What are Radionuclides? Radionuclides are substances such as plutonium, cesium, strontium, tritium, etc., which release high energy electromagnetic rays known as ionizing radiation. Some radionuclides like radium or its decay products are naturally occurring, while many others are inadvertently introduced into the environment from man-made sources. Other sources of ionizing radiation include medical x-rays and nuclear medicine.

        How Can Radionuclides Affect My Health? Radionuclides emit high energy rays which are able to damage biological tissues, potentially resulting in the promotion and growth of cancerous cells. The type of radiation emitted by each radionuclide determines the energy and therefore the penetrating power and cancer potential of the radiation. The route of exposure is also an important determinant of the potential health effects (e.g. the skin is not as delicate as the lining of the lungs).
        How May I be Exposed to Radionuclides? Some radionuclides may be gaseous or particulate, hence exposure will occur through inhalation. The predominant airborne radionuclide is radon, a naturally occurring radioactive gas which originates from radium, a component of practically all soil and rock. Surface waters also contain radionuclides which may be found in the water phase associated with microscopic particles. Naturally occurring radionuclides in river water include tritium, potassium-40, radium, radon and their short term products of decay. These radionuclides may also be present in drinking water.

        While 82% of our total exposure to ionzing radiation is from natural sources, the largest contributor is by far radon gas which makes up 55% of the total. Other natural sources of radiation include cosmic rays from outer space, internal radiation mostly from radioactive potassium already present in our bodies (through food, drink and air) and terrestrial sources. Hence drinking water generally contributes an insignificant amount of exposure from natural sources.

        Are the Levels of Radionuclides in Drinking Water Dangerous to my Health? The average exposure of people in the U.S. to ionizing radiation from all sources is about 360 millirems (rem is a unit of measurement for radiation). Radionuclides in drinking water contribute only a small proportion of the total radiation exposure. Many studies have shown that no significant health effects have occurred when humans have been exposed to less than 20 rems (55 times greater than the average exposure) over a long period of time.


Source: http://www.ci.sf.ca.us/puc/wqfs/tocsec4.htm