D. K. Craig
Westinghouse Savannah River Company
Aiken, SC 29808

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Plutonium is frequently referred to as "the most toxic substance known to man". While there are other chemicals that cause more immediately serious health consequences, plutonium does have the lowest permissible levels for any of the radioactive elements. Concentration limits for plutonium and its compounds are based on its radiotoxicity, specifically carcinogenicity, not on its chemical toxicity. The specific activity of ²³⁹Pu is 61.3 m Ci/mg, the inverse of which is 16.3 m g/m Ci¹, its half-life being 24,400 years. The derived air concentration (DAC) for ²³⁹Pu is 2 x 10^-12 m Ci/cc = 2 x 10^-6 m Ci/m³, which is based on a committed effective dose equivalent of 50 mSv (5 rem)².

In terms of mass per unit volume, the DAC for ²³⁹Pu of 2 x 10^-6 m Ci/m3 is equal to 3.26 x 10^-5 m g/m³. Shorter-lived plutonium isotopes have even smaller mass concentration limits. The most restrictive time-weighted average (TWA) threshold limit values (TLVs) listed³ are 0.001 mg/m³ for calcium chromate, and 0.0005 mg/m³ (i.e., 5 x 10^-1 m g/m³) for strontium chromate. Both of these limits are based on the potent carcinogenic potential of these chemicals, so they form a good basis for comparison with the alpha-emitting plutonium isotopes. The TLV-TWA is 0.002 mg/m³ for both the soluble salts of platinum, and hexachlorobenzene, and 0.001 mg/m³ for calcium chromate (as Cr). The TLV booklet gives "Notice of intended changes (for 2001)" for "Beryllium [7440-41-7] and compounds, as Be" from 0.002 to 0.0002 mg/m³ (i.e., 2 x 10^-1 m g/m³) for the inhalable fraction, also based on its carcinogenic potential. No other chemical substances have lower workplace limits than this new value for Be.

The two relevant plutonium entries in SAX⁵ are duplicated in the appendix.

Clearly, the radiation-based limit for ²³⁹Pu is several (four) orders of magnitude less than that of the chemical with the most restrictive workplace concentration limit. A four order-of-magnitude increase in the DAC for ²³⁹Pu would result in a committed effective dose equivalent of 50,000 rem (i.e., an average dose of 1000 rem per annum), enough to cause acute radiation death in at least some people. Radiation dose rates of this magnitude make it impossible to test the non-radiation biological effects: animals die from the radiation effects long before toxic effects can manifest themselves.

Any overt toxicity of plutonium and its compounds would be overwhelmed by their radiological effects.

1. Radiological Health Handbook, U.S. Department of Health, Education, and Welfare Public Health Service. Revised Edition, January 1970.
2. U.S. DOE; 10 CFR 835, Appendix A, Derived Air Concentrations (DAC) for Controlling Radiation to Workers at DOE Facilities, January 1, 2001.
3. 2001 TLVs^TM and BEIs^TM: Threshold Limit Values for Chemical Substances and Physical Agents, Biological Exposure Indices; American Conference of Governmental Industrial Hygienists (ACGIH), Cincinnati, OH (2001).
4. Documentation of TLVs and BEIs. Sixth Edition. ACGIH, Cincinnati, OH. (1991).
5. SAX’s Dangerous Properties of Industrial Materials, Tenth Edition. Richard J. Lewis, Sr. (Ed.). John Wiley & Sons, Inc. (New York, 2000)

Properties:

A silvery, radioactive metal; chemically reactive. Melting point: 641º, boiling point: 3232º, density: 19.816 @ 20º/4º.

An extremely poisonous radioactive material. The permissible levels for plutonium are the lowest for any of the radioactive elements. This is occasioned by the concentration of plutonium directly on bone surfaces, rather than the more uniform bone distribution shown by other heavy elements. This increases the possibility of damage from equivalent activities of plutonium and has led to the adoption of the extremely low permissible levels given. Radiation Hazard: Artificial isotope 238Pu, T0.5 = 86 Y, decays to radioactive 234U by alphas of 5.5 MeV. Artificial isotope 239Pu, T0.5 = 24,000 Y decays to radioactive 235U by alphas of 5.1 MeV. Artificial isotope 240Up, T0.5 = 6600 Y decays to radioactive 236Pu (Neptunium Series), T0.5 = 13 Y decays to radioactive 241Am by betas of 0.02 MeV. Artificial isotope 242Pu, T0.5 = 3.8 × 105 Y decays to radioactive 238U by alphas of 4.9 MeV. Ignites in air as low as 135ºC. Explosive reaction with carbon tetrachloride. Particles exposed to air and moisture may ignite spontaneously. Corrosion products are usually pyrophoric. When heated to decomposition it emits toxic and radioactive fumes of Pu. See also Plutonium Compounds.

Safety Profile

The toxicity of plutonium compounds is based first upon the very high radiotoxicity of the plutonium atom and secondly upon whatever atoms or combinations of atoms they might contain. Very dangerous! Any disaster which causes quantities of plutonium or plutonium compounds to be scattered about the environment will cause great ecological stress and render areas of the land unfit for public occupancy. Long-term storage in plastic containers is not recommended, as the alpha particles can cause stress cracks and there is a potential for leakage. See also Plutonium.