Primary air pollutants include:
- Sulfur oxides (SOx)
- Nitrogen oxides (NOx)
- Carbon monoxide (CO)
- Volatile organic compounds (VOCs)
- Hydrogen chloride (HCl)
- Hydrogen fluoride (HF)
- Hydrogen sulfide (H2S)
Secondary air pollutants include:
- Nitrogen dioxide
- Sulfuric acid
- Ozone (O3)
- Other photochemical oxidants
= SOx – Sulfur dioxide (SO2) and sulfuric acid vapors are corrosive, colorless air pollutants emitted mainly from the combustion of fuels containing sulfur and industrial wastes containing sulfur.  When emitted to the atmosphere, these contaminants react due to photochemical forces to produce sulfuric acid as well as inorganic and organic sulfate compounds. A large proportion of sulfur dioxide eventually falls to the ground as acid rain or is seized by flora and fauna in the process known as disposition.
= NOx – Nitric oxides (NO) and nitrogen dioxides (NO2) account for NOx, which are created from the combustion of fuels. NO is an odorless air pollutant, and NO2 is an air pollutant with a reddish-brown color that is associated to the brown haze of photochemical smog, common to many urban areas. NO2 contributes to the formation of ground-level ozone, fine particle pollution, and is linked with a number of adverse effects on the respiratory system. 
= CO – CO is a colorless, odorless and partially oxidized compound that contributes to smog formation. It’s formed from the incomplete combustion of fuel and other organic compounds due to insufficient oxygen concentration or too low of temperature, which may occur in automobiles, boilers and industrial furnaces. Other partially oxidized compounds may adhere to particulate matter or remain gaseous.
= VOCs – VOCs are organic compounds that easily volatilize and when released to ambient air, they contribute to photochemical reactions. Organic compounds with a lack of photochemical reactivity are not considered VOCs. They are primarily emitted from the vaporization of organic compounds used as solvents in industrial operation. Such contained and fugitive emissions consist of numerous VOCs with a range of known health effects.
= HCl, HF & H2S – HCl and HF are inorganic acidic gaseous compounds released from combustion industrial processes and well as pollution control devices. Their emitted concentrations are related to the concentrations of chloride and fluoride in the substance being combusted. H2S is a highly toxic chemical that smells like rotten eggs is gaseous form, however the human olfactory bulb quickly acclimates to this odor making it hard to detect even at high concentrations. H2S and other sulfur compounds contribute to the formation of SO2.
= O3 – The oxidant O3 forms in the troposphere due to atmospheric reactions of NOx, VOCs and CO, known as “ground level ozone”. As levels of O3 increase, so do various partially oxidized photochemical oxidants, which are promoted by high temperatures and sunlight intensity. Ozone contributes to what we typically experience as "smog" or haze, which still occurs most frequently in the summertime, but can occur throughout the year in some southern and mountain regions.  Ozone is likely to reach unhealthy levels on hot sunny days in urban environments. Ozone can also be transported long distances by wind. For this reason, even rural areas can experience high ozone levels. Ground level ozone is associated with respiratory difficulties and disease.
= GHGs – Greenhouse Gases (GHGs) are a group of air contaminants that trap heat in our atmosphere. The most important GHGs released from human activity are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and fluorinated gases such as hydrofluorocarbons, perfluorocarbons and sulfur hexafluorides. Different GHGs vary widely in impacts, but may be measured according to their Global Warming Potentials (GWPs).
= HAPs – Hazardous air pollutants (HAPs) are defined in section 112 of the Clean Air Act (CAA) as any air pollutant listed in section (b), which has been amended over time but currently includes 187 pollutants and chemical groups. These chemicals are listed as HAPs due to evidence that they cause or may cause cancer or other serious health effects, or adverse environmental effects.  Although ambient air quality standards are not established for HAPs under the CAA to describe their legally acceptable concentrations in ambient air, they are governed by the EPA through the National Emission Standards for Hazardous Air Pollutant (NESHAP) regulations.