Air pollution is the contamination of Earth’s atmosphere at levels high enough to harm humans, others organisms, or other materials. The major air pollutants are particulate matter, ozone, carbon monoxide, nitrogen oxides, sulfur dioxide, and lead. Primary air pollutants, including carbon monoxide, nitrogen oxides, sulfur dioxide, and particulate matter, enter the atmosphere directly. Secondary air pollutants are harmful chemicals that form from other substances released into the atmosphere. Ozone and sulfur trioxide are examples of secondary air pollutants.