Pollution control is the process of reducing or eliminating the release of pollutants into the environment. It is regulated by various environmental agencies which establish pollutant discharge limits for air, water, and land. Over the years, the air has become contaminated by many air pollutants such as hydrochlorofluorocarbons (HCFC). The air becomes polluted from activities such as driving cars, trucks, burning coal, oil, and other fossil fuels. Air pollution can even come from smaller, everyday activities such as dry cleaning, filling the car with gas, and degreasing and painting operations. These activities add gases and particles to the air we breathe. Air pollution is also not limited to outdoor activities alone. It can occur both outdoors and indoors. Forest fires and volcanic eruptions can produce smoke and gases as well as large amounts of particulate matter that can be emitted into the air. This natural occurrence can cause respiratory problems in both humans and animals, limit the visibility of specific areas, and even change the climate. High levels of pollution adversely affect most of populated regions. Air population is one of several important environmental hazards alongside water contamination, hazardous waste, noise, and others. It is currently an important environmental concern of large cities.