■Oxygen Toxicity

When oxygen is breathed under high pressure, oxygen toxicity can occur. Lip trembling, convulsions, and nausea are common symptoms; coma can occur. As a result, oxygen-only tanks are never used. Even tanks filled with the normal mixture of air present a risk of oxygen toxicity at depths greater than 70m, where special mixes are required.

■Carbon Monoxide

Carbon monoxide is poisonous, so at high pressure even a small amount can be toxic.

Symptoms include headache, confusion, narrowing of the visual field, and translucence of the lips and nail beds. Although carbon monoxide is hard to detect as it is odourless and colourless, if, when doing your pre-dive check, you notice any smell or effect from the tank’s air supply, you should exchange tanks.

■Nitrogen Narcosis

At depths greater than 25m a feeling of intoxication can occur. This intoxication, called nitrogen narcosis, is dangerous because it has an effect of anesthesia. While the effects vary from none to great from person to person, the inability to think safely is a risk. If nitrogen narcosis is suspected, please return to a shallower depth as this will quickly resolve any symptoms.