Lead is a metal that is in the atmosphere as a pollutant. Lead particles are emitted from mines and smelters in the form of lead sulfate and lead oxide.
For many years tetraethyl lead was used as an anti-knocking additive in gasoline, and as a result vehicle emissions contained lead compounds. This practice was phased out in the U.S by 1991. It is estimated that 7 million tons of lead were released into the atmosphere in the U.S. during the 60 years that it was added to gasoline.
Lead is toxic to humans and other animals. Lead poisoning causes central nervous system damage and blood disorders.
The Clean Air Act set standards for lead and atmospheric lead is now monitored by the EPA. It has decreased since lead content in gasoline was reduced. The reference link provided shows graphs of lead concentration in the atmosphere from 1980 onward.