California Code, Health and Safety Code - HSC § 43610

The state board shall set standards for, and certify, exhaust devices to significantly reduce the emission of oxides of nitrogen from 1966 through 1970 model year motor vehicles, as determined by the state board from a representative sampling of such motor vehicles, which the state board has found to be necessary and technologically feasible to carry out the purposes of this division.

In setting standards under this section, the primary consideration shall be the greatest possible reduction of oxides of nitrogen.