Due to its physical and chemical properties, lead (Pb) exists in the environment predominantly in solid form. Consequently upon emission into the air, Pb deposits onto surfaces or exists in the atmosphere as a component of atmospheric aerosol, and usually in the form of various Pb compounds. The National Ambient Air Quality Standard (NAAQS) for Pb pertains to the Pb content of all Pb compounds that may be emitted to air. The major environmental transport pathway for anthropogenic Pb is the atmosphere, in which it can also undergo secondary dispersal via the deposition and resuspension of particles containing Pb. Airborne Pb particles generally have a bimodal distribution with the greater mass of Pb found in the fine fraction, for which deposition is slower and less efficient than for larger particles. Accordingly Pb may be widely dispersed. Wet and dry deposition are the ultimate paths by which Pb particles are removed from the atmosphere. This appendix describes information on sources and emissions of Pb to the atmosphere, and Pb air monitoring data.