Int. J. Environ. Res. Public Health 2012, 9 2014 authorities might allow EPA discretion to consider cumulative effects, and how courts interpreted that EPA statutory provision over time. Acknowledgments Conflict of Interest This research was supported by the National Center for Environmental Research, Office of Research and Development of the U.S. Environmental Protection Agency under Contract No. EP-C-09-009. The views expressed in this article are those of the authors and do not represent official U.S. Environmental Protection Agency policy, nor should any official endorsement be inferred. The authors declare no conflict of interest. References and Notes 1. 2. 3. 4. 5. 6. 7. 8. Human Health Research Strategy; Report No. EPA/600/R-02/050; U.S. Environmental Protection Agency, Office of Research and Development, U.S. Government Printing Office: Washington, DC, USA, 2003. U.S. Environmental Protection Agency. Aggregate and Cumulative Risk. Human Exposure and Atmospheric Sciences. Available online: http://www.epa.gov/heasd/risk/ (accessed on 26 March 2012). Framework for Cumulative Risk Assessment; Report No. EPA/630/P-02/001F; U.S. Environmental Protection Agency, U.S. Government Printing Office: Washington, DC, USA, 2003. Sexton, K.; Linder, S.H. The role of cumulative risk assessment in decisions about environmental justice. Int. J. Environ. Res. Public Health 2010, 7, 4037–4049. Lewis, A.S.; Sax, S.N.; Wason, S.C.; Campleman, S.L. Non-chemical stressors and cumulative risk assessment: An overview of current initiatives and potential air pollutant interactions. Int. J. Environ. Res. Public Health 2011, 8, 2020–2073. According to the U.S. EPA’s Framework for Cumulative Risk Assessment, a stressor is any “physical, chemical, or biological entity that can induce an adverse response” ([3], p. 74). This definition encompasses the concept that stressors may not cause harm directly, but instead may increase vulnerability to harm by other stressors ([3], p. xvii). “For example, chemicals such as toluene can damage the auditory system and have been shown to potentiate the effects of a physical stressor, noise, on hearing loss” ([3], p. 47). Examples of non-chemical stressors include: lack of health care; personal activities (e.g., smoking, diet, and alcohol consumption); natural phenomena (e.g., forest fires, floods); biological pathogens; psychosocial stress; noise; and heat ([3], pp. 51–52; [4,5]). Callahan, M.A.; Sexton, K. If cumulative risk assessment is the answer, what is the question? Environ. Health Perspect. 2007, 115, 799–806. According to EPA’s Framework for Cumulative Risk Assessment, cumulative risk assessment involves an “analysis, characterization, and possible quantification of the combined risks to health or the environment from multiple agents or stressors” ([3], p. 72). Cumulative risk assessments may include both human health and ecological effects ([3], p. 30). Cumulative risk refers to the combined threats from exposure to multiple stressors via all relevant routes [7].