The November 2006 assassination of Alexander Litvinenko in London by means of poisoning with the radioactive element polonium-210 (Po-210) has brought new attention to this material, widely used for industrial purposes, and its potential for malevolent use. Unlike cesium-137, strontium-90, and cobalt-60, radioactive materials that are frequently discussed in the press, Po-210 differs in that it emits clusters of two neutrons and two protons, known as “alpha” particles. This type of radiation has a very short range in air, making it easy to shield against detection by radiation detectors typically used for interdiction at borders and other locations. But alpha radiation can deliver a powerful dose if it is in contact with living tissue. As the Litvinenko case illustrated all too well, if taken into the body in large enough doses, Po-210 can be fatal. This factor has also raised concern about the potential terrorist use of this and other alpha-emitters, if volatilized or otherwise made inhalable.

To assist decision makers in better understanding why this radioactive element was chosen as a “murder weapon,” how it is used in industry, and how it is currently regulated, this article provides a technical introduction to Po-210 and a discussion of its most common civilian use in static eliminators.

Po-210 is a radioactive element that is created by the natural decay of certain other radioactive elements, such as radon-222. It can also be formed artificially by a process known as neutron bombardment in a reactor. The element was discovered by Marie and Pierre Curie in 1898.

Po-210 has a radioactive half-life (the time required to reduce its total amount by half) of approximately 138 days, which is considered to be short in comparison to many other radioactive elements. Although in the long term, this characteristic is beneficial because it speeds the ultimate elimination of Po-210’s hazardous radioactivity, in the near term, the short half-life actually intensifies this hazard, because the frequency of radiation emission is intense. A gram of Po-210 emits about 166 x 109 alpha particles every second. This is about 5,000 times as much radiation as an equal mass of radium.

The radiation most frequently emitted by Po-210 is the alpha particle. As particulate radiation goes, this is a heavy, highly charged form that travels relatively short paths, releasing its energy of motion within tiny volumes of space. Thus, outside the body, alpha particles pose little danger. Within the body, where the element may be in intimate contact with cells and tissues, the release of radiation energy can be damaging, especially at the sub-cellular level where important molecules, such as DNA, may be affected.

Principal Industrial Use
As with some radioactive elements that have found industrial uses, such as radioactive hydrogen (tritium), Po-210 has been “de-regulated” to a certain extent when incorporated into specific-purpose or safety-related devices. For example, Po-210 is commonly used in devices constructed to discharge static electricity. Static electricity can be a nuisance or a deadly hazard depending on the industrial setting. It can cause delicate weighing operations in research laboratories to go awry or it can touch off explosions in powder filled factory rooms. To alleviate these concerns, the static can be discharged by ionizing the local air volume. This can be accomplished by using radioactivity. As radioactive particles give up their energy of motion by transferring it to nearby molecules, in this case molecules of air, the weakly bound outer electrons of the target molecules are stripped away, thus removing negative charges. The net effect is to create positive ions (the stripped molecules) and negative electrons. The ionized air will attempt to become neutral again by finding negative charges. Free electrons will be attracted to any stray positive charges. Thus, the static charge of either polarity can be reduced.

Static eliminators, as these static discharge devices are generally referred to, can contain as little as 200 micro-Curies (about 0.04 micro-gram of Po-210) or as much as 200 milli-Curies (about 45 micro-grams). A “micro” unit is equal to one millionth of a basic unit; a “milli” unit is equal to one thousandth of a basic unit. The purchaser of these devices does not need a specific federal or state radioactive materials license. A small laboratory-grade model used to facilitate precision weighing costs about $35 and is sold through standard laboratory supply catalogs, such as that issued by the VWR Company. [1] This model has a useful lifespan of about a year and a half before the alpha-particle flux becomes too weak to create effective ionization fields. In the United States, NRD, LLC of Grand Rapids, New York, produces many types of static eliminators.

Static eliminators are used in many industries where charge build-up can be a problem, such as biomedical research, the manufacturing of computer disk drives, the printing industry, and other manufacturing areas where friction may cause the separation of materials (and the creation of static electricity). [2] The Po-210 in the eliminators is affixed using a patented process that incorporates small amounts of precious metals with the radioactive element. [3] The source is a welded composite of two metal strips: a silver backing plate covered with a thin gold foil and another gold foil strip containing the Po-210. The foils are pressed together and then gold plated. This encapsulation makes them insoluble and inert to most
chemicals. [4]

Po-210 and other industrially-useful radioactive elements are made by utilizing the flux of neutrons produced in nuclear reactors. To produce Po-210, non-radioactive bismuth-209 (Bi-209) is exposed to a slow (thermal) neutron flux inside the reactor. That transmutes the Bi-209 to Bi-210, which rapidly decays (the half-life is about 5 days) to Po-210. The International Atomic Energy Agency (IAEA) estimates that only about 100 grams are produced per year. [5] The main producer is reported to be the Avangard Facility on the Volga River, 450 miles south of Moscow. [6] Representatives of NRD, LLC would not reveal the source of the company’s supply of Po-210, nor could health physicists at the Centers for Disease Control or Los Alamos National Laboratory confirm Avangard as the primary source of commercially-usable Po-210. [7] According to one specialist, about 8 grams (36 Curies) are shipped from Russia to the United States every month. [8]

Health Effects
In humans, the uptake of sufficient quantities of Po-210 would cause victims to exhibit classic symptoms of radiation sickness. The substance does not represent a hazard if it is maintained outside the body. Alpha particles, being relatively massive and carrying a double positive charge, are easily deflected by intervening matter, such as air molecules. The range in air of the Po-210 alpha particle is less than 4 centimeters (cm). [9] A sheet of paper will block them. As stated above, once inside the body, they are in intimate contact with living tissues and cause biological damage.

Biological damage results from the high energy of the alpha particle, about 5.3 million electron volts, that is largely dissipated within the volume of the tissue. The range of the Po-210 alpha in tissue is about 0.0005 centimeter. [10] Cells with a high rate of turnover are particularly susceptible to radiation. The blood cells and the cells that make up the lining of the intestine are examples of those that reproduce and die relatively quickly. Conversely, the cells of the nervous system are relatively radiation resistant.

The solubility of the Po-210 determines the organs that receive the highest doses. A relatively insoluble chemical form may deliver the maximum dose only to the tissues surrounding the point of entry. Soluble forms will be more readily absorbed into the blood where the Po-210 can be delivered to critical organs and tissues. [11] It is reported that 10 percent of soluble Po-210 absorbed into the body is extracted from the gastrointestinal tract. Of that, about 5 percent is eventually concentrated in the spleen, 30 percent in the liver, and 10 percent in the kidneys. Another 10 percent is concentrated in the red bone marrow (where blood cells are produced), and the remainder (45 percent) is distributed in soft tissue. [12]

The U.S. occupational annual limit of intake for Po-210 is 3 micro-Curies by ingestion. [13] This will result in about 5 rem whole body dose or about 50 rem to organs for which the Po-210 has an affinity. At these occupational dose limits, of course, no significant biological effects are expected. Effects would be expected to begin at 50 to 100 rem whole body dose or to organ doses sufficiently high to mimic the effects of whole body doses of this magnitude. Beyond that, the effects are medically serious and vary depending on the dose. Bone marrow depression, nausea, vomiting, gastro-intestinal bleeding and cell death are typical of high dose acute radiation sickness. (Note: The rem unit and rad unit are not equivalent for alpha radiation. The rem unit includes a factor to account for the higher biological destructiveness of the alpha particle relative to other forms of radiation like X-rays and gamma-rays.)

In the Litvinenko poisoning, one expert estimate of the total organ doses from a soluble form of polonium in the body for 22 days resulted in very high doses to the red bone marrow of 13,000 rads, 32,200 rads to the liver, 58,300 rads to the kidney and 53,200 rads to the spleen. [14] The actual date of the poisoning is in question, which contributes to the uncertainty in these calculations. The estimated intake based on available post-mortem data is 405 milli-Curies or in terms of mass, about 0.09 milli-grams of Po-210 (the specific activity of Po-210 is 4,486 Curies per gram). These organ doses result in symptoms that resemble those of an irradiation by a high dose of external penetrating (gamma) radiation. Animal studies indicate that the lethality of Po-210 is unaffected by the route of introduction into the body e.g., skin absorption vs. injection vs. ingestion vs. inhalation. [15]

Industrial Units Not Easily Misused
Do these commercially-available devices represent a potential source of radioactivity for illicit purposes? A small static eliminator contains about 500 micro-Curies of Po-210. This is about 0.1 microgram or about 800 times less than the Litvinenko estimated intake. A large static eliminator contains about half the amount estimated to have poisoned Litvinenko. The Po-210 used in these devices, however, is chemically difficult to extract. [16] As emplaced in the static eliminators, it is in an insoluble form that would make absorption into the body difficult. The Nuclear Regulatory Commission (NRC) is, as of August 2007, continuing its general license program for these devices. An NRC analysis concludes that vibration and impact normal to commercial use will not compromise these sources and routine conditions of use will not result in ingestion or inhalation of the Po-210. [17] Some security experts believe that the maximum allowed amounts that can be sold without a specific license should be reduced by a factor of 10 to increase the difficulty of illicit use. [18]

Non-Radiological Alternatives
There are non-radiological alternatives to Po-210 static eliminators. Such anti-static technology includes “corona discharge” devices, which use electrical power to create ions. Both Po-210 and corona discharge technologies have advantages and disadvantages for the user. For example, Po-210 static eliminators do not require electrical power. They operate 24 hours per day every day of the year. They have a limited lifetime of about 1.5 years, because the Po-210 decays to stable (non-radioactive) lead-206. [19] These devices are easy to install and because they are cable free, they can be moved to areas where they are needed. Since they are not powered by electricity, they can be used in environments where explosions are of concern.

The disadvantages of Po-210 static eliminators include the fact that very few competitive-market choices are available to purchasers. There is, for example, only one U.S. manufacturer (NRD). This is primarily because the regulatory requirements for radiological manufacturing and distribution dissuade new companies from entering the field. [20] Other disadvantages include the fact that the eliminators must be replaced every 15 to 18 months; time and effort is required to package and properly ship them back to the manufacturer; and employers must deal with the anxiety that some people experience when working near devices that incorporate radioactive material.

Corona discharge devices are radiation-free. However, they require electrical power-supplies and cabling. They can also produce ozone that is detectable by its pungent odor. An expert is usually required to set up a corona discharge system, whereas Po-210 static eliminators can probably be employed with minimal assistance. Corona discharge devices can also produce particles at the emitter head. For clean room environments where computer components are assembled, this can be a problem. New emitter point materials in electrically powered static eliminators, such as silicon and titanium, produce particle concentrations as low as Po-210 eliminators do. [21] Some models are combined with a blower to produce a gentle flow of ionized air that can extend to six feet. This is useful for covering large work areas. Since no radioactive source is present, minimal maintenance is generally needed for these systems, which have a useful lifetime of as long as five years – about four times longer than their radiologically-based cousins. A potential user of static eliminators would thus need to weigh a number of factors before deciding whether a device using Po-210 was optimal for his/her needs.

Conclusion
The investigation into Litvinenko’s death continues, and, along with many other important issues, the origin of the Po-210 used to poison him remains unresolved. A particularly disturbing possibility is that his poisoners obtained the material through a black market that might be prepared to supply other unauthorized persons bent on causing harm on a far larger scale. [22]

For the moment, the NRC has chosen not to further regulate static eliminators, in part because the specific way in which Po-210 is incorporated into these units significantly reduces the potential for its malevolent misuse. The Litvinenko case, however, suggests that other points in the Po-210 supply chain – which includes the production process and transportation of polonium in bulk form – may be vulnerable. According to the IAEA’s Illicit Trafficking Database, since 2004, there have been 14 incidents involving industrial Po-210 sources; three incidents occurred in 2006. The IAEA reported that the “incidents involved the theft, loss, or disposal of static eliminators and air ionizers containing sealed Po-210 sources.” [23] These episodes and the Litvinenko case suggest that U.S. and foreign regulators may need to give further attention to licensing, security, disposition procedures, and other regulatory measures covering this material.

Mark L. Maiello, Ph.D.
(Dr. Maiello is a radiation safety officer with a major pharmaceutical firm)