Much of what is written in the press regarding plutonium is inaccurate. I have often read that plutonium is the most deadly element known to man. This is not true.

I'd like to address two separate issues in answering your question. The first is the toxicity of plutonium as a heavy metal. The second is the hazards associated with the radiations emitted by plutonium and the other isotopes present with plutonium.

1. Plutonium is a heavy metal and as such is toxic. There are many heavy metals that are more toxic however. Since I'm a Radiological Engineer and not an Industrial Hygienist I'll leave the heavy metal toxicity discussion at this point.
2. For the purposes of this discussion we'll assume that the plutonium in question is weapons grade. In this grade of plutonium the desired isotope is Pu-239. Several other isotopes are present however. In addition to the Pu-239 are Pu-238, Pu-240, Pu-241, Pu-242 and Am-241 (americium). The Am-241 is the result of the beta minus decay of Pu-241. Am-241 is primarily an external dose hazard since it's predominant decay mode is to emit a 59.5 keV photon. The plutonium isotopes, with the exception of the Pu-241, are alpha particle emitters.

An alpha particle consists of two protons and two neutrons and is therefore massive with respect to other radiations and has an electrostatic charge of plus two. An alpha particle's mass and charge cause it to deposit it's energy very rapidly and limits it's ability to penetrate materials. Alpha particles are stopped by the layer of dead skin cells covering the body and are therefore not an external concern. If alpha particle emitters enter the body there is no layer of dead cells to protect the living tissues.

There are several ways that plutonium could potentially enter the body. Ingestion, inhalation, and injection are the most common. Ingestion is not a significant hazard since absorption from the gastrointestinal tract is low[1]. Plutonium is transferred to the blood stream through the lungs (inhalation) and can be injected directly (wound). Plutonium absorbed into the blood stream is deposited principally in liver and skeleton[1]. The deposition is divided as follows: 45% to liver, 45% to skeleton, 0.035% to testes and 0.011% to ovaries[1].

Radioactive materials are removed from the body at a rate relative to the effective half life for that isotope. The effective half life is derived from the biological half life and the radiological half life. The radiological half life of plutonium is about 24,000 years and the biological half life is about 20 years for liver and 50 years for skeleton. The effective half life of plutonium deposited in the liver is 20 years and 50 years for plutonium deposited in the skeleton[1]. Plutonium deposited in the gonadal tissue is assumed to be permanently retained[1].

The Annual Limit on Intake for plutonium is based on critical organ dose rather than on whole body dose. The limit for critical organ dose is 50 rem (0.5 Sv) CDE. (CDE stands for Committed Dose Equivalent and is the dose to the critical organ over the next 50 years.) A critical organ dose of 50 rem (0.5 Sv) is not expected to cause greater than a 1.0E-4 (1 in 10,000) increase in the incidence of cancer.

A fairly conservative dose conversion factor that I have used when dealing with weapons grade plutonium is 4.6E+8 rem/Ci (1.2E-4 Sv/Bq). Using this factor it would take an inhalation of 1.1E-7 Ci (4.0E+3 Bq) to cause a CDE of 50 rem (0.5 Sv). This roughly equates to 2.9E-7 g (0.29 ug). Twenty- nine hundredths of a microgram may not seem like much material, but in terms of inhalation it is quite allot. It is extremely rare for a plutonium worker to receive an inhalation of this magnitude and the general public is at far less risk.

In regard to the statement that 1 pound of Pu would kill everyone on Earth... One pound of plutonium would be enough to give 1.6E+9 persons a CDE of 50 rem (0.5 Sv) {which could result in 1.5E+5 additional cancers} *IF* and ONLY IF the material was pulverized into particles of respirable size and the material could be adequately dispersed in the atmosphere. A few decades ago the United States and other countries engaged in atmospheric nuclear bomb testing. These tests released many pounds of radioactive isotopes, including plutonium, to the atmosphere. Although there is some evidence of increased incidence of cancer among "downwinders", there have not been 1.5E+5 cancers in excess of the number expected. So it would be practically impossible to kill everyone on earth with one pound of plutonium.

References

[1] ICRP Publication 30 Part 4, 1988, Pergamon Press