Cigarettes & Radiation…

One theory on lung cancer which brings both air-borne radon and cigarettes to a comparable assessment is the amount of radiation contained in both. Because tobacco is grown in soils fertilized by Phosphorous (radioactive) enriched materials, polonium-210 and lead-210 (radioactive) are resident in the tobacco leaves. When smoked, this radiation is transferred to the smoker’s lungs, and then exhaled to the environment. If contained in a room or car, this radiation filled smoke can be inhaled by non-smokers, causing lung damage. Two articles on the subject are…

Radioactivity in Cigarette Smoke
Winters-TH, Franza-JR
New England Journal of Medicine, 1982; 306(6): 364-365

To the Editor: During the 17 years since the Surgeon General's first report on smoking, intense research activity has been focused on the carcinogenic potential of the tar component of cigarette smoke. Only one definite chemical carcinogen -- benzopyrene -- has been found. Conspicuous because of its absence is research into the role of the radioactive component of cigarette smoke.

The alpha emitters polonium-210 and lead-210 are highly concentrated on tobacco trichomes and insoluble particles in cigarette smoke (1). The major source of the polonium is phosphate fertilizer, which is used in growing tobacco. The trichomes of the leaves concentrate the polonium, which persists when tobacco is dried and processed.

Levels of Po-210 were measured in cigarette smoke by Radford and Hunt (2) and in the bronchial epithelium of smokers and nonsmokers by Little et al. (3) After inhalation, ciliary action causes the insoluble radioactive particles to accumulate at the bifurcation of segmental bronchi, a common site of origin of bronchogenic carcinomas.

In a person smoking 1 1/2 packs of cigarettes per day, the radiation dose to the bronchial epithelium in areas of bifurcation is 8000 mrem per year -- the equivalent of the dose to the skin from 300 x-ray films of the chest per year. This figure is comparable to total-body exposure to natural background radiation containing 80 mrem per year in someone living in the Boston area.

It is a common practive to assume that the exposure received from a radiation source is distributed throughout a tissue. In this way, a high level of exposure in a localized region -- e.g. bronchial epithelium -- is averaged out over the entire tissue mass, suggesting a low level of exposure. However, alpha particles have a range of only 40 um in the body. A cell nucleus of 5 to 6 um that is traversed by a single alpha particle receives a dose of 1000 rems. Thus, although the total tissue dose might be considered negligible, cells close to an alpha source receive high doses. The Po-210 alpha activity of cigarette smoke may be a very effective carcinogen if a multiple mutation mechanism is involved.

Radford and Hunt have determined that 75 per cent of the alpha activity of cigarette smoke enters the ambient air and is unabsorbed by the smoker, (2) making it available for deposit in the lungs of others. Little et al. have measured levels of Po-210 in the lungs of nonsmokers that may not be accounted for on the basis of natural exposure to this isotope.

The detrimental effects of tobacco smoke have been considerably underestimated, making it less likely that chemical carcinogens alone are responsible for the observed incidence of tobacco-related carcinoma. Alpha emitters in cigarette smoke result in appreciable radiation exposure to the bronchial epithelium of smokers and probably secondhand smokers. Alpha radiation is a possible etio- logic factor in tobacco-related carcinoma, and it deserves further study.

Thomas H. Winters, M.D.
Joseph R. Di Franza, M.D.
University of Massachusetts Medical Center
Worcester, Ma 01605

How much radiation dose is received by a cigaret
te smoker?

Interestingly, this subject was initially investigated some 40 years ago by scientists at the School of Public Health at Harvard University. Working with physicians in the neighboring Harvard teaching hospitals, they were able to obtain lungs taken during autopsies of smokers who had died from lung cancer. The School of Public Health scientists carefully analyzed samples from selected areas of these lungs and found that they contained relatively high concentrations of 210Po (polonium-210), a naturally occurring radionuclide that the International Commission on Radiological Protection considers to be one of the most hazardous of all radioactive materials. In fact, it is far more hazardous than 239Pu (plutonium-239). Of particular significance was that the Harvard studies showed that this radionuclide tended to concentrate in "hot spots" at bifurcations of segmental bronchi within the lungs, precisely the areas where lung cancer originates among cigarette smokers.

Armed with this information, studies were conducted to determine the source of the 210Po. Although the initial assumption was that it was taken up by the tobacco plant from the soil, the investigations revealed that it was deposited on the leaves of the plants (which are large and sticky) from the air. Just as the decay of naturally occurring radium in the soil often results in the presence of relatively high concentrations of radon and its radioactive decay products in the air inside buildings, the decay of radium in the soil outdoors results in the presence of radon and its decay products in the surrounding air. Whereas radon is a gas, its radioactive decay products are solids. Enhancing the adherence of these decay products to the tobacco leaves is the fact that they are electrically charged and readily adhere to any surface with which they come into contact. When a smoker lights a cigarette, the 210Po is volatilized and, when he/she inhales, it is deposited in the lungs.

Based on careful assessments of the concentrations of 210Po in the lung tissues, it was estimated that the "hot spots" received an annual dose of about 160 millisievert (about 16,000 millirem), two of the more common units for expressing doses from ionizing radiation. To provide perspective, it is useful to compare this dose to the limit stipulated, for example, by the US Environmental Protection Agency for members of the US public. Making this difficult in this case, however, is that the annual dose limit for members of the public (1 millisievert, or 100 millirem) is expressed in terms of a dose to the whole body, whereas, as noted above, the dose to a smoker is limited to a very small portion of the body.

Nonetheless, in a report published in 1987, the National Council on Radiation Protection and Measurements (NCRP 1987) sought to make such a comparison, the tentative outcome of which suggested that the annual dose to a smoker (when converted into an equivalent dose to the whole body) was more than 10 times the annual dose limit for a member of the public. Having provided this estimate, however, the NCRP went on to state that they would prefer not to make such a comparison. That is to say, the comparison to the annual whole-body dose limit may not be completely valid.

In a similar manner, the scientists at Harvard, while acknowledging that the dose to a smoker was high, were quick to recognize that 210Po was only one of a multitude of carcinogenic compounds in cigarettes. For this reason, they were hesitant to cite 210Po as a primary source of lung cancer among cigarette smokers. Based on these considerations, their conclusion was that "we believe 210Po may be an important factor in the initiation of bronchial carcinoma in humans" (Little et al. 1965).

Dade W. Moeller, CHP, PhD

Little JB, Radford EP Jr, McCombs HL, Hunt VR. Distribution of Polonium210 in pulmonary tissues of cigarette smokers. New England Journal of Medicine 273: 1343-1351; 16 December 1965.

National Council on Radiation Protection and Measurements. Radiation exposure of the US population from consumer products and miscellaneous sources. Bethesda, MD: NCRP; NCRP Report No. 95; 1987.