Photochemical smog is formed as a result of the reaction of sunlight with hydrocarbons and nitrogen oxides. Ozone, nitric oxide, acrolein, formaldehyde and peroxyacetyl nitrate (PAN) are common components of photochemical smog.

While ozone is toxic in nature, both NO₂ and O₃ are oxidizing agents. They react with the unburnt hydrocarbons in air to produce formaldehyde, PAN, and acrolein.