Cambridege Environmental

58 Charles Street
Cambridge, MA 02141
Phone: 617-225-0810

411 Aviation Way
Frederick, MD 21701
Phone: 301-695-0251

Probabilistic Risk Assessment

Risk assessment confronts uncertainties and variabilities at every step. Typical risk assessments attempt to account for them by choosing extreme values in order to be conservative; but this may lead to compounding levels of conservatism and unreliable results. In such cases, where more extensive analysis is needed, we use probabilistic methods such as Monte Carlo simulations to quantify uncertainties and variabilities and to characterize risk more completely and reliably. Sometimes the uncertainties or variabilities can be adequately summarized using analytical techniques (e.g. using means and standard deviations, together with evidence-based ideas about the probability distributions that apply); but in most cases numerical methods are necessary. In addition to performing probabilistic assessments, we offer consulting services to determine whether such advanced techniques can or should be used, whether others should have used them in specific situations, and to assess the validity and accuracy of such assessments when they have been performed. Members of our staff have been working in this area since the mid-1970's, and are nationally recognized experts in risk assessment and related fields.

Sample Projects

Report to the federal Commission on Risk Assessment and Risk Management

Under contract with the Commission on Risk Assessment and Risk Management, Cambridge Environmental designed and conducted probabilistic assessments of cancer risks due to drinking water contaminated with part-per-billion concentrations of the chemicals 1,1-dichloroethylene and vinyl chloride, and to occupational exposures to the latter chemical. In particular, we were asked to design and implement methods that could provide regulators and other decision-makers with information that went considerably beyond "upper-bound" point-estimates generated by default methods. We conducted case studies based on holistic uses of the underlying dose-response data generated from rodent bioassays, and on thorough evaluations of the variabilities and uncertainties in key input parameters and models. We also discussed the qualitative distinctions between carcinogens, such as vinyl chloride, known to cause cancer in workers, as opposed to chemicals, such as 1,1-dichloroethylene, known to cause cancer only in laboratory rodents. Explicit recognition of the needs of environmental regulators and others was also central to our work.

PCBs in the Kalamazoo River

For a private company, Cambridge Environmental performed a probabilistic health risk assessment for people presumed to ingest fish from a river contaminated with polychlorinated biphenyls (PCBs). Our assessment took full account of all the available measurements on the exposed population, and incorporated the variability among members of that population and the uncertainties inherent in these measurements. Rather than evaluate hypothetical populations (as is done in screening-level assessments), we examined the population of anglers who actually ate fish from the Kalamazoo River, a population that had been extensively characterized in a survey designed for that purpose. We also quantified and took account of the uncertainties and variabilities in the dose-response data upon which the carcinogenic potencies of the PCBs are estimated. Our assessment provided risk-estimates applicable to individuals and to various populations, including hunters and gardeners in the surrounding flood plain.