TABLE OF CONTENTS A variety of toxic gases are produced, processed, stored, and transported. Examples are chlorine for water purification and manufacture of plastics and solvents, hydrogen fluoride and hydrogen sulfide in refinery operations, ammonia in refrigeration and manufacture of fertilizers, hydrogen chloride in manufacture of a range of chemicals and pharmaceutical products.
The physiological effects of exposure to a toxic gas depend on both the concentration and the duration of exposure. The concentration of a toxic gas at a particular location remote from the escape point depends on the weather at the time: the wind direction, wind speed, and the "atmospheric stability" (explained later) all are important.
In calculating the risks to people from a postulated toxic gas escape, the steps are:
calculate the concentration at the required distance for a range of wind and weather conditions;
for each calculated concentration, and with the estimated duration of exposure, estimate the physiological effect and the probability of fatality per such exposure;
using the probability of occurrence of each of the selected wind and weather combinations, calculate the probability of fatality per leak; and
using the estimated frequency of the leak, calculate the risk of fatality per year.
If gas escapes continuously from a single point, it moves with the wind, following the continually varying path of the wind and gradually dispersing in both the horizontal and vertical planes due to...
