Official websites use .gov
A
.gov website belongs to an official
government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you've safely
connected to the .gov website. Share sensitive
information only on official, secure websites.
Illustration of the relationship between exposure source points (open circles) on a highway (heavy line) and the location of a residence (large solid circle) along with the corresponding axes. Solid circles on highway lines demark segments, and short lines intersecting highways demark subsegments.
