For non-cancer health effects, we aim to identify a threshold level at which the lowest possible health effects or no adverse health effects occur. Depending on whether that is a low level or no adverse effect at all, above which adverse effects start to occur, we refer that as LOAEL, and NOAEL.  Here, the aim of the EHRA process is to identify the lowest level of exposure that will result in an acceptable level of health risk that can be mitigated. In order to do this, for the exposure (chemicals or other agents), the level of exposure at which health effects manifest are studied. Some health effects are significant and serious; others are non-serious. Some health effects manifest at very low levels of exposure: these health effects are sensitive health effects. Some of the health effects are very sensitive; the most sensitive health effect such as skin lesions may or may not be serious. On the other hand, there may be serious health effects such as cardiovascular effects that are manifest at doses higher than that lead to the manifestation of the highly sensitive yet trivial or non-serious health effects. These effects can be critical health effects. The critical health effects are identified from the hazard identification steps and then these significant or critical health effects are studied with respect to the dosage in which they manifest.
The lowest dose at which the critical yet non-cancers health effects that manifest and the ones that are not beyond their baseline risks is referred to as no observed adverse effects level (NOAEL). For some combinations of exposure and disease conditions, a NOAEL is not possible. There are other situations, where the health effects are manifest but these manifest at certain levels of exposure. These are referred to as lowest observed adverse effect level (LOAEL). The NOAEL and LOAEL are threshold levels. Thus, for non-cancerous health effects, the focus is on finding the apparent safe levels of exposure rather than finding out conducting a risk assessment. With cancerous health effects, the aim is to eliminate risks at all levels. In risk assessments that involve cancerous health effects, the dose response graph is studied and the specific levels of the dosage in which the effects are manifested are charted. As many dose response charts are studied in the contexts of animal studies where very high doses are involved, these doses are unsustainable or impractical in human beings. As a result, most dose-response studies in humans for carcinogens are studied using extrapolation from large dose studies to low or ultra low dose studies. The goal of these extrapolation exercises is to see what would be the result in terms of health effects if the dosage is at the level of human scale (this is in the form of mg or micrograms/Kg of body weight or for inhalation agents, on a scale of mg or weight per L of air).
Assuming that they have a linear dose response effect (based on epidemiological studies), see the figure below: