Never Share Your SCUBA Mouthpiece with a Free Diver!!!!

You may recall (or not) during your SCUBA training that you were warned not to share air with a

snorkeler or free diver. But, do you recall why? And, do you recall that this simple act could be deadly?

The deadly part of this scenario is called air embolism, i.e., over-inflation of the free diver’s lungs that

can cause injury during ascent. This is precisely the same injury that can affect a SCUBA diver who does

not breathe (or at least exhale) during ascent.

As either of these individuals ascends towards the surface without breathing, the air inside the lungs

expands…with potentially lethal consequences.

Let’s consider a simple example of a diver at 33 feet below the surface (2 atmospheres pressure). As

he/she surfaces the pressure reduces to 1 atmosphere, and the volume of the air doubles…potentially

injuring the lungs.

A simple form of Boyles Gas Law applies as follows:

(P1) (V1) = (P2) (V2) or, Pressure times Volume at location 1 = Pressure times Volume at location 2.

To keep the math very simple let’s measure pressure in atmospheres and measure volume in lungs-fulls.

At the surface the diver breaths one lungs-full at one atmosphere. Pressure and volume of the air don’t

change.

For a known change in pressure situation, we solve the Boyle’s Gas Law equation for V2 and get V2 =

(P1) (V1)/P2.

If we again consider the diver who fills his/her lungs with air at 33 feet and ascends without exhaling, we

can plug in values as follows and solve the equation: V2 = (2 atm) (1 LF)/(1 atm) = 2 Lungs-full. Since one

diver can’t hold two lungs-full in volume, injury results.

Please don’t let it happen to you or your acquaintances.

Be Safe. Dive Safe.

Dave Wills