1. Terminology  AEGL – Acute Exposure Guideline Levels – Threshold exposure limits for the general public that are applicable to emergency exposure. T here are three guideline levels; Level 1 – irritating, Level 2 – irreversible harm, Level 3 – fatal  IDLH – Immediately Dangerous to Life and Health – Workplace exposure limits that are meant to protect workers when they are exposed to a toxic chemical in t he course of their work. T hese guidelines are developed by NIOSH and enforced by OSHA  LTEL – Long Term Exposure Limit – T he maximum exposure permitted over an 8 - hour period.  OEL – Occupational Exposure Limit – T he acceptable concentration of a hazardous su bstance in workplace air.  PEL – Permissible Exposure Limits – The maximum amount or concentration of a chemical that a worker may be exposed to under OSHA regulations.  PPM – Parts Per Million – how many parts a certain molecule or compound makes up within the one million parts of the whole solution.  REL – Recommend Exposure Limits – A n occupational exposure limit recommended by NIOSH.  STEL – Short Term Exposure Limits – The maximum exposure permitted over a 15 - minute period.  TLV – Threshold Limit Values – Guidelines (not standards) prepared by ACGIH to assist industrial hygienists making decisions regarding safe levels of exposure to various hazards found in the workplace.  TWA – Time Weighted Average – The average workplace exposure to any hazardous cont aminant or agent.