(1) No person shall cause or contribute to concentrations of lead in the ambient air which exceed the following standard:

(a) ninety-day average: 1.5 µg/m³ of air, 90-day average, not to be exceeded.

(2) For determining compliance with this rule, lead shall be measured by the high-volume method as more fully described in 40 CFR Part 50, Appendix B, and by the atomic absorption method as more fully described in 40 CFR Part 50, Appendix G, both incorporated by reference in ARM 17.8.202, or by an approved equivalent method.