1.2 Measurements of ozone precursor substances must include at least nitrogen oxides, and appropriate volatile organic compounds (VOC). A list of volatile organic compounds recommended for measurement is given below.
| Ethane | 1-Butene | Isoprene | Ethyl benzene |
| Ethylene | trans-2-Butene | n-Hexane | M+p-Xylene |
| Acetylene | cis-2-Butene | i-Hexane | o-Xylene |
| Propane | 1.3-Butadiene | n-Heptane | 1,2,4-Trimeth. Benzene |
| Propene | n-Pentane | n-Octane | 1,2,3-Trimeth. Benzene |
| n-Butane | i-Pentane | i-Octane | 1,3,5-Trimeth. Benzene |
| i-Butane | 1-Pentene | Benzene | Formaldehyde |
| 2-Pentene | Toluene | Total non-methane hydrocarbons |