185 The influence of measurement uncertainty and environmental factors in the assessment of workplace exposure

According to good occupational hygiene practices, measurement uncertainty (U) should be included when testing TWA workplace atmosphere measurement against OEL. In some countries such as the US (OSHA) and Belgium, this is included in legislation: law enforcement tests the TWA lower confidence limit LCL=TWA*(1-U) exceedance of the OEL while employers test the TWA upper confidence limit UCL=TWA*(1+U) compliance with the OEL. International standards such as EN482 and ISO-20581 help establish U, UCL and LCL. Current exposure assessment strategies promoted by IOHA members, such as AIHA (2015), EN689 (2018) and BOSH/NVvA (2022), ignore measurement uncertainty. This may be due to historical work (Nicas, 1991), which shows that in similar exposure groups (SEG), lognormal long-term daily environmental variability exceeds U. However, Monte-Carlo simulations, which will be shown in this lecture, show that the influence of U on the lognormal, one-sided upper confidence limit of the 95th percentile (UTL95,##%) can be significant in occupational hygiene practice with sample sizes often smaller than n=6. Also in the last decade, several strategies promote a simplified screening/preliminary test (French Code du Travail, EN689:2018 §5.5.2, BOSH/NVvA 2011/2022). In these tests, the highest result (TWAmax) of 3 to 5 measurements is compared with the OEL or with a defined fraction f(OEL). In line with the above, these TWAmax tests should use TWAmax*(1-U) ≤ f(OEL) for compliance and TWAmax*(1+U) > OEL for exceedance. IOHA member organisations are advised not only to align the numerical test schedules for compliance, but also to include measurement uncertainty in the OEL tests.