At ground level, it's a toxic pollutant that can damage health. Nitrogen oxides: Nitrogen dioxide NO2 and nitrogen oxide NO are pollutants produced as an indirect result of combustion, when nitrogen and oxygen from the air react together. The natural sources include volcano, forest fire and pollens.
Photo: Pollution solution: an electrostatic smoke precipitator helps to prevent air pollution from this smokestack at the McNeil biomass power plant in Burlington, VT.
Ozone is a gas that stops harmful ultraviolet rays from the sun. Any gas could qualify as pollution if it reached a high enough concentration to do harm. Again polluted air acts as a better insulator which prevents heat to get escaped back into the space. Air pollution generally enters human body through the respiratory system and thus causes many respiratory disorders.