Autocorrelation of short-term and daily average exposure levels in workplaces

Based on workplace exposure data for 16 worker-chemical combinations, the degree of autocorrelation in a series of short-term time-weighted average (TWA) exposure concentrations (7.5-min, 15-min, 30-min, and 60-min TWAs) was investigated. For 7.5-min and 15-min TWAs, consecutive values generally exhibited no correlation or positive correlation, but not negative correlation. Consecutive 30-min TWAs and 60-min TWAs showed either positive correlation, negative correlations, or no correlation. Autocorrelation functions displayed various patterns that depended on the exposure time series. The mean of the autocorrelation coefficients across all worker-chemical combinations was similar to an exponential function, which signifies that the degree of autocorrelation, on average, decreased as the interval between two averaging periods increased. Autocorrelation in a series of 8-hr TWAs also was analyzed for 10 worker-chemical combinations. In general, little autocorrelation was observed. Based on this analysis, the authors discuss a sampling strategy that, on average, would minimize the degree of correlation between measurements of short-term TWAs.