I will again reference my 30 years of teaching emergency response courses and the surprising lack of hazard awareness so many “certified technicians” lack. The one that get a lot of attention is the relationship between TOXIC and PHYSICAL hazards when the HAZMAT is both toxic and flammable. I have shared many tips over the years, but the following is one that every safety professional should get tattoed onto their forearm:
1% = 10,000 ppm
So, let’s look at that relationship and think that we measure Lower Explosive Limits (LEL) in percentage and most often use ppm to measure inhalation hazards from vapors and gases. We can begin to see the relationship. This is a key relationship to understand when working in known hazardous atmospheres.
We tend to focus on the acute risks when dealing with flammable liquids and gases: ignition of the vapor/gas and the consequences of that event. And YES, we need to be well aware of that hazard. But remember this: Most of the flammables we work around have LELs less than 5%; most are in the 2-3% range. Those percentages translate into the tens of thousands of parts per million (ppm).
Take gasoline for example:
The FP of Gasoline is -50°F, meaning that it will almost always be above its FP at standard temperature and pressure (68F and 14.7 psi).
Gasoline’s LEL is 1.2% or, in terms of ppm, 12,000 ppm.
This means that we only need 1.2% of the volume of air to be gasoline vapors, and we have enough to ignite. Keep in mind that gasoline vapors are heavier than air, so they will accumulate at ground level and achieve their LEL at ground level—AND NOT evenly dispersed throughout the height of the space.
But here is the kicker…
Gasoline’s TLV is 300 ppm averaged over an 8-hour workshift or in terms of % that is 0.03%. A far cry from the LEL of 1.2%
In other words, we may not be in a flammable atmosphere, but we are in a hazardous atmosphere and need to protect the workers against the TOXIC hazards of gasoline.