One of the top questions I get via SAFTENG and in my PRCS or Hazardous Atmospheres training courses is... Why did OSHA use 10% LEL to define a hazardous atmosphere?
There is an official and logical reason, both of which should be understood.
First, the "official" reason:
OSHA's official wording:
Concentrations above 10% of the LEL are considered a risk for fires or explosions and are classified as Immediately Dangerous to Life and Health (IDLH) by OSHA and NIOSH.
NIOSH took a different approach and stated that:
Lower explosive limit (LEL): It was decided to restrict the “routine” entry into a possible explosive atmosphere to concentrations no greater than 10% of the LEL. [Note: SCP-derived IDLHs were set at 100% of the LELs if there were no known serious health hazards below these values. However, OSHA considers concentrations in excess of 10% of the LEL to be a hazardous atmosphere in confined spaces [29 CFR 1910.146(b)].]
IDLH is determined using both physical and toxic hazards. The physical hazard is quantified as 10% of the hazardous material's LEL.
The logical explanation is that by using 10% of the LEL, we have created a 90% safety factor. And inside a space with "limited or restricted means for entry or exit," TIME is a critical safety aspect when it comes to escaping from that space BEFORE we have achieved the LEL. Remember, most of these spaces required us to climb, contort, slide, and slither into them due to their limited or restricted means of entry. And if there are 2-3 (or more) entrants inside the space and we get an alarm at 10% of the LEL, how long will it take to achieve 100% of the LEL vs. how long will it take the entrants to climb, contort, slide, and slither OUT of the space?
If we are using OSHA's minimum expectations to define a "hazardous atmosphere," we need to consider doing better. Throw in the fact that many Entry Supervisors have not been adequately trained in the use of a calibrated direct-reading instrument and its LIMITATIONS; we can have workers in 40% of the LEL, and the meter reading will be below 10%.
SAFTENG members can read my dozen or so articles on the limitations of these meters and why our entry programs should be using 2% LEL - not OSHA's 10%.
