Common Contaminants in Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 254

(ATSDR 2004c), and sometimes exists as an acid mist. This mist may cause skin and lung
irritation, and skin burns can occur if you are exposed to a highly concentrated mist for a
prolonged period (ATSDR 2004c).
7.8.7 Radioactive Compounds
Radioactive decay occurs when an unstable atomic nucleus spontaneously loses energy by
emitting ionizing particles and radiation. This decay, or loss of energy, results in an atom of
one type (parent nuclide) transforming into an atom of a different type (daughter nuclide).
All elements with an atomic number greater than 80 possess radioactive isotopes, and all
isotopes of elements with an atomic number greater than 83 are radioactive.
Some radioactive compounds deserving special attention include the following
(Kathren 1991):
• Beryllium
• Calcium
• Carbon
• Potassium
• Cadmium
• Cesium
• Iodine
• Strontium
• Palladium
• Tin
• Radon
• Radium
• Thorium
• Uranium
• Plutonium
Radon (CAS registry number 10043-92-2) is the most common radioactive compound pres-
ent in urban areas, and it has the potential for adverse human health effects (ATSDR 2000).
USEPA (2009b) classifies radon as a human carcinogen, and exposure to radon for a long
period of time at elevated concentrations may cause cancer.
Radon is a decay product of uranium, found naturally in the Earth's crust. It is one
of the heaviest substances existing as a gas under normal conditions of pressure and
temperature. The highest average radon concentrations in the United States are found in
Iowa, southeastern Pennsylvania, and Appalachian Mountain areas. During the decay
process, alpha, beta, and gamma radiation is released. Alpha particles can travel only
short distances and cannot penetrate your skin. Beta particles can penetrate through your
skin but not your whole body. Gamma particles can penetrate your whole body. Radon is
normally present at very low levels in outdoor air but may be present at higher levels in
indoor air, especially in basements and buildings with poor ventilation and in well water
(ATSDR 2008c).