Ozone is a secondary emission and is formed in the atmosphere from primary emissions. The effects of ozone varying depending on where it is located; in the upper regions of earth’s atmosphere it occurs naturally and shields the earth from the sun’s ultraviolet rays. However, at ground level it poses a threat to human health as a pollutant, to be precise it is a respiratory irritant. Ozone is formed when VOC’s and nitrogen oxides react in the presence of sunlight, which is why ozone is typically highest on sunny days in urban environments. The sources of ozone forming compounds include emissions from vehicles, construction equipment, industry, and some consumer products, however, ozone is never directly emitted itself. Another factor to consider, is that ozone is more prevalent outdoors than indoors, and should be considered when engaging in outdoor physical activities.