Ground-level ozone, a major component of smog, is formed in the atmosphere when nitrogen oxides and volatile organic compounds react in the presence of sunlight. Produced by vehicle exhaust, industrial emissions, gasoline vapors, and chemical solvents, ozone is one component of photochemical smog.

Health effects include chest pain, coughing, nausea, throat irritation, and congestion, irritation of the respiratory system, reduced lung function, aggravated asthma, inflamed and damaged cells that line the lungs, aggravated chronic lung diseases, and permanent lung damage.

Fine particulate matter is a complex mixture of very small liquid droplets of solid particles in the air. It may come from factory and utility smokestacks, vehicle exhaust, wood burning, mining, construction activity, and agriculture. These fine particles are easily inhaled deeply into the lungs where they can be absorbed into the bloodstream or remain embedded for long periods of time.

Health effects include wheezing, coughing, and respiratory irritation and exposure may trigger asthma attacks. It is especially harmful to those with asthma and chronic obstructive pulmonary disease (COPD), including chronic bronchitis and emphysema.