In a previous post, I discussed atmospheric hazards related to oxygen deficient and enriched environments. One of the ways that an atmosphere can become oxygen deficient is from another gas being introduced into the area and pushing all the oxygen out. We refer to this process as oxygen displacement. But just how much of that other gas is present? That is the answer we are going to answer today.

Let’s do the math

In order for our oxygen concentration to decrease 1% (I.E., from 20.8% to 19.8%), there needs to be five times that amount, or 5% of another gas pushing it out. The reason for that is because oxygen is only 1/5 of the normal atmosphere. Let’s remember that we can also express percent of atmosphere in terms of parts per million (PPM), with 1% equaling 10,000 PPM.


Step 1 – Subtract your current oxygen percentage from 20.8 to determine how much it decreased.

Step 2 – Multiply that number by 10,000 to convert it to PPM.

Step 3 – Multiply that number by 5 to account for the fact that oxygen is only 1/5 of normal atmosphere.

Practical application

Observe the readings on the detector and calculate how much of another gas is present in the confined space.

Step 1 – 20.8% – 19.1% = 1.7%

Step 2 – 1.7% x 10,000 = 17,000 PPM

Step 3 – 17,000 PPM x 5 = 85,000 PPM

That is A LOT of another gas! It could easily be well above the immediately dangerous to life and health (IDLH) level for that gas. If your detector does not have a sensor capable of detecting the displacing gas, you may have no indication that it is present other than your oxygen percentage decreasing. The moral of the story is that any drop in your oxygen percentage may indicate something bad is present!

Bill Elder
Owner / Lead Instructor

Elder Technical Rescue Services, LLC