Ozone is a pollutant well known for producing foliar damage in ozone- sensitive ponderosa and Jeffrey pines. Destroying the photosynthetic cells of these pines compromises their health and ability to resist other forest stressors such as insects, pathogens, and drought.

Ozone is also well understood as a health hazard to humans. As such, it is a criteria pollutant, which is regulated by state and federal agencies. There are measured levels that reflect good, moderate, or unhealthy stages. These levels determine an advisory for people to consider when doing any outdoor activities. Currently, Yosemite National Park exceeds the state and federal 8-hour standard (unhealthy) approximately 5 times per summer, but even at the moderate stage, unusually sensitive people are advised to restrict their outdoor activity. Given the increased growth and development in the adjacent San Joaquin Valley, it is suspected that the number of exceedance days will increase.