Reports
Developing Comparisons for Risks Due to Plutonium Inhalation and Carbon Tetrachloride Exposure
by David Albright and Lauren Barbour
May 14, 1999
Part 1: Background
The Historical Public Exposure Studies on Rocky Flats examine the potential exposure to contaminants and the related health risks posed to residents living near Rocky Flats between 1952 and 1989. The first phase of the exposure study investigated past operations at Rocky Flats and identified the materials of concern, release points and quantities released. Phase 2 of this study has investigated in depth the doses and risks from materials released.
The results of Phase 2 of the Historic Public Exposure study will be presented quantitatively; the risk from the materials released will be expressed as ranges of numbers. These values, however, can be difficult to comprehend. As a result, comparisons of the risks associated with exposure at Rocky Flats to more familiar everyday risks are used to foster more complete public understanding. However, comparisons must be selected carefully so that they do not inadvertently minimize or maximize the reported dose or risk. In order to accurately communicate the findings of the health study, this report analyzes a variety of common risks and subsequently recommends appropriate risk comparisons for the plutonium and carbon tetrachloride releases at Rocky Flats.
Part 2: About Risk
There are many definitions of risk, but generally, risk is the probability that negative effects will result from a specific activity. While discussions of risk may evoke visions of skydiving or alligator wrestling, people accept risks even when engaging in the most mundane tasks. We weigh the benefits of driving to the grocery store against the possibility of death in a car accident; we balance the risk of cancer posed by a high fat diet with the pleasure of eating potato chips.
Epidemiologists compile statistics on the numbers and causes of deaths and from this data calculate risks. For example, the probability of dying in a car accident is calculated from the number of motor vehicle deaths in a year compared to the total number of drivers; this number is the annual risk of driving a car. Likewise, but less straightforward, carcinogens can be evaluated as to their probability of causing cancer. This number is the risk of developing cancer as a result of exposure to a chemical or a source of radiation.
To establish a point of reference for risk in general, Tables 1 and 2 list the risks of several everyday activities that can lead to accidental death. The risks these activities posed both within Colorado and the United States were compiled from data provided by the National Safety Council and the state of Colorado. For each activity, the number of fatalities both nationally and within Colorado was divided by the number of participants in order to determine the annual risk of death from each activity. This annual risk was then multiplied by the estimated duration of the risk, for a lifetime risk of death.
Table 1 contains the risks of dying in home accidents, vehicle accidents, natural disasters and skiing accidents for 1996. While these are illustrative of the risks each activity poses, the number of deaths from a specific cause can vary widely from year to year. To show the variability in risks, Table 2 contains the risks certain activities posed in 1997. For example, 251 Coloradans died from home accidents in 1996, while 302 died from the same causes in 1997. This increase in fatalities resulted in about a 15% increase in the annual risk, but surely when averaged over several years this variation in the risk of death would not be so great.
In addition to risks of accidental death, people are exposed daily to background radiation which poses a risk of causing cancer or genetic effects. This radiation comes from natural and artificial sources. Among the natural sources, radon supplies the largest radiation exposure. Radon is a colorless odorless radioactive gas produced by the radioactive decay of radium, which originates from uranium 238 in soils and rocks. Radon itself decays into alpha-emitting radioactive solids which can be deposited in the lungs when inhaled. Table 3 presents a comparison of concentrations of radon in the US and select Colorado counties. Due to local geology, Colorado has significantly higher than average indoor radon levels. The EPA considers indoor radon levels greater than 4.0 picocuries/L to be hazardous. Table 3 also shows the risks of dying from cancer caused by radon; Table 4 presents the risk of cancer incidents, or contracting cancer, due to exposure to radon.
Other natural sources include cosmic, terrestrial and internal radiation. Cosmic radiation consists of both the energetic protons, alpha particles and electrons that strike the atmosphere and the secondary particles they generate as they pass through the air. A person’s exposure to cosmic radiation is greater at higher altitudes. Terrestrial radiation is predominantly gamma radiation received from the radioactive elements in rocks and soil. This exposure is related to the local geology and composition of the soil, but on average consists of gamma radiation from airborne radon (and its daughters) and potassium 40 and thorium 232 (and its daughters) in soil. Table 5 presents both the terrestrial and cosmic radiation doses for the United States and Colorado and the lifetime risks they pose. Because the soil in Colorado is higher in radioactive elements than other US soils, there is greater terrestrial radiation here. And towns, like Denver, at high altitudes will be subject to more cosmic rays than cities at sea level. Internal radiation exposure comes from natural radionuclides deposited within the body which are breathed, or ingested with food and water.
Medical radiation is one source of artificial background radiation. It includes exposure from diagnostic x-rays and nuclear medicine. Radiation from consumer products is a second artificial source. It includes radioactive elements in building materials, airport inspection systems and smoke detectors.
Table 6 summarizes the average annual background radiation doses the US population receives from natural and artificial sources. A risk of dying from cancer caused by exposure to this background radiation was then calculated. Table 7 presents the same information, except that it details the doses from background radiation that Denver residents receive.
In order to develop risk comparisons for the risks posed by materials releases from Rocky Flats, we first generated a list of risks and activities Americans routinely face and then evaluated those risks as to their relevance. After generating a list of common risky activities, we divided them into two categories: voluntary and involuntary. Table 8 contains examples of these types of risks. Voluntary risks are undertaken with full knowledge of the possible consequences and are consensual. Before participating in these activities, e.g. flying across the country or scuba diving, the public is aware of the chance of death and accepts this risk. Cancer or death are the possible consequences of these voluntary actions.
In contrast, involuntary risks are risks that are unwittingly taken or are outside the public’s control. Solar radiation and being struck by lightning are two examples of involuntary risks. Involuntary risks come in a number of types; they can be a chronic risk from a natural phenomenon or a man-made source, or be an immediate risk from an “act of god” or sudden/imposed event.
Although we have created a dichotomy of voluntary and involuntary risk in order to aid the comparisons of different types of risk, no categorization scheme is perfect. Ultimately the perception of a risk depends upon many factors. For example, risks from radon or x-rays may be viewed as either voluntary or involuntary depending on the circumstances. Nevertheless, these categories are typical of those used in risk comparisons.
Risk must also be evaluated in terms of the emotional reaction it evokes. The level of public anger or fear about a risk is called “outrage”. Public perception of a risk as involuntary, uncontrollable, immoral, or unfamiliar may lead to outrage. This emotional response must be taken into account when risks are communicated. In this report, we have assessed each risk as to its potential to cause outrage.
Part 3: Developing appropriate risk comparisons for Plutonium exposure at Rocky Flats
In determining appropriate risks for comparison to plutonium exposure at Rocky Flats, we generated a list of risks similar to that posed by plutonium. Plutonium is a long-lived alpha-emitting material. Phase 1 of the Historical Public Exposure study determined that the principal off-site risk from Rocky Flats plutonium was via direct inhalation.
We divided these similar risks into voluntary and involuntary categories (see Tables 9 and 10). We assigned each activity an “outrage” value, based on its potential to cause public anger, fear, defensiveness, frustration or suspicion. Risks were also rated as to their appropriateness to this study on a scale of 1 to 5, where one is the most appropriate and five is the least appropriate. Since the main danger of inhaling plutonium is that it lodges in the lungs and emits alpha radiation to the surrounding tissue, hazards that would effect the body similarly were found to be most appropriate.
From this assessment we found that few risks are comparable to the historical off-site plutonium risks from operations at Rocky Flats. Table 11 summarizes the risks that received a scoring of one or two in appropriateness: radon, plutonium fallout from nuclear tests, polonium 210 in second-hand smoke, and plutonium accidents or releases from other sites. Because local citizens were not informed of the site’s operations, voluntary risks and “acts of god” are not similar to the risks associated with plutonium production at Rocky Flats and so are not effective comparisons. Radon is the only risk in Table 11 that is both appropriate and not typically associated with an outrage factor.
We recommend both the chronic exposure to radon and plutonium fallout as risks with which the exposure to plutonium from Rocky Flats should be compared in a detailed quantitative manner. Radon is a natural source of radiation and has no outrage associated with it. Plutonium fallout from nuclear tests does contain a factor of outrage.
The risks from polonium 210 in second-hand smoke and plutonium accidents at other sites will be complicated to calculate; they will require many decisions about the exposure scenario which may introduce error and bias. Further, if the public suspects that there are exterior motives effecting the results, then the risk comparisons will be undermined.
Presenting plutonium risk comparisons
Based on Table 11, the following specific plutonium risk comparisons can be analyzed:
Part 4: Developing appropriate risk comparisons for carbon tetrachloride exposure at Rocky Flats
Carbon tetrachloride is a solvent that was used to remove grease and to clean components and equipment at Rocky Flats. It is a carcinogen in animals, but not a proven carcinogen in humans; to be cautious the EPA classifies carbon tetrachloride as a probable human carcinogen. Carbon tetrachloride does, however, cause liver damage. People are most commonly exposed to carbon tetrachloride by breathing it, drinking contaminated water or from contact with contaminated soil.
For carbon tetrachloride risk comparisons we contrasted the risk of liver cancer from exposure to carbon tetrachloride with the risk of liver damage or cancer from other agents. Like the risk comparisons for plutonium exposure, the risk comparisons for carbon tetrachloride were divided into voluntary and involuntary categories ( Tables 12 and 13). The involuntary risks stem from either chronic exposure or immediate sources and are categorized accordingly.
As in the plutonium assessment, we determined the appropriateness of the risks to the health study and the outrage these risks might cause. In general terms, the carbon tetrachloride exposure at Rocky Flats was the result of unannounced releases of a liver toxin to the environment. We based the appropriateness score on how closely each activity resembled that scenario. As with plutonium, we found that few risks are comparable to the historical off-site exposure to carbon tetrachloride from operations at Rocky Flats. While none of the risks received a score of one, Table 13 summarizes the risks that received a scoring of two or three: carbon tetrachloride background levels, working with vinyl chloride before OSHA, involuntary exposure to household chemicals or pesticides, and carbon tetrachloride accidents at other sites. All of these risks are associated with outrage as well. The voluntary activities (Table 12) which can lead to liver damage, are not similar to the inadvertent exposure to carbon tetrachloride at Rocky Flats and thus are not appropriate comparisons.
Benzene is one of the few chemicals which is a proven human carcinogen. Several studies have shown that exposure to benzene has led to an increased incidence of leukemia. Although benzene and carbon tetrachloride have different effects on the human body and a direct comparison between the two chemicals is not valid, a comparison of benzene and carbon tetrachloride may highlight the risks the public takes with a known carcinogen. Benzene is an industrial chemical used as a solvent and as a starting material in pharmaceutical development; it is also a gasoline additive. Common sources of exposure, besides occupational contact, are from drinking water, cigarettes and the combustion of gasoline.
There are no federal standards specifically for emissions of carbon tetrachloride to the air. The EPA has set a maximum permissible level of carbon tetrachloride in drinking water at 5 parts per billion (ppb). The EPA maximum permissible level of benzene in drinking water is also 5 ppb.
Presenting carbon tetrachloride risk comparisons
Based on Table 14, we list specific carbon tetrachloride risk comparisons below:
Part 5: Conclusions
Our work assumes that the principle audiences for these risk comparisons are the general public and the media. Scientists, government officials, and other specialists may require other risk comparisons. However, our belief is that risk comparisons for those three audiences are easier to develop.
In presenting both plutonium and carbon tetrachloride risks, we do not recommend that the committee use voluntary risk comparisons, except as an aid to further understanding of risk and risk comparisons. Since the public’s exposure to plutonium and carbon tetrachloride was the result of unannounced releases over more than 35 years, it is not appropriate to compare these risks to those of voluntary activities.
For plutonium exposure, we recommend drawing two comparisons: compare the public exposure to plutonium from Rocky Flats to risks from both chronic exposure to radon and plutonium fallout from weapons tests. The next steps are to develop quantitative comparisons for presentation to the public.
For carbon tetrachloride exposure, we recommend drawing the comparison between carbon tetrachloride background levels and the off-site exposure the public received. It may also be illustrative to equate the carcinogenic properties and risks of exposure to benzene and the risks posed by exposure to carbon tetrachloride at Rocky Flats.
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