Time weighted average (TWA) is the average workplace exposure to any hazardous contaminant or agent using the baseline of an 8 hour per day or 40 hours per week work schedule. The TWA reflects the maximum average exposure to such hazardous contaminants to which workers may be exposed without experiencing significant adverse health effects over the standardized eight-hour work period. The TWA is expressed in units of parts per million (ppm) or mg/m3.
The time weighted average for exposure to any substance may be used when the concentration and time of exposure varies. It is also applicable to short-term samples, such as a 15-minute TWA. TWA considers particular variables, dose rate, and duration.
For example, if a worker is exposed to different doses of a chemical vapor for different amounts of time, we may find TWA to determine the worker’s average amount of exposure to that particular chemical.
TWA may be calculated as follows:
TWA= (t1c1+t2c2+…+tncn) / (t1+t2+…+tn)
Where ci = concentration during nth interva and ti = duration of nth interval.