1. Intensity, where the detection threshold level is defined as the lower limit of the perceived intensity range (by convention the lowest concentration that can be detected in 50% of the cases in which it is present).

2. Quality, where the recognition threshold level is defined as the lowest concentration at which the sensory effect can be recognized correctly in 50% of the cases.

3. Acceptability and annoyance, where the nuisance threshold level is defined as the concentration at which not more than a small proportion of the population, less than 5%, experiences annoyance for a small part of the time, less than 2%.