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Billowing smoke from this year’s historic wildfire season in California has caused hazardous air conditions across the state, prompting air quality alerts and forcing many residents to take refuge indoors to avoid unhealthy exposure to bad air.

Hazy skies and thick, smoky air aren’t just symptoms of the fire — they present their own dangers, even when wildfires themselves remain very far away. Poor air quality can have disastrous effects on people’s health: like coughing, sore throats, extreme wheezing among people with respiratory disease, and cardiovascular illness. Prolonged exposure to bad air can even work its way into your lungs and blood stream.

Conditions in the San Joaquin Valley are particularly bad because of the natural geography and weather patterns of the region, experts say, even though the state’s major fires are elsewhere. As smoke drifts into the valley, mountains and the climate trap the pollution, which then gets pushed toward the ground because of the heat. Currently the Ferguson Fire and the Mendocino Complex Fires are dumping smoke into the region.