From Wikipedia:
Lead content

To enhance the machinability of brass, lead is often added in 
concentrations of around 2%. Since lead has a lower melting point than 
the other constituents of the brass, it tends to migrate towards the 
grain boundaries in the form of globules as it cools from casting. The 
pattern the globules form on the surface of the brass increases the 
available lead surface area which in turn affects the degree of 
leaching. In addition, cutting operations can smear the lead globules 
over the surface. These effects can lead to significant lead leaching 
from brasses of comparatively low lead content.[9]

Silicon is an alternative to lead; however, when silicon is used in a 
brass alloy, the scrap must never be mixed with leaded brass scrap 
because of contamination and safety problems.[10]

In October 1999 the California State Attorney General sued 13 key 
manufacturers and distributors over lead content. In laboratory tests, 
state researchers found the average brass key, new or old, exceeded the 
California Proposition 65 limits by an average factor of 19, assuming 
handling twice a day.[11] In April 2001 manufacturers agreed to reduce 
lead content to 1.5%, or face a requirement to warn consumers about lead 
content. Keys plated with other metals are not affected by the 
settlement, and may continue to use brass alloys with higher percentage 
of lead content.[12][13]

Also in California, lead-free materials must be used for "each component 
that comes into contact with the wetted surface of pipes and pipe 
fittings, plumbing fittings and fixtures." On January 1, 2010, the 
maximum amount of lead in "lead-free brass" in California was reduced 
from 4% to 0.25% lead. The common practice of using pipes for electrical 
grounding is discouraged, as it accelerates lead corrosion.[14][15]

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