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HEALTH ALERT From the |
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Radioactive Tobacco
[Radiologic Technology]
[Northern Light Technology LLC]
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Source: Radiologic Technology
Date: 01-02/1996
Document ID: LW19971007060004181
Subject(s): Tobacco--Physiological
aspects; Carcinogenicity
testing--Analysis;
Carcinogens--Physiological
aspects
Citation Information: (v67 n3) Start Page:
p217(6) ISSN: 0033-8397
Author(s): Kilthau, Gustave F.
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[Radiologic Technology]
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Cancer risk in relation to radioactivity in tobacco.
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Leaf tobacco contains minute amounts of lead 210 (210Pb) and polonium
210 (210Po) both of which are radioactive carcinogens and both of
which can be found in smoke from burning tobacco. Tobacco smoke also
contains carcinogens that are nonradioactive.
People who inhale tobacco smoke are exposed to higher concentrations of
radioactivity than nonsmokers. Deposits of 210Pb and alpha
particle-emitting 210Po form in the lungs of smokers, generating
localized radiation doses for greater than the radiation exposures humans
experience from natural sources. This radiation exposure, delivered 'to'
sensitive tissues for long periods of time, may induce cancer both alone
and synergistically with nonradioactive carcinogens.
This article explores the relationship between the radioactive and
nonradioactive carcinogens in leaf tobacco and tobacco smoke and the risk
of cancer in those who inhale tobacco smoke.
Almost all externally-induced cancer in humans is reported to be caused by
cigarette smoking, alcohol and some foods.[1] In the 1960s it was reported
that leaf tobacco and tobacco smoke contained radioactivity,[2-6] and it
was noted that people who inhale tobacco smoke retain smoke-borne
radioisotopes in their lungs.[2,4,6]
Leaf tobacco contains minute quantities of radioactive isotopes that pose a
radiation exposure hazard to those who intentionally or passively inhale
tobacco smoke. This article reviews scientific literature documenting
radioactivity in leaf tobacco, radioactivity in tobacco smoke, the
concentrations of radioactivity on and within the tissues of those who
inhale tobacco smoke, the radiation doses to organs and tissues, and the
significance of these exposures toward cancer initiation in those who
inhale the smoke. Because synergistic effects toward cancer initiation
exist between smoke-borne radioactivity and inhaled nonradioactive
carcinogens, the article also discusses some of the nonradioactive
carcinogens in tobacco.
Discussion
Identification of 210Pb and 210Po
In Tobacco Smoke and in Smokers
In the 1960s, investigators reported that lead 210 (210Pb) and
polonium 210 (210Po) are present in both gaseous and particulate
phases of tobacco smoke.[2-4] Both radioisotopes descend from radium 226
and its decay product, radon 222. Lead 210 decays by beta particle emission
to bismuth 210, which then decays by beta particle emission to 210Po.
Polonium 210 emits high energy alpha particles (5.3 million eV) and gamma
radiation (550,000 eV) when it decays, becoming stable lead 206.[7,8]
Tar in tobacco smoke traps 210Po on lung epithelium, particularly at
the bifurcations of peripheral bronchioles, leading to very significant
localized radiation doses.[2,4] It also was reported[2] that as low as 36
rem exposure to bronchial epithelium of a smoker during 25 years of smoking
is significant to the induction of lung cancer due to the coincidental
presence of nonradioactive carcinogens in the smoke.
Some investigators[4] believe the quantities of nonradioactive carcinogens
in tobacco smoke are too small, by themselves, to generate the lung cancer
rates caused by smoking. Supporting this belief, it has been shown[2,5]
that the urine of smokers contains about six times more 210Po than
the urine of nonsmokers, and that the rate of bladder cancer among smokers
increases in relationship to how much they smoke. Nonradioactive
carcinogens in tobacco tar are not found in the urine of smokers, no matter
how heavily they smoke.[5]
210Pb and 210Po in Leaf Tobacco
The amount of 210Pb and 210Po radioactivity in leaf tobacco is
minute per gram of tobacco. This low concentration of radioisotopes,
however, can accumulate into significant concentrations in and on the
tissues of those who inhale the smoke from burning tobacco.
Tobacco plants absorb 210Pb and 210Po from the soils in which
they grow.[9-13] In addition, tobacco plants gather naturally present radon
222 descendants from the surrounding air.[14,15] Tobacco leaves have sticky
trichomes, or "hairs," on both sides.[9,14] Radon daughter products collect
on aerosols in the atmosphere which, in turn, are captured on the sticky
surfaces of the trichomes. This provides an additional concentration of
210Pb on leaf surfaces beyond its concentration within the whole
leaf.[14] It has been shown[16] that tobacco leaf trichomes capture
atmospheric aerosols, polymerize with them in the heat of burning tobacco
and are present in that form in cigarette smoke.
The 210Po content of tobacco from several countries has been
measured. One report[17] on the radioactivity of tobacco grown in India
indicated that a single Indian-grown tobacco cigarette had a 210Po
complement of up to 0.4 pCi. Another group from India[13] found a great
difference between the 210Po content of Indian-grown tobacco and
tobacco from the United States. The 210Po in Indian tobacco averaged
0.09 pCi per gram, whereas the 210Po in tobacco grown in the United
States averaged 0.516 pCi per gram--about 5 1/2 times as much
radioactivity.
Although such sizable differences in radioactivity concentration in leaf
tobacco may be related to variations in natural fallout, natural soil
radioactivity or absorption differences due to soil pH, another factor may
be responsible. It has been noted[9,19] that modern tobacco farming takes
advantage of special fertilization methods, and that tobacco leaf grown in
soil with low nitrogen levels is "more flavorful" than tobacco leaf grown
in soils high in nitrogen.
To grow this quality of tobacco, farmers in "developed" countries such as
the United States usually fertilize their tobacco fields with chemically
manufactured fertilizer high in phosphate content. Tobacco farmers in
poorer countries do not. The phosphate portion of this fertilizer is made
from a rock mineral, apatite, that is ground to powder, dissolved in acid
and further processed (personal communication, Mobil Mining and Minerals,
Houston, Texas, 1995). Apatite rock contains radium and its many descendant
radioisotopes, including both radioactive lead and polonium.(210Po)
When this type of fertilizer is spread onto tobacco fields year after year,
soil nitrogen is depleted, providing a "more flavorful" smoking tobacco.
The higher the phosphate level of the fertilizer used, the higher the
concentration of 210Pb and 210Po in the tobacco leaves.[19]
A measurement of the 210Po content of mainstream cigarette smoke from
U.S.-grown tobacco is reported to be 0.0263 pCi per cigarette;[20] which is
about 0.1 pCi per milligram of smoke. Other investigators[14] have measured
the 210Po concentration in the mainstream tobacco smoke of one
cigarette as approximately 0.036 pCi, with a corresponding measurement of
0.81 pCi of 210Pb per gram of dry condensate derived from the whole
smoke.
The filtration of mainstream tobacco smoke by ordinary commercial cigarette
filters has a negligible effect on the concentration of radioactivity in
the smoke inhaled into the lungs of smokers.[2,6,19,20] It has been
estimated that the intake of 210Po by a typical smoker is about 0.72
pCi per pack of 20 cigarettes.[21] In another study,[14] it was noted that
210Pb specific activities of 100 pCi per milligram of pyrolized
glandular heads of tobacco leaf trichomes in tobacco smoke often are
reached or exceeded.
It also has been reported[14] that radioactive lead and polonium are
adsorbed onto tobacco smoke particles vented into room air from burning
tobacco, where they remain suspended and available until inhaled as
"secondhand" smoke by anyone present in the room.
Concentrations of 210Pb and 210Po
In the Bodies of Those Who Inhale Tobacco Smoke
Compared to nonsmokers, heavy smokers essentially have four times greater
radioisotope density throughout their lungs.[21] It has been estimated[22]
that the 210Po content of blood in smokers averages 1.72 pCi per
kilogram and, in nonsmokers, 0.76 pCi per kilogram. Concentrations of
210Pb and 210Po in rib bones and alveolar lung tissue were
found to be twice as high in ax-smokers as in nonsmokers, even a year after
cessation of smoking.[21]
In smokers, the concentration of 210Po directly on epithelial tissue
at segmental bifurcations of bronchioles is two orders of magnitude greater
(i.e., 100 times greater) than is its concentration overall within their
lungs,[23] which already is four times higher in heavy smokers than it is
in nonsmokers.[21] Other investigators[24] found that the lungs, blood and
livers of smokers contained significantly more 210Po than did those
of nonsmokers.
Dosage from the Radioactivity
From Inhaled Tobacco Smoke
Polonium 210 emits alpha particles upon its decay. Alpha particles have
penetrations limited to about 40 microns or less in animal tissue,[8,25,26]
the aggregate diameter of only several typical cells. Alpha particle
radiation has a very destructive effect on animal tissue because virtually
all of its very high ionizing energy is expended within the tissue. Due to
its double positive charge, limited range in tissue and enormously high
energy, an alpha particle can produce huge numbers of ion pairs in
substances with which it interacts. For example, 20,000 ion pairs can be
produced per alpha particle per centimeter path length in air.[8] DNA
chromosome damage by alpha particle radiation is much greater, by 100
times, than by exposure of DNA to other types of radiation.[19]
The radiation dose from 210Po alpha particle radiation has been
measured as 82.5 millirads per day for heavy smokers.[27] Extrapolating
this measurement, doses of 30.1 rads per year and 752.5 rads per 25 years
of smoking two packs of cigarettes per day are calculated. Such a radiation
exposure dose rate is about 150 times higher than the approximately 5
rem([dagger]) per 25 years received from natural background radiation
sources.[2] Interestingly, many of the lung cancers contracted by cigarette
smokers are adenocarcinomas, a type of lung cancer that can be caused by
alpha particle radiation from 210Po.[19]
In localized areas of tissue that surround deposits of insoluble
210Pb particles, the dose rate from 210Po alpha particle
radiation can be from 100 to 10,000 times that of natural background
radiation sources.[19] The "low-polonium" tobacco grown in India provides
its users with a lung burden of about 24 millirads a day[13]--or 219 rads
during 25 years of smoking. This is about 40 times the exposure rate from
natural background radiation inhaled from the earth's atmosphere.[2] Other
researchers[2,4,14,26,28] have estimated that a range of dose rates from
210Po alpha particle exposure of lung epithelial tissue in smokers is
from 165 rem to 1000 rem over a period of 25 years.
Carcinogenicity of Low Dose Rate Radiation Exposure
Those who directly inhale tobacco smoke receive alpha particle bombardment
totalling many rads over many years of smoking.[14,23,27,28] The frequency
of harmful effects from low dose rate radiation exposure is proportional to
the total dose received over time.[29] The risk of cancer initiation for
any cumulative radiation dose increases significantly at lower dose rates
in accordance with the lengthening of exposure periods[14] such as those
experienced by smokers. Investigators have shown that inhalation of tobacco
smoke causes more DNA damage in smokers than in nonsmokers.[30] DNA damage
is reported to be associated with cancer initiation.[19]
Lung cancer has been induced in test animals using less than one-fifth the
210Po exposure experienced by a two-pack-a-day smoker during 25 years
of smoking.[31] It has been suggested[19,32] that 210Po accounts for
many, if not all, cigarette smoke-induced lung cancers.
Polonium 210 is a "bone seeker." In other words, bone tissue avidly takes
up available 210Po.[8] Ionizing radiation delivered to bone marrow at
relatively low rates (e.g., 7 rads to 13 rads total exposure) has been
reported[33] to induce the onset of leukemia at relatively high rates per
red of exposure.
Despite this evidence, there has been doubt concerning the role of
radioactivity in general and 210Po, specifically, as prime cancer
initiators in those who inhale tobacco smoke.[26,34] Because polonium is
water soluble, could it linger in the lungs long enough to cause cancer?
It might ordinarily be thought that inhaled 210Pb particles and
210Po would be readily cleared from the lungs by ciliary action or be
otherwise excreted, even though tobacco smoke inhalation results in
decreased ciliary activity in the bronchioles.[35,36] However, a continuing
alpha particle bombardment from 210Po is caused by pockets, or
concentrations, of insoluble 210Pb and polymerized tobacco leaf
trichome-210Pb entities because the 210Pb decays to become
210Pb[7,8] in or on the affected tissue.[14,15,19] Another impediment
to clearance of 210Po from the lungs of those who inhale tobacco
smoke is the "locking down" of the radioisotope by tobacco tar present in
the smoke.[2,4,23,26,32]
Much of the experimental work performed to assess the carcinogenicity of
tobacco smoke has been done using mouse skin assays in which tobacco smoke
distillates are placed onto shaved areas of mouse skin to look for
development of cancer.[34] However, because of the unique mechanics of
210Pb and 210Po deposition in the lungs, mouse skin assays are
inadequate for assessing the role of those radioisotopes in lung cancer
initiation.[26]
Synergistic Effects from Tobacco Smoke
Radioactive Contamination of Meat and Dairy
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