A manufacturer ran mixers on a mezzanine where air sampling showed respirable crystalline silica above OSHA's permissible exposure limit of 50 micrograms per cubic meter as an 8-hour time-weighted average, so the area was posted as a regulated area. Maintenance workers needed to enter 8 to 10 hours after mixing stopped to perform equipment checks and would leave before mixing resumed. The company asked whether those workers could enter the posted regulated area without respirators.
OSHA explained that while the silica standard requires respirators for employees entering regulated areas, "the key determination is whether employee exposure in the area exceeds the PEL or can be expected to exceed the PEL." Areas can function as temporary regulated zones: when silica-generating activities are not occurring and exposures will not exceed the PEL, the employer need not treat the space as a regulated area. If exposure assessments show maintenance workers' exposures during non-silica work will not exceed the PEL, they need not wear respirators for their checks, even though the same mezzanine is a regulated area during mixing operations (29 CFR 1910.1053(e)(4), 1910.1053(g)(1)).
There is an important exception: if the maintenance activities themselves could disturb settled silica dust and push exposures over the PEL, for example cleaning mixers or replacing filters, respirators remain mandatory. OSHA also reiterated that engineering controls must be prioritized over respiratory protection whenever feasible (29 CFR 1910.1053(d)(1)).