As USEPA (2000a) puts it, each health risk assessment should be accompanied with a section addressing the uncertainties involved. Therefore, health risk characterization should include a section on the uncertainties with toxicity assessment and another section on the uncertainties with human exposure assessment. However, it is this lecture's opinion that a comprehensive listing of all the uncertainties involved is neither practical nor warranted. These days, even regulators less familiar with dermal exposure calculation appreciate the presumption that the actual human dermal absorption rate for many chemicals is lower than the regulatory default (generally 50% or 100% of the applied dose), or is typically lower than the one derived from a rat study.

There is also no real need to determine if a workday is or is not longer than the typical default of 8 hours. Underlying these arguments is the fact that human exposure is actually a function of several variables, of which daily duration and dermal absorption are usually the minor two. The mean exposure rate from a chemical-specific study could easily be doubled or halved if the monitoring were repeated on a different group of test subjects, or using a different protocol. Or the air emission rate could be quite different if it were measured on a different day, in a different monitoring location, or using a different sampling method.

Therefore, exposure appraisal or risk characterization should only be limited to discussing those elements that are extraordinary, or specific to the particular chemical or exposure scenario under study.