Lead is a heavy metal pollutant which is now present in the environment
at several orders of magnitude higher than natural levels.
Patterns of Emission
Beginnning
with the Industrial Revolution, human actions caused an increase in lead
emissions to the environment. Lead is released in different amounts
(see chart) by industrial processes such as mining; combustion of wood,
coal, and oil; and incineration.
Worldwide lead emissions to the
atmosphere by category of use in 1983.
Image from Smith and Flegal (1995).
The introduction of alkyl lead into
gasoline as an anti-knock additive in the 1920's caused a sharp increase
in lead pollution. 
The
peak of lead pollution occured in 1966, and levels remained fairly constant
until 1972, when the Clean Air Act began phasing out lead gasoline additives.
Since that time, emissions from gasoline declined to very low levels, causing
a general decrease in emissions.
Lead consumption in gasoline in the U.S.
Image from Smith and Flegal (1995).
Patterns of Deposition
Lead released into the atmosphere
can take several forms. Once in the atmosphere, it deposits either
as dry fall out, or in solution/suspension with rainwater. 
Most
lead pollution settles out near its source, with high concentrations found
near factories and highways. However, some lead incorporates into
aerosol compounds and spreads great distances. (The glacier statistic.)
Storm patterns
across continental United States funnel into the northeast, bringing high
concentrations of lead and other pollutants. Within the Northeast,
higher lead levels are found along highways, near factories, and in high
altitude areas (Johnson et. al, 1982).
Image from Johnson et. al (1982).
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