The major components of photochemical smog are nitrogen oxides, ozone $\left({\mathrm{O}}_3\right)$, $\mathrm{PAN}$ (peroxyacetylnitrate), and chemical compounds that contain the-$\mathrm{CHO}$ group (aldehydes). $\mathrm{PAN}$ and aldehydes can cause eye irritation and plant damage if their concentrations are sufficiently high.

Classical smog occurs in a cool, humid climate. Its components include smoke, fog and sulphur dioxide. It is reducing in nature.