The Organization for Economic Co-operation and Development defines secondary pollution as "pollution caused by reactions in air already polluted by primary emissions (from factories, automobiles and so forth)." Secondary pollution is just as harmful to humans, animals and plant life as primary pollution. There are several recognized types of secondary pollution.
There are two types of ozone--upper ozone and lower ozone. Upper ozone shields humans, plants and animals on Earth from radiation caused by the sun. Lower ozone is caused when hydrocarbons and nitrogen oxides that are released by manufacturing plants and other industrial activity mix with sunlight. Ozone is a well-known secondary pollution that affects many people with upper-respiratory illnesses, is toxic to plant life and acts as a greenhouse gas that causes global warming. Ozone is the most damaging secondary pollution in North America.
Smog is another common secondary pollution. Smog is caused when car emissions and industrial pollution interact with sunlight. Smog affects plant life and the health of animals and humans.
Secondary pollution is created by landfills in the form of water contamination. When landfills are improperly contained, chemicals are leached into the water supply.
Gas may also be generated in landfills if not properly handled. Methane, ammonia, carbon dioxide and hydrogen sulfide are all created by toxic substances found in landfills.
Proper containment and covering landfills with soil are both effective, if not perfect, ways to prevent secondary pollution.
CFCs were safely used for years in cleaning products and as coolant for refrigerators. Though not toxic during initial use, CFCs dissolve into the atmosphere and destroy the upper ozone. The upper ozone protects Earth from radiation generated by the sun, and the depletion of the upper ozone can increase the risk of cancer in humans.
Acid rain is caused by sulfuric and nitric acids. These acids are created by burning fossil fuels and some industrial processes. The acids change the pH of rain or snow from neutral to very acidic. Though not directly damaging to humans, acid rain affects fish and aquatic animals and has ruined some lakes to the point that no fish survive in those waters.
Many countries are taking aggressive measures to combat pollution. By preventing primary sources of pollution, secondary pollution is abated as well. Setting limits on emissions, increasing mileage of vehicles and creating new sources of energy such as solar, wind and biofuels all inhibit pollution.