§ 197.515 - Permissible exposure limits (PELs).
The permissible exposure limits (PELs) for personal exposure are as follows:
(a) The time-weighted average exposure limit (TWA).
(b) The short-term exposure limit (STEL). Exposures at the STEL must not be repeated more than four times a day. There must be at least 60 minutes between successive exposures at the STEL.