In case you haven’t heard, a massive wildfire is currently raging across the drought-parched landscape of Butte County in Northern California.

Despite the misleadingly innocent name of the Camp Fire, the blaze has ravaged 57,500 hectares (142,000 acres) in the 11 days since it ignited outside the small rural town of Paradise, near Chico. Having claimed the lives of at least 77 people (almost 1,000 more are currently reported as missing) and destroyed more than 12,000 structures, the Camp Fire is now the deadliest and most destructive wildfire in state history.

Adding to the fire’s devastating human impact, winds have been blowing the toxic smoke west, directly into the highly populated Sacramento region and Bay Area. At the time of writing, the air quality index in San Francisco and the East Bay ranged between about 200 to 320 – hazardous levels considered to represent a population-wide health emergency. The inland East Bay community of Tracy was measured at 404, second only to areas immediately near Paradise.

Based on National Weather Service guidelines, people living in these communities have been encouraged to stay inside, and many schools and businesses were closed on Friday, when smoke pollution peaked.

The air quality index (AQI) measurement being used by monitoring platforms such as the Environmental Protection Agency (EPA)-run AirNow and the privately operated PurpleAir is based on the concentration of a category of particulate matter called PM2.5. Though smoke may appear gas-like, it is actually composed of aerosolized solid and liquid particles that are the byproducts of the combustion. Depending on what is being burned, the resulting particulate matter will vary in toxicity (i.e. a tire fire is more dangerous than a wood bonfire), however, all smoke is dangerous due to the high proportion of small particles that easily absorb through your lung tissue into your bloodstream and enter individual cells.

As the name hints, the PM2.5 category includes all liquid droplets and solid particles that measure 2.5 micrometers in diameter (about one-thirtieth the width of a human hair) or smaller.

“These microscopic particles can get into your eyes and respiratory system, where they can cause health problems such as burning eyes, runny nose, and illnesses such as bronchitis,” the EPA writes. “Fine particles also can aggravate chronic heart and lung diseases – and even are linked to premature deaths in people with these conditions.”

According to the World Health Organization’s Air Quality Guidelines, the safe limit for PM2.5 concentration in the air is 10 µg/m3 per day averaged across the year, and no more than 25 µg/m3 during one 24-hour period. As reported by SFGate, the PM2.5 concentration in the Bay Area was 225 µg/m3 the day after the Camp Fire ignited, and has likely stayed near or even surpassed that level in many California regions throughout the past week. To put it in a different perspective, Vox cites research that likens smoking one cigarette to an air pollution of 22 μg/m3 for one day; thus, residents of the North Bay and northeast foothills have been inhaling the equivalent of eight cigarettes.

To protect yourself from the health effects of wildfire smoke if you do venture outdoors, the EPA recommends covering your face with an N95 respirator or P100 mask. The filters on these masks block out most particles of 2.5 micrometers or below.