Which gas is primarily responsible for causing decompression sickness?

Decompression sickness, often referred to as "the bends," is primarily caused by nitrogen. When divers descend, they breathe in a mixture of gases, including oxygen and nitrogen, at higher pressures. As the pressure increases, more nitrogen is dissolved in the body's tissues. During the ascent, if a diver rises too quickly, the pressure decreases, and nitrogen that was previously dissolved in the tissues comes out of solution and forms bubbles. These bubbles can cause various symptoms, from joint pain to more severe issues affecting the nervous system or lungs.

While oxygen is essential for respiration and carbon dioxide is a byproduct of metabolism, they are not the primary gases responsible for decompression sickness. Helium is sometimes used in gas mixtures for deep dives to minimize narcosis and reduce the amount of nitrogen exposure, but it is not the main culprit for decompression sickness either. Nitrogen is the key gas that leads to these potentially dangerous physiological effects in divers who do not ascend slowly enough or who do not use appropriate decompression protocols.