Public concern about risks associated with radioactive-waste disposal, recent large-scale outbreaks of serious disease from microorganisms such as Cryptosporidium in drinking water, and disasters from natural hazards such as floods, earthquakes, and hurricanes are reminders that chemicals do not constitute the only environmental threats to public health. In many situations, people (and ecosystems) are exposed to combinations of radiation, chemicals, and infectious agents--a mixtures problem (see section 3.3). In many others, comparisons and tradeoffs among types of risk are necessary (for example, risks associated with byproducts of drinking-water disinfection versus those associated with microorganisms contaminating drinking water). Concurrent chemical, radiation, and microbial exposures will have to be evaluated in some situations when the risk-management framework is implemented.

It is surprising that environmental protection seems to be focused so predominantly on chemicals and so little on ionizing radiation and microorganisms. There is no doubt about the many serious health effects of exposure to radiation and microorganisms, whereas the effects of many regulated chemicals are of uncertain importance for humans. Nell Ahl, director of the risk-analysis program at the Department of Agriculture, expressed concern to the Commission about the disproportionate official emphasis placed on chemical hazards, in view of the public outrage that is rightly engendered by the deaths and other effects caused by microbial contamination of food, such as the recent, toxic E. coli contamination of under-cooked hamburger. The public-health consequences of exposing patients and workers to ionizing radiation and of exposing the general population to infectious agents are so well established that they might be in the category of "familiar" risks, which psychologists have shown are far less frightening to the general public than "unfamiliar" or "dreaded" risks, even when the estimated magnitudes of the former are much higher. Small estimated risks from radiation, especially from potential radiation releases from nuclear power plant operations or wastes, continue to attract considerable public concern. For example, in testimony before the Commission in St. Louis, Kay Drey, of the Missouri Coalition for the Environment, expressed a deep concern about our country's ability to manage its radiation hazards and particularly the anticipated decommissioning of nuclear power plants at the end of their useful lives.

FINDING 3.6.1: Risk-assessment methods for radiation hazards are well established, and regulatory strategies for occupational and environmental radiation exposures have been in place for many years. An elaborate standards process uses extragovernmental organizations, such as the National Council for Radiation Protection and Measurement and the International Council for Radiation Protection; lead agencies are the Nuclear Regulatory Commission, Department of Energy, EPA Office of Air and Radiation, and FDA Division of Radiological Health. Scientists and regulators dealing with chemical hazards or with radiation hazards have been remarkably independent of each other and have given little attention to medical, industrial, disposal, nuclear power, and nuclear weapons production settings when radiation exposures and chemical contamination co-exist.

RECOMMENDATION: A concerted effort should be made to relate the methods, assumptions, mechanisms, and standards for radiation risks to those for chemicals, to enhance the comparability of risk-management decisions and investments.

RATIONALE

The radiation-protection literature began with devastating accounts of early neglect of the health hazards of a new technology--the use of Roentgen rays (x rays), discovered in 1895 and immediately introduced into medical practice. Pioneering scientists and workers developed internal cancers and radiation burns of the skin. Radiation can affect genes, chromosomes, cell survival, and regeneration of rapid turnover tissues. The skin, bone marrow, intestine, oocytes, spermatogonia, lens, and respiratory tract are most typically affected.

Natural sources of ionizing radiation include cosmic rays; radium and other radioactive elements in the earth's crust; internally deposited potassium-40, carbon-14, and other radionuclides normally present in living cells; and inhaled radon and its progeny. The doses received from cosmic rays vary appreciably with altitude, so exposure is twice as high in Denver as at sea level and 100 times higher at jet-aircraft altitudes. The largest exposures come from inhaled radon-222, a colorless, odorless, alpha-particle-emitting gas formed by the radioactive decay of radium-226 in the earth. Human exposure to radon varies--according to its concentration in indoor air--by more than a factor of 10. Smokers expose themselves to polonium-210--another decay product of radium--in tobacco at up to 0.2 Sv/year, or 20 rems/year.

There appears to be a disparity between the levels of risk that are considered negligible for radiation exposures and those considered negligible for chemical exposures. In the case of chemicals, exposure limits are generally set to keep incremental upper-bound lifetime cancer risks for workers below one per thousand and for the general population below a range of one per 10,000 to one per million. In the case of radiation, the current occupational exposure limit is a whole-body-equivalent external dose of 50 mSv/year or 5 rems/year (10CFR20, 1990 revisions), which is equivalent to a lifetime cancer risk of more than one in ten, assuming a linear dose-response relationship. That risk is far above those associated with lifetime exposure to chemical carcinogens at the level of their occupational standards. However, radiation-exposed workers are constantly monitored and large exposures are detected almost immediately and corrected. Health physicists have assured us that protective actions and the application of ALARA (as low as reasonably achievable) workplace practices lead to actual exposures for workers that are much smaller than occupational standards. Monitoring and job change lead to similarly lower actual exposures for chemically exposed workers.

The limit for unrestricted radiation exposure of a member of the public is now set at 1 mSv/year effective dose equivalent (100 mrems), only one fiftieth of the occupational exposure limit. In contrast, the difference between occupational and general population exposure limits for chemicals is usually much greater than a factor of 50.

FINDING 3.6.2: Methods for anticipating and assessing microbial hazards on a population basis (versus a clinical basis) are less developed than those for chemicals or for radiation; microbial risks generally are not evaluated using the dose-response modeling techniques used to evaluate chemical and radiation risks.

RECOMMENDATION: Refinement and application of epidemiologically based and other types of risk assessment methods for microbiologic hazards, and the collection of data and monitoring to validate and support those methods, should be encouraged.

RATIONALE

The emergence and resurgence of infectious agents ranging from the HIV-AIDS and Ebola viruses to tuberculosis mycobacteria, and the importance of antibiotic resistance mechanisms as a result of medical and veterinary overuse of antibiotics, have revived interest in the public-health aspects of infectious diseases. As seen in recent outbreaks of diarrhea caused by Cryptosporidium in Milwaukee's drinking water and of kidney failure in children caused by E. coli-contaminated hamburger meat in Seattle, our inability to assess health risks associated with microorganisms and inattention to risk reduction can lead to disaster. Those deaths, unlike theoretical low-dose cancer risks, are observable and countable. However, as with chemicals, most exposures to pathogens are below those associated with death or disease, because the body has effective defense mechanisms, so long as white blood cell and immune systems are intact. Empirical studies usually cannot produce sufficient information to assess a dose-response relationship in people, so methods for microbial risk assessment have increasingly relied on indirect measures of risk based on analytic models that estimate the extent of human exposure and the probability of human responses to exposure (Eisenberg et al. 1996a). Variation in susceptibility should be an important and specifiable aspect of infectious-disease risk assessments.

Models for assessing microbial risks include static models based on individual risks and population-based models that account for changes over time (Haas 1983, Haas et al. 1993, Eisenberg et al. 1996a,b). However, such epidemiologic factors as secondary spread of waterborne microorganisms, the effects of such host factors as the development of immunity, and the risk of death or disease resulting from bacterial contamination of meat at the slaughterhouse or from infected food handlers cannot be described fully with traditional dose-response modeling techniques. Static and dynamic models have complementary attributes, and different microbial-risk problems are likely to require different methods or combinations of methods.

A systematic examination of the applicability of existing models and of the need for new models to assess microbial risks is needed. Monitoring and the collection of data on the characteristics of microorganism behavior, toxicity, dose-response relationships, and risks comparable with data on chemical hazards are also essential. (See discussions of risk associated with waterborne microorganisms in section 6.1.4 and of risks associated with food-related pathogens in section 6.4). The International Life Sciences Institute recently convened a working group that is seeking an appropriate risk-assessment paradigm for microorganisms in drinking water and defining the important data gaps and research needs.