The variability of exposure over time: a prospective longitudinal study.

Four hundred and forty personal air measurements were carried out on 54 workers, employed in the main processes in six different factories (6-13 in each). Potential exposure was to lead, benzene and dust. Ten randomly repeated hygiene surveys were carried out over 1 year. In order to estimate the magnitude of the variability in workers' exposure over time, its sources, the variance between workers and the variance within a worker, a nested unbalanced analysis-of-variance model was fitted to the log-transformed data. Of the total exposure variance, the within variance of a worker's exposure over time was 51% (geometrical standard deviation, GSD = 3.1) and the between workers, factories and air contaminants variance was 49%. The exposure variance between all the workers was due mainly to variance between workers within the same factory (67%). Outdoor locations, mobility of the worker and mobility of the sources of exposure result in a positive influence on both the variance between (26%, ANOVA) and the variance of a worker over time (39%, regression). These variables are therefore important in the sampling strategy of workers' exposure. For valid compliance testing and assessment of workers' exposure the mean and the within- and between-variance of the workers' exposure over time should be considered. The exposure should be measured several times a year randomly in order to prevent workers misclassification. To assess yearly exposure, a GSD = 3.1 can be used to calculate confidence limits for the arithmetic mean of worker's exposure measurements, in circumstances similar to those in this study.