Many hazardous chemicals have defined exposure limits called Permissible Exposure Limits or PELs. These exposure limits can be based on an 8-hour Time Weighted Average (TWA) exposure, a 15-minute Short Term Exposure Limit (STEL), or an upper limit exposure (Ceiling). Where specific limits are given, the lab standard requires employee exposures not exceed these limits. If an overexposure is suspected, an exposure assessment is necessary. If the assessment indicates that an employee could reasonably have been exposed to a hazardous chemical in a manner that might have caused harm, the person is entitled to medical services. Services include a medical consultation and, if determined to also be needed, a medical examination.