Reduction A change in a chemical characterized by the gain of electrons. Reference Dose (RfD) The EPA estimate of a lifetime daily exposure level for humans that is likely to be without risk of harmful effects.  RfD's are acceptable safety levels for chronic non-carcinogenic and developmental effects.  The process used to derive an RfD is a modification of that used to derive an ADI. Relative Risk (RR) A statistical calculation of the ratio of disease in an exposed population to that of an unexposed population. Reproductive Toxicity Toxicity of the male or female reproductive system.  Toxic effects may include damage to the reproductive organs or offspring. Respiratory Toxicity Toxicity of the upper (nose, pharynx, larynx, and trachea) or lower (bronchi, bronchioles, and lung alveoli) respiratory system. Retrospective Cohort Study An epidemiology study in which cohorts are identified according to past exposure conditions and follow-up proceeds forward in time RfD see Reference Dose Risk The probability that a hazard or effect will occur at a specific level of exposure. Risk Assessment The process by which the probability that an adverse effect will occur at a defined exposure level is determined. Risk Characterization The final stage in the risk assessment process which involves the prediction of the frequency and severity of effects in exposed populations. Risk Management The process of weighing policy alternatives and selecting the most appropriate regulatory action based on the results of risk assessment and social, economic, and political concerns.