Section 50.16 - National primary and secondary ambient air quality standards for lead(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) A reference method based on appendix G of this part and designated in accordance with part 53 of this chapter or;(2) An equivalent method designated in accordance with part 53 of this chapter.(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. 73 FR 67052 , Nov. 12, 2008