Why Polonium is Highly Radiotoxic?

Polonium (named after Poland, the native country of Marie Curie) is a rare and highly radioactive element (symbol Po and atomic number 84). It was discovered in 1898 by Marie Curie and Pierre Curie. Chemically, polonium is similar to Bismuth. It is one of the radioactive daughter products of uranium (U-238) series and hence occurs in uranium ores, along with another alpha emitter of Radium isotope, Ra-226. It is found in uranium ores at about 0.1 mg per metric ton (1 part in 10¹⁰), which is approximately 0.2% of the amount of radium found in the ore. Because of its small abundance, the separation of polonium is very difficult. However, it can now be produced by irradiation of bismuth (Bi-209) by high energy neutrons. Major part of the legal Po-210 available is produced in Russia.

 

The time required for the decay of one-half of the Po-210 activity (called half-life) is 138 days. It emits alpha particles during decay and the decay product is stable lead isotope – Pb-206. Its parent, Bi-210 has a half-life of 5 days and it emits beta particles. The energy of the alpha particle emitted by Po-210 is 5.3 million electron volts (MeV)!

Because of its shorter half-life, specific activity, i.e., radioactivity per gram of the material is very high. As a comparison, as small as one milli-gram of Po-210 emits as many alpha particles as 5 gram of Ra-226, with a half-life of 1600 years!. A 1 gram sample of Po-210 will spontaneously get heated up to above 500 degree C, generating about 140 watts of power. The uses of Po-210 are in space probes, antistatic devices and as neutron and alpha source.

While handling polonium, it easily gets airborne. If a polonium sample is heated to 55 degree C, 50% of the material gets vaporized. This is in-spite of the fact that the melting point and boiling point of Polonium is high at 254 degree C and 962 degree C respectively.  

Polonium is handled in specially designed air-tight boxes called Glove Boxes, with adequate negative pressure inside so that the contaminated air inside the box is not coming out into the working environment. High performance gloves are used to prevent any possible diffusion of polonium inside the gloves and contaminate the hands of the working personnel. Use of torn/damaged gloves should never be used. The inside air of the glove box is double-filtered through high efficiency particulate air (HEPA) filters before its release into the environment. Appropriate monitoring of the workplace and individual handling polonium are carried out to ensure radiological safety.

Polonium is highly radiotoxic in the human body because its high-energy alpha particle emissions. In the body, there is no biological role for polonium. By mass basis, Po-210 is 250,000 times more toxic than hydrogen cyanide. The lethal dose (LD₅₀) is as low as 1 micro-gram for an average adult.

Polonium can be easily absorbed (transcutaneous diffusion) through intact skin. Other modes of intake into the body are inhalation of the airborne polonium or ingestion of the polonium contaminated food or water. The target organ for polonium in the body, include spleen, liver, bone marrow and thymus. In view of the nature of the handling hazard, any work with polonium is strictly regulated by national regulatory body.