40 CFR §50.16

1. (a)The national primary and secondary ambient air quality standards for lead (Pb) and its compounds are 0.15 micrograms per cubic meter, arithmetic mean concentration over a 3-month period, measured in the ambient air as Pb either by:
   1. (1)A reference method based on appendix G of this part and designated in accordance with part 53 of this chapter or;
   2. (2)An equivalent method designated in accordance with part 53 of this chapter.
2. (b)The national primary and secondary ambient air quality standards for Pb are met when the maximum arithmetic 3-month mean concentration for a 3-year period, as determined in accordance with appendix R of this part, is less than or equal to 0.15 micrograms per cubic meter.