User:ChyranandChloe/Workshop 18

Radioactive carcinogens

In addition to chemical, nonradioactive carcinogens, tobacco and tobacco smoke contain small amounts of lead-210 (²¹⁰Pb) and polonium-210 (²¹⁰Po) both of which are radioactive carcinogens. Lead 210 is a product of the decay of radium-226 and, in turn, its decay product, radon-222; lead 210 then decays to bismuth-210 and then to polonium 210, emitting beta particles in both steps. Tarry particles containing these elements lodge in the smokers' lungs where airflow is disturbed; the concentration found where bronchioles bifurcate is 100 times higher than that in the lungs overall. This gives smokers much more intense exposure than would otherwise be encountered. Polonium 210, for instance, emits high energy alpha particles which, because of their large mass, are considered to be incapable of penetrating the skin more than 40 micrometres deep, but do considerable damage (estimated at 100 times as much chromosome damage as a corresponding amount of other radiation) when a process such as smoking causes them to be emitted within the body, where all their energy is absorbed by surrounding tissue. (Lead 210 also emits gamma rays).

The radioactive elements in tobacco are accumulated from the minerals in the soil, as with any plant, but are also captured on the sticky surface of the tobacco leaves in excess of what would be seen with plants not having this property. As might be expected, the radioactivity measured in tobacco varies widely depending on where and how it is grown. One study found that tobacco grown in India averaged only 0.09 pico-Curies per gram (3.3 Bq/kg) of polonium-210, whereas tobacco grown in the United States averaged 0.516 pCi per gram (19.1 Bq/kg). Another study of Indian tobacco, however, measured an average of 0.4 pCi (15 mBq) of polonium 210 per cigarette, which also would be approximately a gram of tobacco. One factor in the difference between India and the United States may be the extensive use of apatite as fertilizer for tobacco in the United States, because it starves the plant for nitrogen, thereby producing more flavorful tobacco; apatite is known to contain radium, lead 210, and polonium 210. This would also account for increased concentration of these elements compared to other crops, which do not use this mineral as fertilizer.

The presence of polonium-210 in mainstream cigarette smoke has been experimentally measured at levels of 0.0263–0.036 pCi (0.97–1.33 mBq), which is equivalent to about 0.1 pCi per milligram of smoke (4 mBq/mg); or about 0.81 pCi of lead 210 per gram of dry condensed smoke (30 Bq/kg). The amount of polonium 210 inhaled from a pack of 20 cigarettes is therefore about 0.72 pCi (27 mBq). This seems to be independent of any form of filtering or 'low tar' cigarette. This concentration results in a highly significant increase in the body burden of these compounds. Compared to nonsmokers, heavy smokers have four times greater radioisotope density throughout their lungs. The polonium 210 content of blood in smokers averages 1.72 pCi per kilogram (64 mBq/kg), compared to 0.76 pCi per kilogram (28 mBq/kg) in nonsmokers. Higher concentrations of polonium 210 are also found in the livers of smokers than nonsmokers. Polonium 210 is also known to be incorporated into bone tissue, where the continued irradiation of bone marrow may be a cause of leukemia, although this has not been proved as yet.

Research by NCAR radiochemist Ed Martell determined that radioactive compounds in cigarette smoke are deposited in "hot spots" where bronchial tubes branch. Since tar from cigarette smoke is resistant to dissolving in lung fluid, the radioactive compounds have a great deal of time to undergo radioactive decay before being cleared by natural processes. Indoors, these radioactive compounds linger in secondhand smoke, and therefore greater exposure occurs when these radioactive compounds are inhaled during normal breathing, which is deeper and longer than when inhaling cigarettes. Damage to the protective epithelial tissue from smoking only increases the prolonged retention of insoluble polonium 210 compounds produced from burning tobacco. Martell estimated that a carcinogenic radiation dose of 80-100 rads is delivered the lung tissue of most smokers who die of lung cancer.

In other experiments, the alpha particle dosage from polonium 210 received by smokers of two packs a day was measured at 82.5 millirads (0.825 mGy) per day, which would total 750 rads (7.5 Gy) per 25 years, 150 times higher than the approximately 5 rem (50 mSv) received from natural background radiation over 25 years. Other estimates of the dosage absorbed over 25 years of heavy smoking range from 165 to 1,000 rem (1.65 to 10 Sv), all significantly higher than natural background. In the case of the less radioactive Indian tobacco referred to above, the dosage received from polonium 210 is about 24 millirads (0.24 mGy) a day , totalling 219 rads (2.19 Gy) over 25 years or still about 40 times the natural background radiation exposure. In fact, all these numbers of total body burden are misleadingly low, because the dosage rate in the immediate vicinity of the deposited polonium 210 in the lungs can be from 100 to 10,000 times greater than natural background radiation. Lung cancer is seen in laboratory animals exposed to approximately one fifth of this total dosage of polonium 210.

Whether the quantities of these elements are sufficient to cause cancer is still a matter of debate. Most studies of carcinogenicity of tobacco smoke involve painting tar condensed from smoke onto the skin of mice and monitoring for development of tumors of the skin, a relatively simple process. However, the specific properties of polonium 210 and lead 210 and the model for their action, as described above, do not permit such a simple assay and require more difficult studies, requiring dosage of the mice in a manner mimicking smoking behavior of humans and monitoring for lung cancer, more difficult to observe as it is internal to the mouse.

Some researchers suggest that the degree of carcinogenicity of these radioactive elements is sufficient to account for most, if not all, cases of lung cancer related to smoking. In support of this hypothetical link between radioactive elements in tobacco and cancer is the observation that bladder cancer incidence is also proportional to the amount of tobacco smoked, even though nonradioactive carcinogens have not been detected in the urine of even heavy smokers; however, urine of smokers contains about six times more polonium 210 than that of nonsmokers, suggesting strongly that the polonium 210 is the cause of the bladder carcinogenicity, and would be expected to act similarly in the lungs and other tissue. Furthermore, many of the lung cancers contracted by cigarette smokers are adenocarcinomas, which are characteristic of the type of damage produced by alpha particle radiation such as that of polonium 210. It has also been suggested that the radioactive and chemical carcinogens in tobacco smoke act synergistically to cause a higher incidence of cancer than each alone.

However, the view that polonium 210 is responsible for many cases of cancer in tobacco smokers is disputed by at least one researcher.