So the amount of ozone in the air increases. With nitric oxide reacting with hydrocarbons, ozone may accumulate to damaging levels. These are referred to as secondary pollutants, because they are formed by the reaction of primary pollutants, nitrogen oxides and hydrocarbons, emitted by burning fossil fuels. The energy from the sun moves the reactions along. This forms photochemical smog, the brown gunk we see in the sky, especially on hot sunny days.
Photochemical smog can cause eye irritation and poor visibility.
Strong oxidants such as ozone can damage the lungs. The oxidants irritate the linings of lungs. Damage to the lungs may stress the heart. Health damage is worse for people with existing lung and heart conditions. Other health implications may include loss of immune system function, increased susceptibility to infections, and fatigue.