Ozone (O₃) is created through a photochemical reaction between atmospheric oxygen, hydrocarbons and/or carbon monoxide, oxides of nitrogen, and sunlight. At high concentrations at ground level in the lower atmosphere, O₃ is an air pollutant that can harm the respiratory systems of people and animals and damage plant tissue. The primary sources of the precursor gases in the Tahoe Basin include on-road motor vehicles, residential fuel combustion, motorized watercraft, off-road equipment, solvent, and fuel evaporation, and off-road recreational vehicles. Ozone can also be transported into the Tahoe Basin from outside sources. Regional programs such as reducing automobile use through transit and bike paths and cleaner-burning wood stoves aim to reduce ozone levels.