# HAZWOPER Air Monitoring: Oxygen, LEL, Toxics and Direct-Reading Instruments HAZWOPER guide · 2 min read · Updated 2026-11-03 Air monitoring tells you what is in the air, how much of it, and whether workers' PPE and work zones are adequate. OSHA 29 CFR 1910.120(h) requires monitoring during initial site entry and periodically after that, whenever there may be exposure above permissible limits, a change in conditions or a new operation. ## What to monitor first On initial entry, check hazards in this order, because each one can kill fastest: 1. **Oxygen**: below 19.5% is oxygen deficient (atmosphere-supplying respirator needed). Above 23.5% is oxygen enriched and raises fire risk. 2. **Flammability**: measured as a percent of the lower explosive limit (LEL). Combustible gas readings are only accurate when oxygen is normal. 3. **Toxic gases and vapors**: such as hydrogen sulfide, carbon monoxide and organic vapors, compared against exposure limits and IDLH values. 4. **Radiation**, where suspected. ## Common action levels | Reading | Typical action | | --- | --- | | O2 < 19.5% | Oxygen deficient: SCBA or supplied air, or don't enter | | O2 > 23.5% | Oxygen enriched: fire hazard, stop work and find the source | | Combustible gas < 10% LEL | Continue with monitoring | | 10-25% LEL | Continue with extreme caution and continuous monitoring | | > 25% LEL | Explosion hazard: withdraw and reassess | | Toxics at or above PEL/REL | Upgrade PPE or use controls per the site plan | | Toxics at or above IDLH | Atmosphere-supplying respirator or withdraw | These follow widely used EPA guidance. Your site safety and health plan sets the actual action levels for your site. ## Direct-reading instruments - **Multi-gas (4-gas) meters** measure O2, LEL, H2S and CO - **Photoionization detectors (PID)** measure many volatile organic compounds in ppm. Response depends on the lamp energy (eV) and the chemical's ionization potential, and readings need correction factors for specific compounds - **Flame ionization detectors (FID)** measure organic vapors including methane, which PIDs don't detect well - **Colorimetric detector tubes** give a reading for one specific chemical - **Aerosol and dust monitors**, **radiation meters** and **mercury vapor analyzers** for specific hazards Direct-reading instruments give real-time results for decisions on site. Personal sampling with pumps and lab analysis documents actual worker exposure against PELs. ## Bump testing and calibration - **Bump test** before each day's use: expose the instrument to a known gas to make sure the sensors and alarms respond - **Calibrate** per the manufacturer, and whenever a bump test fails - Record bump tests and calibrations - Know each instrument's limits: interfering gases, humidity, temperature, sensor poisoning and response time ## Where to monitor Sample at the breathing zone, then at low and high points depending on vapor density. Heavier-than-air gases collect in trenches, pits and basements; lighter gases rise. Monitor before entering confined spaces, and continuously during work that could release contaminants, such as drum opening or excavation. ## Frequently asked questions ### What oxygen level is considered oxygen deficient? OSHA considers an atmosphere with less than 19.5% oxygen by volume to be oxygen deficient. Above 23.5% is oxygen enriched. ### What is the difference between a PID and an FID? Both detect organic vapors. A PID uses ultraviolet light and does not respond well to methane. An FID burns the sample in a hydrogen flame and detects methane and most organic compounds. ### How often should a gas meter be bump tested? Before each day's use is the common industry and manufacturer recommendation, with full calibration per the manufacturer and whenever a bump test fails. 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 Air Monitoring and Gas Detection Training](https://www.hazwoperosha.com/courses/osha-air-monitoring-and-gas-detection-training/) 2 Hours · **$49.99** [HAZWOPER Micro-Module: Air Monitoring & Direct-Reading Instruments](https://www.hazwoperosha.com/courses/hazwoper-micro-module-air-monitoring-and-direct-reading-instruments/) 20 Min · **$14.99** [OSHA 40 Hour HAZWOPER Training](https://www.hazwoperosha.com/courses/osha-40-hour-hazwoper-training/) 40 Hours · **$249.99** ## 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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