The STEL is the maximum average concentration to which workers can be exposed for a short period, typically 15 minutes, without suffering adverse health effects. STELs are designed to protect against acute effects from brief exposures. Exposures above the TWA but below the STEL are allowed, but the STEL must not be exceeded at any time during the workday, and there are limits on how often and how long such exposures can occur. These are listed as specifications in the exposure tables.