San Francisco Bay Area has some of the worst air in the US, according to a 2019 study from American Lung Association. The frequency and intensity of wildfires in Northern California have become highly unusual and partially to blame.
From April to October, warmer temperatures and longer days can cause ozone to build up to unhealthy levels.
Exposure to air with excess ozone can make it difficult or even painful to breathe, inflame airways, and aggravate lung diseases such as asthma, emphysema, and chronic bronchitis. [Source: Curbed SF]