The harmful constituents of photochemical smog are, NO₂, O₃, PAN and aldehydes. The PAN and aldehydes cause  eye irritation. NO₂  and ozone  are strong oxidants and cause damage to elastomeric/ rubber materials and plants.

Photochemical Reactivity of Hydrocarbons

The exhaust gases of gasoline engines contain more than 150 different hydrocarbons and their derivatives. Some hydrocarbons are more reactive than the others. The photochemical reactivity of hydrocarbons has been measured in terms of the rate at which the specific hydrocarbon causes oxidation of NO to NO₂. To determine the rate of photo-oxidation, NO in presence of the specific hydrocarbon is irradiated by ultra violet radiations in a reaction chamber and the buildup of NO₂ in terms parts per billion/per minute is recorded. Another photochemical reactivity scale has been defined in terms of ozone formation. Reactivity of different classes of hydrocarbons based on formation of NO₂ is given in Table 1.4
It has been noted that the reactivity of a given hydrocarbon depends also on the initial concentrations of pollutants in the environment in which a particular hydrocarbon is added when emitted.  A reactivity termed as incremental activity has been determined in terms of ozone formed. It is defined as the change in ozone formation rate   when specific VOC is added to the base reactive organic gas mixture in the environment divided by the amount of the specific VOC added. This reactivity is considered to bge of more practical relevance.

*based on NO₂ formation rate for the specific hydrocarbon relative to that for 2,3 dimethyl-2-benzene