Are We Warmer Diving with Nitrox

Nitrox offers a number of benefits for divers, but one that's generated some controversy is the notion that breathing Nitrox keeps us warmer underwater.

First, none can argue with a person's experience. If some individuals claim to feel warmer when diving Nitrox, then it is difficult to argue with that. However, many other factors influence warmth, such as whether the diver has just eaten, the level of insulation worn, and the old "psychological factors," which can influence us all.

Humans cool about four times faster in water than in air of the same temperature. Body temperature remains stable only in water that's about 95oF. When we get colder, the main response is peripheral vasoconstriction to reroute the blood away from the cold skin. We also increase heat production via shivering and burning energy metabolically. In extreme cold, both cold and pain receptors are stimulated, which leads to metabolic changes in the skeletal muscle and liver cells; we would increase our consumption of oxygen (O2) and production of carbon dioxide (CO2).

Various studies using nitrous oxide (N2O) as an analogue for nitrogen narcosis have demonstrated a narcosis-induced decrease in the shivering response. Due to the inhibitory effect of narcosis on shivering, divers are predisposed to heat loss when diving with compressed air or other gases with a high partial pressure of nitrogen.

Both intermittent and prolonged exposure to hyperoxia (high PO2) have been shown to suppress the metabolic response to cold in animals, but that reaction hasn't been observed in humans immersed in cold water.

Experiments conducted with N2O in humans have revealed that sub-anaesthetic levels of narcosis increase thermal comfort during mild hypothermia. Divers perceive their body temperature to be higher than when in identical conditions without narcosis. In other words, if you are narked, you may feel warmer, but you're not.

Does one feeling warmer when diving Nitrox? There is no clear reason why that should be the case, and there seem to be no published studies of Nitrox's effect on thermoregulation. One could speculate, however, that if a diver claims to feel less narcosis with Nitrox, then the shivering response might not be as blunted by narcosis, so he would maintain heat production. Future research will, no doubt, look more closely at these nonthermal factors and their interactions with each other.

Diving with a closed circuit rebreather -- now that does indeed reduce the heat lost through ventilation because the breathing gas is recirculated.

A version of this article, by Lynn Taylor, Ph.D., appeared in Dive New Zealand.