The level of distinct odor awareness (LOA) represents the concentration above which it is predicted that more than half of the exposed population will experience at least a distinct odor intensity, and about 10% of the population will experience a strong odor intensity. The LOA should help chemical emergency responders in assessing public awareness of the exposure due to odor perception. The LOA derivation follows the guidance given by Ruijten et al. (2009).
The odor detection threshold (concentration at which 50% of people detect an odor; OT50) for H2S was calculated to be 0.0006 ppm (Ruijten et al. 2009).
The concentration (C) leading to an odor intensity (I) of distinct odor detection (I = 3) is derived with the Fechner function:
For the Fechner coefficient, the default of kw = 2.33 is used because of a lack of chemical-specific data:
The resulting concentration is multiplied by an empirical field correction factor. It takes into account that in everyday life factors such as sex, age, sleep, smoking, upper airway infections, and allergy as well as distraction increase the OT50 by a factor of 4. In addition, it takes into account that odor perception is very fast (about 5 seconds) which leads to the perception of concentration peaks. On the basis of current knowledge, a factor of 1/3 is applied to adjust for peak exposure. Adjustment for distraction and peak exposure lead to a correction factor of 4/3 = 1.33.