# Exposure Limits Explained: PEL, REL, TLV, STEL, Ceiling and IDLH HAZWOPER guide · 2 min read · Updated 2026-11-03 HAZWOPER training levels, PPE selection, medical surveillance and air monitoring all depend on **exposure limits**: airborne concentrations a worker can be exposed to without expected harm. Here are the terms you'll see on safety data sheets and in site plans. ## Who sets the limits | Limit | Set by | Legal status | | --- | --- | --- | | PEL (Permissible Exposure Limit) | OSHA | Legally enforceable (29 CFR 1910.1000 tables and substance-specific standards) | | REL (Recommended Exposure Limit) | NIOSH | Recommendation, often stricter than the PEL | | TLV (Threshold Limit Value) | ACGIH | Guideline, updated yearly; adopted by some states and contracts | | IDLH (Immediately Dangerous to Life or Health) | NIOSH | Used for respirator selection and emergency decisions | Many PELs date from 1971 and are less protective than current RELs and TLVs. HAZWOPER uses PELs and, where there is no PEL, "published exposure levels". ## Types of limits - **TWA (time-weighted average):** the average concentration over an 8-hour shift (OSHA) or a 10-hour shift (NIOSH) - **STEL (short-term exposure limit):** a 15-minute average that shouldn't be exceeded at any time in the shift - **Ceiling (C):** a concentration that must never be exceeded, even briefly - **Skin notation:** the chemical can be absorbed through skin, so airborne limits alone don't protect workers - **IDLH:** a concentration that could cause death, immediate or delayed permanent health effects, or prevent escape. Only atmosphere-supplying respirators (SCBA or supplied air with escape) are used at or above IDLH ## Routes of exposure 1. **Inhalation:** the most common route at hazardous waste sites 2. **Skin and eye absorption:** many solvents and pesticides pass through skin 3. **Ingestion:** usually from contaminated hands, food or tobacco, which is why eating and smoking are banned in the exclusion zone 4. **Injection:** punctures from contaminated sharp objects ## Acute vs chronic effects **Acute** effects show up during or shortly after a short, high exposure, such as dizziness from solvent vapors or burns from a corrosive. **Chronic** effects come from repeated exposure over months or years, such as cancer, liver or kidney damage, or nerve damage. Some substances have both. ## Using limits on site [Air monitoring](https://www.hazwoperosha.com/guides/air-monitoring/) readings are compared against these limits to choose [PPE levels](https://www.hazwoperosha.com/guides/ppe-levels-a-b-c-d/), set action levels in the [site plan](https://www.hazwoperosha.com/guides/site-safety-and-health-plan/) and decide who needs [medical surveillance](https://www.hazwoperosha.com/guides/hazwoper-medical-surveillance/). A respirator's assigned protection factor times the exposure limit gives the maximum concentration it can be used in. ## Frequently asked questions ### What is the difference between a PEL and a TLV? A PEL is OSHA's legally enforceable limit. A TLV is a guideline published by ACGIH that is updated regularly and is often more protective. ### What does IDLH mean? Immediately Dangerous to Life or Health: a concentration that could cause death or serious permanent harm or prevent escape. Only SCBA or supplied air with escape is used at or above IDLH. ### What is a STEL? A short-term exposure limit: a 15-minute time-weighted average exposure that should not be exceeded at any time during the workday. This guide summarizes federal OSHA, Cal/OSHA and EPA rules for training purposes. It is not legal advice. Always check the current regulation text and your site safety and health plan. ## Get trained [OSHA 40 Hour HAZWOPER Training](https://www.hazwoperosha.com/courses/osha-40-hour-hazwoper-training/) 40 Hours · **$249.99** [OSHA Air Monitoring and Gas Detection Training](https://www.hazwoperosha.com/courses/osha-air-monitoring-and-gas-detection-training/) 2 Hours · **$49.99** [OSHA 24 Hour HAZWOPER Training](https://www.hazwoperosha.com/courses/osha-24-hour-hazwoper-training/) 24 Hours · **$169.99** ## Test yourself 10 free questions with answers. [Air monitoring quiz](https://www.hazwoperosha.com/quizzes/air-monitoring-quiz/) ## More HAZWOPER guides - [What Is HAZWOPER? 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