The table below lists the limits set by the United States Nuclear Regulatory Commission (NRC) on concentration of in air and water of some isotopes commonly found around nuclear facilities. These standards apply to facilities licensed by the NRC (such as commerical uranium processing facilities and nuclear power plants), and are applied to members of the public. Occupational exposure limits for nuclear facility workers are higher.

These concentration limits are dependent upon the particular radionuclide chosen and are calculated such that a dose limit of 50 millrems per year is not exceeded for each radionuclide. They assume that a given radionuclide is the only one inhaled or ingested; allowable concentrations are proportionately reduced if more than one radionuclide is present.

These standards exclude background radiation, which is defined as "radiation from cosmic sources; naturally occurring radioactive materials, including radon (except as a decay product of source or special nuclear material) and global fallout as it exists in the environment from the testing of nuclear explosive devices. 'Background radiation' does not include radiation from source, byproduct, or special nuclear materials regulated by the Commission."1 In other words, background radiation is counted in addition to these allowable limits. However, if another facility is located nearby (such as multiple plants in a complex), the combined emissions from these facilities must be lower than the allowable limit.

US NRC CONCENTRATION LIMITS FOR AIR AND WATER
Radionuclide	Solubility	Air Conc. Limits (pCi/l)	Water Conc. Limits (pCi/l)	Health effects
Hydrogen-3 (Tritium)	Insoluble Soluble	100 100	1,000,000* 1,000,000	Low-energy beta-emitter. When in the form of water, it can become organically bound by replacing hydrogen atoms in a body's cells. As water, it crosses the placenta and irradiates fetuses when pregnant women are exposed.
Strontium-90	Insoluble Soluble	0.006 0.03	- 500	Beta-emitter. Behaves like calcium and concentrates in bones.
Iodine-131	Insoluble Soluble	0.2 0.2	1000 1000	Beta-emitter. Concentrates in the thyroid, esp. via the milk pathway.
Cesium-137	Insoluble Soluble	0.2 0.2	1000 1000	Beta- and gamma-emitter. Resembles potassium and collects in muscles.
Radon-222	no decay products with decay products	10 0.1	- -	Damage mostly from short-lived alpha-emitting decay products deposited in bronchial walls and can cause lung cancer.
Radium-226	Insoluble Soluble	0.0009 0.0009	60 60	Alpha-emitter. Similar to calcium, concentrates in bones. Primary route of exposure is ingestion.
Natural uranium	Insoluble Somewhat soluble Soluble	0.00009 0.0009 0.003	- - 300	Primarily alpha-emitter, also chemically toxic, esp. to kidney. Inhalation or ingestion increases chances of lung and bone cancer.
Plutonium-239	Insoluble Somewhat soluble Soluble	0.00002 0.00002 -	- 20 -	Alpha-emitter. Main health danger comes from inhalation of fine particles or incorporation in cuts.
Americium-241	Insoluble Soluble	0.00002 0.00002	20 20	Alpha- and gamma-emitter. Decay product of pu-241; of special concern for workers handling reactor-grade pu.
Source: Nuclear Regulatory Commission, 10 CFR Part 20, Appendix B (Washington, D.C.: US Government Printing Office, 1994). * Drinking water standards set by the Environmental Protection Agency (EPA) are based on a limit of 4 millirem per year via drinking water pathway only. So allowable concentrations under the EPA would be generally less than 1/10 of those given in the table. In the case of a few radionuclides, like tritium, the allowable limit is even lower (20,000 pCi/l).