The ZMCM is highly photo-reactive, as its high ozone levels confirm. The increasing contamination by ozone is due to the reaction of different precursor components, such as hydrocarbons and nitrogen oxides with the hydroxyl free radical (OH radical dot) in the atmosphere, as explained by Carter (1994) defining the concept of ‘Maximum Incremental Reactivity’. This concept states what the contribution due to the concentration variation of each organic species present in the atmospheric mixture could be, in the ozone production under certain given environmental conditions. The mere atmospheric presence of compounds which can block the oxidation cycle of organic compounds, such as PANs or H2O2, is as important as ozone in atmospheric photo-chemistry. This is the case in Mexico City.