In determining threshold quantities for toxic chemicals, OSHA used the Gaussian dispersion model. OSHA made the following assumptions:
- average conditions of 4.3 m/sec. wind speed and D stability with urban dispersion coefficients;
- continuous steady-state release for one hour;
- no liquid pools; all released chemicals in vapor or gaseous state;
- chemical release is at ambient temperature and at ground level;
- chemical gas or vapor cloud is neutrally buoyant; and
- no design features prevent downwind dispersion.
The calculated threshold quantities were rounded by OSHA to further simplify the standard.
The Gaussian dispersion model (also known as a Gaussian plume model) is a mathematical model used to predict the dispersion of pollutants in the atmosphere. It assumes that pollutant concentration follows a normal (Gaussian) distribution, spreading out in a bell-shaped curve pattern. This model is widely used due to its simplicity and ability to predict concentrations over a range of distances from the source.