The main task of hazard identification (HI) is to assess and characterize all (potential) toxicological endpoints. The ultimate objective is to determine the endpoint(s) of concern, typically in terms of severity, effect, or both. The toxic effects via different routes of exposure may or may not be the same. Sometimes the endpoints of concern do not represent the most severe effects. They are usually those low enough to drive the risk mitigation process.

The procedures for when and how to consider different routes of exposure are beyond the scope of this lecture. Those who have a special interest in aggregate or cumulative exposure/risk assessment are referred to the regulatory guidance documents drafted by USEPA (1999a, 2000b). It is also important to note that the weight-of-evidence and the credibility or integrity of the toxicity studies all play an important role in HI.

Once the endpoints of concern are identified, the next critical step is to (quantitatively) determine the highest dose at which no statistically as well as no biologically significant adverse effect is expected to occur relative to the control group. This highest dose is referred to as NOEL (no observed effect level) or NOAEL (no observed adverse effect level), which is the level just below the LOEL (lowest observed effect level). As the National Research Council (1994) puts it, the difference between NOEL and NOAEL rests on the definition of adverse effect only. LOEL is the lowest dose at which there begins to show significant increase in the observable effect.