California Code, Health and Safety Code - HSC § 40921.5

(a) For purposes of classifying ozone nonattainment areas pursuant to Sections 40918, 40919, 40920, and 40920.5, the terms moderate, serious, severe, and extreme shall be defined as follows:

(b) For the purposes of classifying carbon monoxide nonattainment areas under Sections 40918 and 40919, the terms moderate and serious shall be defined as follows:

(c) The state board shall determine the ambient concentration of each nonattainment area consistent with the designation criteria established pursuant to subdivision (e) of Section 39607. Classifications for ozone shall be based upon the calendar years 1989 to 1991, inclusive. Classifications for carbon monoxide shall be based upon the 1989-90 and 1990-91 winter seasons.