Objective: Common variation is a statistical process-control term for variability associated with usual operating conditions. Special variation occurs when usual operating conditions are disrupted. The objective was to explore the implications for preventive occupational medicine practice of common and special variation in air-level exposure.
Methods: Illustrations are derived from US and UK beryllium facility databases.
Results: Special variation may be missed in finite sampling sets, giving a very inaccurate indication of the highest air levels experienced on the job. Depending on the toxicologic model, failure to assess special variation influences the meaningfulness of aspects of occupational prevention, from medical surveillance through risk management.
Conclusions: Jobs and tasks should be characterized for special variation in addition to traditional air sampling. Both special variation and common variation should be considered in occupational medicine preventive practice.