retired social worker Vaughn Whalen said gray haze still obscured the blue sky there, making the sun look eerie, dull, and orange. “I’m a tougher old fella,” he said, when asked about how he was dealing with the smoke. “But a friend of mine who lives nearby has asthma. She was telling me that the smoke is in the house. Her eyes are burning. Her chest hurts. She has difficulty breathing.”

The most immediate threat from the 22 devastating wildfires currently roaring through California are the immediate fire zones. At least 21 people have died there; more than 600 people have gone missing; and thousands of buildings have been destroyed. But beyond the fire zones, millions of Californians are facing a secondary, more insidious threat: polluted air, rife with tiny particles small enough to penetrate deep into the circulatory system. Those potentially deadly particles are creating unhealthy air as far as 70 miles away from fire zones, according to Bay Area Air Quality Management District spokesman Tom Flannigan. But people closer to the fire zones are even more at risk, since the air in those regions could also be tinged with toxic heavy metals like arsenic, cadmium, copper and lead, as the smoke picks up chemicals from burned-up plastic, cars, and building materials.

“These are unprecedented conditions,” Flannigan said, estimating that four to five million people living in the Bay Area are breathing toxic air outdoors. “We’ve measured some of the highest air pollution ratings in the Bay Area ever recorded.” For context, Flannigan pointed to Beijing, China, perhaps the poster city for air pollution. “When they measure in Beijing on their worst days, they’re around 500 on the Air Quality Index,” he said. “And those are the kind of readings we’ve been seeing here.”

The particles within this smoke pose the biggest short-term risk to human health. It’s been extensively proven that high-dose exposure to so-called “fine particulate” pollution, or PM2.5, can trigger death, particularly in people with pre-existing conditions like asthma or heart disease. And breathing in smoke can make anyone—even healthy people—experience chest pain, dizziness, or shortness of breath. In California, those effects are already turning up: At least 20 people from outside immediate fire zones had visited UC San Francisco’s hospital facilities due to symptoms from smoke inhalation as of Tuesday evening, according to UCSF spokesperson Elizabeth Fernandez.