Why are snorkels longer than 15 inches not recommended for use?

Snorkels longer than 15 inches are not recommended primarily due to their increased dead air space. This refers to the volume of air within the snorkel that does not participate in gas exchange during breathing. A longer snorkel can trap a significant amount of air that remains stagnant during inhalation and exhalation, which can result in inefficient breathing. The diver may have to inhale more effortfully to get fresh air, leading to a less comfortable and potentially more tiring experience.

The dead air space in longer snorkels also makes it more challenging for divers to access the clean, fresh air at the water's surface efficiently. When using a snorkel with excessive dead air space, a diver may risk hypoxia, as they could be breathing mostly stale air, especially if they are submerged or if there are long periods between breaths.

In this context, while the other options may have some relevance, they do not specifically address the primary concern regarding snorkel length as effectively as the issue of dead air space does. This fact highlights the importance of snorkel design in ensuring optimal breathing efficiency for underwater activities.