When fires burn in uninhabited wildlands, their corrosive effects can be carried hundreds of miles by the wind, causing stinging eyes, burning throats and severe coughing. Particles in smoke can trigger asthma attacks and heart attacks. Smoke also contains cancer-causing substances and gases.

Local air districts issue warnings to residents to wear masks and avoid outside exercise. Hospital emergency rooms report increased numbers of patients seeking help for respiratory problems, and school closures can keep as many as a million children home as even indoor air quality deteriorates.