Ozone (O3) is a gas composed of three oxygen atoms. Ozone found at ground level is commonly referred to as smog and has serious health impacts. Ozone in the stratosphere is not harmful; it helps to block ultraviolet (UV) rays.

Ground-level ozone is at its highest on hot summer days
when there is little wind. When inhaled, ozone irritates the respiratory system and can cause shortness of breath, wheezing, coughing, and chest pain, as well as exacerbate allergies and respiratory diseases such as asthma. Ozone not only aggravates the respiratory system temporarily; prolonged inhalation of unsafe levels of ozone can reduce lung function and development in children and permanently damage lung tissue (link). Recent studies show that ozone can actually cause asthma in children who are active outdoors in smoggy areas (link). ​Learn how to protect yourself from ozone ​here.​

Ozone is generally not released directly into the air. Rather, it forms through chemical reaction in the presence of heat and sunlight. Pollutants called nitrogen oxides (NOx) and volatile organic compounds (VOCs) are main two chemical ingredients. Learn about sources of Ozone ​here.


Particulate matter smaller than ten microns in diameter, known as PM10 and PM2.5 are about one-seventh the diameter of a human hair. The smaller the particle, the easier it enters the deepest part of the lungs. PM2.5 and ultrafine PM are small enough to enter the bloodstream. Blood transports the pollution throughout the body – to organs, tissues, and cells – and can block blood-flow to heart and brain. Ultrafine PM is so small, it enters and travels within the cells of our body where it can disrupt basic cellular functions.

PM, especially PM2.5, is the deadliest pollutant. Nationwide, air pollution causes between 50,000 and 100,000 premature deaths per year, and particulates account for a majority of these (link). In fact, PM accounts for more deaths than homicides and automobile accidents combined every year(link)​. Non-fatal health impacts of PM include reduced lung function, heart problems, and aggravation of respiratory illnesses, such as asthma, bronchitis, emphysema, chronic obstructive lung disease, and pneumonia. PM can accelerate death from other causes, such as lung cancer ​(link)​, and exposure to the small particles, even for short periods of time, can cause heart damage and trigger heart attacks and strokes (link). ​Learn how to protect yourself from PM ​here.​

Learn about sources of PM​ ​here.


Toxic Air Contaminants (TACs) are toxic gasses that escape from industrial processes, like oil and gas production and refining, the application of pesticides, and chrome plating and metal manufacturing. Depending on the toxic, the amount produced, and the proximity of the emissions to sensitive receptors, there are varying health effects.
TACs do not travel far, and thus are usually restricted to localized areas. Communities near large, confined animal feedlots are often exposed to ammonia (link), communities near oil and gas operations are exposed to formaldehyde and benzene (link), communities near major roadways and highways or near distribution centers and high truck traffic are exposed to

toxic diesel PM (link) and communities near land designated as heavy-industrial could be exposed to a range of toxics depending on the facilities located there. Toxics can have cancer and non-cancer effects. For instance, ammonia from dairies and fertilizers aggregates different human systems but is not considered a carcinogen. However benzene and formaldehyde from oil and gas operations are known carcinogens, as is Diesel PM.
See a map of the most toxic census tracts in California here (link). See a map of major toxic facilities here (link).