Answer to Question #3137 Submitted to "Ask the Experts"

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

References
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.