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Selecting and Prioritizing Contaminants for Assessment
Air and water contaminants are identified by the National Aeronautics and Space Administration (NASA) through a comprehensive assessment of potential sources of contaminants aboard spacecraft (NRC 1992, 2000; Kahn-Mayberry et al. 2011). The process is complex, requiring an understanding of all the materials and components onboard spacecraft, the environmental control and life-support system of space vehicles, processes that might occur in space, experiments to be performed, and a variety of other scenarios. Major contaminants are chemicals produced from off-gassing of cabin materials, components, and equipment; metabolic waste products from crew members; chemicals formed in the water treatment system; chemicals added to the water supply to retard bacterial growth; and compounds formed by chemical and physical processes in the cabin air. Scenarios considered are continuous or frequent releases from routine operations and activities; inadvertent, accidental, or emergency releases; and releases from experiments performed on the space station. Information about known and new sources of contaminants has been accumulated over decades of spaceflight experience and from environmental sampling and monitoring (Kahn-Mayberry et al. 2011).
Because NASA has insufficient resources to conduct comprehensive risk assessments of all potential air and water contaminants, it is important that priorities be established to focus on the chemicals that pose the greatest potential human health risk. The earlier guidance from the National Research Council (NRC) on setting spacecraft water exposure guidelines found that there are many approaches to setting priorities for choosing candidates for formal risk assessment (NRC 2000). The approaches range in complexity and each has its advantages and disadvantages. The three approaches to priority setting considered by the previous committee included an ad hoc approach, an ad hoc approach with factors specified, and a formal system with parameters, weights, and interrelationships specified. Key elements of each approach include the following:
- Ad Hoc Approach: Candidate chemicals are proposed as the chemicals become of interest to NASA. They might be identified through screening of
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potential sources of atmospheric or water contaminants or from monitoring of the environmental control and life-support systems. Periodically a chemical is chosen from the list of candidates by NASA on the basis of informed judgment using subjective and qualitative information. The parameters or the data elements on which candidates are chosen are not specified and the candidates are not weighed against each other in a quantitative sense.
- Ad Hoc Approach with Factors Specified: A slightly more formal approach to setting priorities is for NASA to specify the parameters it considers in setting priorities. Such parameters might include evidence of exposure, magnitude of routine and accidental exposure, short- and long-term health effects, ability to monitor and control exposure, and the need to have the chemical on board a spacecraft.
- Formal System: Priorities are based on a specified set of parameters, a formula is used to combine scores for various parameters, and the relationship and weighting of the parameters are specified. The formula could be a simple sum of scores of various parameters or a more formal complex formula in which parameters are given unequal weights and their relationships are other than additive. Parameters that might be relevant for NASA’s purposes include likelihood of routine exposure, medical intelligence from ground-based or flight-based experience, likelihood of unusual exposure, severity of toxicity, design requirements (e.g., the capacity for controlling and eliminating exposures), special spaceflight considerations, and spaceflight experience (NRC 2000).
Previous NRC guidance has encouraged flexibility in selecting chemicals as a means to increasing the effectiveness of risk assessment. The process should allow for new information on changes in parameters, changes in information on specific chemicals, and the addition of new chemicals for consideration. For example, a series of priority rankings based on changes in parameters considered, their weighting, or their relationships could be developed. Priority rankings could then be compared in a way akin to sensitivity analysis in mathematical risk assessment.
Similar conclusions about available schemes were drawn by other NRC committees formed to provide the US Environmental Protection Agency with assistance in establishing a priority-setting process for drinking water contaminants (NRC 1999, 2001). Those committees evaluated existing schemes and explored the development of alternative approaches and found that there are no sharp boundaries between the types of schemes, as all involve a mix of data and to some degree expert judgment. The main differences are the specific mix of parameters considered and the extent to which explicit or implicit judgments come together to produce reliable results. The committee concluded that the output of most prioritization schemes is so uncertain that they are only useful in making preliminary screening assessments or classifications and should not be used for sorting contaminants in a specific order.
FINDINGS AND RECOMMENDATIONS
The options for choosing candidate chemicals for risk assessment remain the same as previously available. One choice is subjective and is based on informed expert judgment. The second approach provides a slightly more formal approach, in which parameters for making the decision are specified but their weights and interrelationships are not. The third approach is more formulaic and involves specifying and quantifying the elements that are considered and using a weighting system for ranking contaminants.
Recommendation: The committee endorses NASA’s use of a combination of these approaches to select chemicals for risk assessment. The process should be described to support the selection process.
REFERENCES
Khan-Mayberry, N., J.T. James, R. Tyl, and C. Lam. 2011. Space toxicology: protecting human health during space operations. Int. J. Toxicol. 30(1):3-18.
NRC (National Research Council). 1992. Guidelines for Developing Spacecraft Maximum Allowable Concentrations for Space Station Contaminants. Washington, DC: National Academy Press.
NRC. 1999. Setting Priorities for Drinking Water Contaminants. Washington, DC: National Academy Press.
NRC. 2000. Methods for Developing Spacecraft Water Exposure Guidelines. Washington, DC: National Academy Press.
NRC. 2001. Classifying Drinking Water Contaminants for Regulatory Consideration. Washington, DC: National Academy Press.