Air pollution is caused by both natural and human-made pollutants, including particulate matter, gases, and biological molecules. Common air pollutants are particulate matter, nitrogen oxides, sulfur dioxide, carbon monoxide, ozone, and chlorofluorocarbons. Nitrogen oxides are produced by combustion and harm the respiratory and cardiovascular systems. Carbon monoxide reduces oxygen in the blood and chlorofluorocarbons were widely used but deplete the ozone layer. Smog results from emissions reacting with sunlight and air, reducing visibility and potentially causing respiratory issues and heart/lung disease.