A 4-gas monitor reports oxygen, flammable gas as a percentage of the lower explosive limit (LEL), hydrogen sulfide and carbon monoxide. This checker compares your readings with OSHA's definition of a hazardous atmosphere and the alarm setpoints most monitors use out of the box.
The limits used
| Gas | Acceptable range used here | Reference |
|---|---|---|
| Oxygen | 19.5% to 23.5% | OSHA: below 19.5% is deficient, above 23.5% is enriched |
| Flammable gas | Below 10% LEL | OSHA: above 10% of the LFL is a hazardous atmosphere |
| Hydrogen sulfide | Below 10 ppm | Common alarm; OSHA general industry ceiling 20 ppm; construction PEL 10 ppm; NIOSH ceiling 10 ppm; IDLH 100 ppm |
| Carbon monoxide | Below 35 ppm | Common alarm; OSHA PEL 50 ppm; NIOSH REL 35 ppm; IDLH 1,200 ppm |
Reading the results
- One reading out of range means don't enter. Ventilate, isolate or clean, then retest.
- Normal air is about 20.9% oxygen. A reading of 20.0% is still "acceptable" but means something is displacing or consuming oxygen. Find out why.
- Test at all levels before entry. A good reading at the opening says nothing about the bottom of the space. See atmospheric testing.
- Your permit's acceptable entry conditions control. Some programs set tighter limits, and other contaminants need their own.
Sources and official references
Last reviewed: October 25, 2026 · Free general education based on OSHA 29 CFR 1910.146 and 29 CFR 1926 Subpart AA. It does not replace the site-specific training, written program and hands-on evaluation your employer must provide, and it does not issue a certificate. Nothing you enter on this page is saved or sent.