Which gas is less soluble in blood and therefore poses less risk during rapid ascents?

Helium is less soluble in blood compared to other gases like nitrogen, oxygen, and carbon dioxide. This characteristic is particularly important for divers, especially when considering rapid ascents. When a diver ascends quickly, the body experiences a decrease in ambient pressure, which can lead to the formation of gas bubbles if the dissolved gases in the body fluids come out of solution too quickly.

Helium's lower solubility means that it can be eliminated from the body more rapidly than nitrogen, which tends to be more soluble. This reduces the risk of decompression sickness, commonly known as "the bends," during fast ascents. Additionally, divers often use helium in gas mixtures for deep dives because it minimizes the narcotic effects that nitrogen can have at high pressures.

Oxygen and carbon dioxide have higher solubility in blood than helium, which means they present greater risks during rapid ascents. Nitrogen, while less soluble than carbon dioxide or oxygen, is still more soluble than helium and contributes to the risks associated with decompression when ascending too quickly.