Pollution in California relates to the degree of pollution in the air, water, and land of the U.S. state of California. Pollution is defined as the addition of any substance (solid, liquid, or gas) or any form of energy (such as heat, sound, or radioactivity) to the environment at a faster rate than it can be dispersed, diluted, decomposed, recycled, or stored in some harmless form. The combination of three main factors is the cause of notable unhealthy levels of air pollution in California: the activities of over 39 million people, a mountainous terrain that traps pollution, and a warm climate that helps form ozone and other pollutants. Eight of the ten cities in the US with the highest year-round concentration of particulate matter between 2013 and 2015 were in California, and seven out of the ten cities in the US with the worst ozone pollution were also in California. Studies show that pollutants prevalent in California are linked to several health issues, including asthma, lung cancer, birth complications, and premature death. In 2016, Bakersfield, California recorded the highest level of airborne pollutants of any city in the United States.

The Federal Clean Water Act defines water pollution as "dredge spoil, solid waste, incinerator residue, sewage, garbage, sewage sludge, munitions, chemical wastes, biological materials, heat, wrecked or discarded equipment, rock, sand, cellar dirt, and industrial, municipal, and agricultural waste discharged into water." In 2011, an Environmental Protection Agency (EPA) study showed that water quality standards were not met on 1.6 million acres of California's 3 million acres of lakes, bays, wetlands, and estuaries. The Porter-Cologne Water Quality Control Act governs the water quality regulation in California.

There is also an effect on agricultural sector of extreme weather, sea level rise, and wildfires. After the 2024 election there was a change of government interaction with global climate policies. Now in 2025 president Donald Trump withdrew the United States from the Paris Agreement. With Clean Air Act (CAA) there is a limit of certain containment pollutions in efforts to help clean the air. This limits many industrial and chemical plants in the amount of releasing chemical pollutants.

In 1943, people first recognized episodes of smog-causing irritated eyes, burning lungs, and nausea, and this led people to walk the city's streets wearing masks to shield the thick air. Beginning in 1967, a group of California politicians and leaders teamed up to unify statewide efforts to address the severe air pollution, creating The Mulford-Carrell Air Resources Act, which formed the California Air Resources Board (CARB). That same year, the Federal Air Quality Act of 1967 was enacted, which allowed California to set more stringent air quality rules due to its unique conditions of geography, weather, and growing population. Despite dramatic progress, air pollution in the United States, and California in particular, continues to harm people's health and environment. Under the Clean Air Act (United States) of 1970, the EPA works with state and local governments and federal agencies to reduce air pollution and limit the damage that it causes. The Act was amended in 1977 and 1990, establishing emission standards that require the maximum degree of reduction of hazardous air pollutants.

The CARB continues to work with local governments, the business sector, and the public to address California's air quality problems. In the past decade, California has become a global leader in climate change by entering agreements with several nations and linking cap-and-trade programs with Quebec. Many programs have been established to reduce greenhouse gas emissions, including a Zero Emission Vehicle mandate, which is planned to clean up the transportation sector and add about 1.5 million hydrogen fuel cell vehicles to the roads by 2025. The cap-and-trade program has allowed billions of dollars to be invested in reducing greenhouse gases in cities throughout California. While California has taken significant action to decrease pollution, the state remains behind the rest of the nation. The ever-growing population, abundance of cars, and sunny weather continue to foster a pollution-friendly environment.

According to the American Lung Association's recent "State of the Air 2017" report, California is a leader in air pollution among other states, with the highest ozone levels. The top three cities in the country with the highest recorded levels of ozone (smog) levels were Los Angeles-Long Beach, Bakersfield, and Fresno-Madera. More than ninety percent of Californians live in counties with unhealthy levels of air quality. A majority of the contaminants that make up air pollution derive from wildfires, industrial facilities, and transportation, most notably human contribution of vehicles.

The state of air pollution has been improving in Los Angeles, although it remains the city with the worst ozone pollution. With a population of roughly over 10 million, the Los Angeles area is a large basin with the Pacific Ocean to the west, and several mountain ranges with 11,000-foot peaks (3,400 m) to the east and south, Los Angeles is particularly susceptible to high ozone levels. Diesel engines, ports, motor vehicles, and industries are main sources of air pollution in Los Angeles. Frequent sunny days and low rainfall contribute to ozone formation, as well as high levels of fine particles and dust. The strong relationship between air quality index (AQI) and ozone levels may be found on air pollution maps.

Smog is formed when primary pollutants, usually emitted from vehicles, react with ultraviolet rays to create secondary pollutants. When these primary and secondary pollutants combine, it creates the thick haze that can be seen hovering over the city. While ozone can protect the environment from powerful ultraviolet rays, an overabundance of this ozone can be extremely harmful to the health of people and wildlife. As the city rests in a basin between mountains and an inward-sweeping wind from the Pacific Ocean, the ozone, smog, and other toxic particles stagnate in the middle of the Los Angeles Atmosphere with nowhere to escape. Stagnant atmospheric conditions are fairly typical for Los Angeles atmosphere due to frequent atmospheric inversion (that holds multiple pollutants within over the city in the form of "pollution dome"). The effect of "pollution dome" leads to enhanced concentration of greenhouse gases as well such as methane and carbon dioxide. Researchers have shown that carbon dioxide enhancement in the "dome" over Los Angeles is one of the strongest in the world and it is easily detectable from the satellite observations.