How is exposure calculated in relation to ppm, site pressure, and temperature?

The correct formula for calculating exposure in relation to parts per million (ppm), site pressure, and temperature involves adjusting the ppm value based on the specific conditions present at the site. This requires taking into account the pressure and temperature at that site relative to standard conditions.

Using the formula Exposure = ppm site (760/Psite)(Tsite/298), the components can be broken down as follows:

1. ppm site: This signifies the concentration of hazardous substances by volume in air as measured at the location in question.

2. Pressure adjustment: The term (760/Psite) adjusts the concentration to standard atmospheric pressure (760 mmHg). Since gas volumes are affected by pressure, when the site pressure is less than standard pressure, this term appropriately increases the ppm value to account for a greater volume of gas under lower pressure.

3. Temperature adjustment: The term (Tsite/298) adjusts for temperature deviations from standard temperature conditions (298 Kelvin, or 25 degrees Celsius). This adjustment is necessary because gases expand or contract with changes in temperature. Therefore, cooler temperatures would increase the concentration of gas, while higher temperatures would decrease it.

By combining these factors, the formula accurately expresses the effective exposure in the given environmental conditions