Radiation has always been a part of
the natural environment.
Natural Radiation - Naturally-occurring
radiation accounts for approximately 80 per cent of
our radiation exposure. Background radiation comes
from natural sources always present in the
environment, including solar and cosmic radiation,
radiation emitted from the earth's crust, radon, and
internal radiation from the foods we eat and drink.
Man-Made Radiation - Man-made radiation
sources include medical (x-rays and diagnostic or
therapeutic materials), industry, consumer products
(such as smoke detectors and television sets), and
atmospheric testing of nuclear weapons.
The radionuclides present in our environment can
give both internal and external doses. Internal dose
is received as a result of the intake of
radionuclides. The major routes of intake of
radionuclides for members of the public are
ingestion and inhalation. Ingestion includes the
intake of the radionuclides from drinking milk and
water, and consumption of food products. Inhalation
includes the intake of radionuclides through
breathing dust particles containing radioactive
The background radiation level for an average
American is about 360 mrem per year.
Calculate your radiation dose (http://www.epa.gov/rpdweb00/understand/calculate.html)