2.3 Decompression sickness

If you think back to William Trubridge and his free-diving record in the Introduction, you will recall that he swam up from a depth of 102 metres in just over 2 minutes, a rate of 51 m min⁻¹, without suffering any ill effects on his return to the surface. By contrast, scuba divers are advised not to ascend faster than 9 m min⁻¹ to avoid developing decompression sickness (‘the bends’).

This discrepancy between free-divers and scuba divers lies in the differences in partial pressures of gases that are inhaled under atmospheric pressure and under compression. When you breathe from a scuba tank, the air has the same pressure as the pressure of water at that depth. The pressure of water is much higher than air; for example, at 20 m below the surface of the water, the pressure exerted by water on the body is about three times that experienced on dry land.

The high pressure can cause some of the N₂ gas in the air to dissolve into the blood. As the diver swims back up to the surface, the PN₂ in the blood is higher than in the surrounding water, so N₂ will be released from the blood and into the alveoli to be exhaled. If the change in pressure happens too quickly, the N₂ will not have time to be exhaled and instead will form air bubbles (similar to what happens when you open a shaken can of fizzy drink). These bubbles can cause severe pain in joints and muscles and in extreme cases, death due to embolism.

Free-divers experience the same effects of PN₂ as scuba divers at deep depths. However, because free-divers are not breathing pressurised air as they dive, their lungs actually get compressed (down to a quarter of their original size) by the high pressure of the water. As the divers ascend, their lungs will slowly expand back to their original volume.

So, as the divers return to the surface, the PN₂ in the lungs decreases relative to the PN₂ in the blood and N₂ diffuses into the alveoli, thereby decreasing the chances that it will form bubbles in the tissue.