Radon is a naturally occurring gas that
seeps out of rocks and soil. Radon comes from uranium that has been in the
ground since the time the earth was formed, and the rate of radon seepage is
very variable, partly because the amounts of uranium in the soil vary
considerably. Radon flows from the soil into outdoor air and also into the air
in homes from the movement of gases in the soil beneath homes. Outside air
typically contains very low levels of radon, but it builds up to higher
concentrations indoors when it is unable to disperse. Some underground mines,
especially uranium mines, contain much higher levels of radon.
Although radon is chemically inert and
electrically uncharged, it is radioactive, which means that radon atoms in the
air can spontaneously decay, or change to other atoms. When the resulting
atoms, called radon progeny, are formed, they are electrically charged and can
attach themselves to tiny dust particles in indoor air. These dust particles
can easily be inhaled into the lung and can adhere to the lining of the lung.
The deposited atoms decay, or change, by emitting a type of radiation called
alpha radiation, which has the potential to damage cells in the lung. Alpha
radiations can disrupt DNA of these lung cells. This DNA damage has the
potential to be one step in a chain of events that can lead to cancer. Alpha
radiations travel only extremely short distances in the body. Thus, alpha
radiations from decay of radon progeny in the lungs cannot reach cells in any
other organs, so it is likely that lung cancer is the only potentially
important cancer hazard posed by radon.
For centuries, it has been known that
some underground miners suffered from higher rates of lung cancer than the
general population. In recent decades, a growing body of evidence has causally
linked their lung cancers to exposure to high levels of radon and also to
cigarette smoking. The connection between radon and lung cancer in miners has
raised concern that radon in homes might be causing lung cancer in the general
population, although the radon levels in most homes are much lower than in
most mines. The National Research Council study, which has been carried out by
the sixth Committee on Biological Effects of Ionizing Radiation (BEIR) VI, has
used the most recent information available to estimate the risks posed by
exposure to radon in homes.
The most direct way to assess the risks
posed by radon in homes is to measure radon exposures among people who have
lung cancer and compare them with exposures among people who have not
developed lung cancer. Several such studies have been completed, and several
areunder way. The studies have not produced a definitive answer, primarily
because the risk is likely to be very small at the low exposure encountered
from most homes and because it is difficult to estimate radon exposures that
people have received over their lifetimes. In addition, it is clear that far
more lung cancers are caused by smoking than are caused by radon.
Since a valid risk estimate could not be
derived only from the results of studies in homes, the BEIR VI committee chose
to use the lung-cancer information from studies of miners, who are more
heavily exposed to radon, to estimate the risks posed by radon exposures in
homes. In particular, the committee has drawn on 11 major studies of
underground miners, which together involved about 68,000 men, of whom 2,700
have died from lung cancer. The committee statistically analyzed the data to
describe how risk of death from lung cancer depended on exposure. In this way,
the committee derived two models for lung cancer risk from radon exposure.
In converting radon risks from mines to
homes, the committee was faced with several problems. First, most miners
received radon exposures that were, on the average, many times larger than
those of people in most homes; people in a few homes actually receive radon
exposures similar to those of some miners. It was necessary for the committee
to estimate the risks posed by exposures to radon in homes on the basis of
observed lung cancer deaths caused by higher exposures in mines. The committee
agreed with several earlier groups of experts that the risk of developing lung
cancer increases linearly as the exposure increases; for example, doubling the
exposure doubles the risk, and halving the exposure halves the risk.
Furthermore, the existing biologic evidence suggests that any exposure, even
very low, to radon might pose some risk. However, from the evidence now
available, a threshold exposure, that is, a level of exposure below which
there is no effect of radon, cannot be excluded.
The second problem is that the majority
of miners in the studies are smokers and all inhale dust and other pollutants
in mines. Because radon and cigarette smoke both cause lung cancer, it is
complicated to disentangle the effects of the 2 kinds of exposure. That makes
it especially difficult to estimate radon risks for nonsmokers in homes using
the evidence from miners. A final problem is that the miners were almost all
men, whereas the population exposed to radon in homes also includes men,
women, and children.
The committee used the information from
miners and supplemented it with information from laboratory studies of how
radon causes lung cancer. Then, with facts about the US population, including
measurements of radon levels in homes, it estimated the number of lung-cancer
deaths due to radon in homes. In 1995, about 157,400 people died of lung
cancer (from all causes including smoking and radon exposure) in the United
States. Of the 95,400 men who died of lung cancer, about 95% were probably
ever-smokers; of the 62,000 women, about 90% were probably ever-smokers.
Approximately 11,000 lung cancer deaths are estimated to have occurred in
never-smokers in 1995.
The BEIR VI committee’s preferred
central estimates, depending on which one of the two models are used, are that
about 1 in 10 or 1 in 7 of all lung-cancer deaths—amounting to
centralestimates of about 15,400 or 21,800 per year in the United States—can
be attributed to radon among ever-smokers and never-smokers together. Although
15,400 or 21,800 total radon-related lung-cancer deaths per year are the
committee’s central estimates, uncertainties are involved in these
estimates. The committee's preferred estimate of the uncertainties was
obtained by using a simplified analysis of a constant relative risk model
based on observations closest to residential exposure levels. The number of
radon-related lung-cancer deaths resulting from that analysis could be as low
as 3,000 or as high as 32,000 each year. Most of the radon-related lung
cancers occur among ever-smokers, and because of synergism between smoking and
radon, many of the cancers in ever-smokers could be prevented by either
tobacco control or reduction of radon exposure. The committee’s best
estimate is that among the 11,000 lung-cancer deaths each year in
never-smokers, 2,100 or 2,900, depending on the model used, are radon-related
lung cancers.
Radon, being naturally occurring, cannot
be entirely eliminated from our homes. Of the deaths that the committee
attributes to radon (both independently and through joint action with
smoking), perhaps one-third could be avoided by reducing radon in homes where
it is above the "action guideline level" of 148 Bqm-3 (4
pCiL-1) to below the action levels recommended by the Environmental
Protection Agency.
The risk of lung cancer caused by
smoking is much higher than the risk of lung cancer caused by indoor radon.
Most of the radon-related deaths among smokers would not have occurred if the
victims had not smoked. Furthermore, there is evidence for a synergistic
interaction between smoking and radon. In other words, the number of cancers
induced in ever-smokers by radon is greater than one would expect from the
additive effects of smoking alone and radon alone. Nevertheless, the estimated
15,400 or 21,800 deaths attributed to radon in combination with
cigarette-smoking and radon alone in never-smokers constitute an important
public-health problem.
