Humans contribute to air pollution by releasing harmful substances into the earth's atmosphere. Over many decades, harmful substances have accumulated to produce areas with severely polluted air. Polluted air impacts human health in many ways.

• High $NO_2$ levels irritate the respiratory system.
• High BC (black carbon) levels are associated with respiratory and cardiovascular disease. BC may also be represented by $BC_{ff}$ (fossil fuels) or $BC_{bb}$ (biomass burning).
• $PM_{10}$ and $PM_{2.5}$ both measure fine particulates in the air, which irritate the respiratory system.
• $O_3$ (ozone) in exceedingly high concentrations causes airway inflammation and may reduce lung function.
• High outdoor CO levels may reduce oxygen transport to the heart.
• High THC (total hydrocarbons) and high NMHC (non-methane hydrocarbons) levels are toxic and may cause cancer.
• Low formaldehyde levels are enough to irritate the respiratory tract. Prolonged exposure may cause cancer.
• High AOD levels (>1) indicate heavy pollutants in the air.
• High AQI values (100-500) indicate poor air quality conditions (ranging from unhealthy for sensitive groups to hazardous).
• BTEX values (benzene, toluene, ethylbenzene, xylene) indicate levels of hazardous and often carcinogenic compounds dangerous to human health.