Sex Differences in Diving: Fact or Folklore.

The first female Navy diver made history in 1975 when graduating from the Navy’s Deep Sea Diving School after Navy divers were exclusively male for more than 100 yr. The physical differences between men and women have often been cited to discourage women from becoming divers. However, the public image of women being too frail to physically perform the same as men has been shattered despite longstanding exclusionary views within this male dominated community V opinions which remain today (see the comments following this article (25)) even though not endorsed by the Department of Defense. A recent implementation plan states that the first female enlisted Navy Sea, Air, and Land Teams (SEALs) could be assigned to units in fall 2017, and the first female SEAL officers could be in place by 2018 (25). Yet, varying views about the ability/ safety of women as divers remain. The greatest potential physiological differences between male and female divers include physical performance, decompression sickness (DCS), and thermal balance. Disparate qualities of research and much supposition, though, make it difficult to weed out fact from fiction. On average, women have lower maximal aerobic capacities and produce less speed and power than men. Multiple layers of underclothing, dry suit, mask, water-filled tube suit with power supply and pump, rebreather or SCUBA tanks, and weights to counteract buoyancy can easily force a diver to carry an additional 150 lb of gear today (and 250 lb when women first became Navy divers! ). Additionally, dive suits, facemasks and air tanks are designed for men, forcing women divers to adapt gear for a better anatomic fit (40). These challenges could increase physical demand before water entry and make movement underwater cumbersome. Because many combat diving roles involve light physical activity levels on the sea bottom, power and speed may not be as critical. Conversely, manual dexterity/accuracy and fine motor skills, which have been suggested to be better in women by some (16,35,36) but not others (24), may be more valuable skills. Regardless, the physical performance standards required for divers are sex-neutral, so a female diver, who can pass these standards, should not have any greater risk of reduced physical performance while diving than a man. It also is possible female divers could have better overall performance than men. Numerous diving duties require buoyancy and because women have greater subcutaneous fat stores, this may prove advantageous in some diving jobs. Furthermore, not only has the energy expenditure of surface swimming been shown to be less in woman than men even after correcting for body size (33), but women divers also expend less energy when swimming underwater using predominantly fins and scuba gear compared to men (34). This was attributed to less drag in female divers who remained more horizontal than men and suggests women are more economic divers if men do not proactively position their diving weights to adjust attitude. Consequently, if male divers consume greater amounts of oxygen (i.e., gas), this results in shorter sustainable dive times than women. Therefore, overall, because training adaptations are similar in men and women and women have some inherent advantages, it is possible that female divers could physically outperform male divers. After early work found women were four times more likely than men to get altitude DCS based on incidence in the US Air Force from 1966 to 1977(4), the concept of women being more susceptible to DCS was born; later work, though, revealed no such differences (41). These results may not be comparable to DCS in hyperbaric exposures because diving and altitude exposures result in different gas saturation profiles. A 1979 questionnaire suggested that women divers reported a 3.3-fold greater incidence of either suspected or diagnosed DCS (1), but women often self-report symptoms more readily than men (10) and numerous subsequent epidemiological studies found that DCS risk in diving was independent of sex (28,29). The Naval Diving and Salvage Training Center examined the logs of 878 dives and found no increased risk of DCS among female divers; in fact, men had a 1.3% higher risk than women (43). Similar results were found in two retrospective cohort studies of dive masters in which DCS risk was independent of sex (22,34). The exact etiology of DCS is not known, but is believed to result from an event cascade initiated by nitrogen bubble formation (15). The current model suggests that while bubbles are required for DCS, they are not sufficient for causing DCS. The few randomized controlled studies on DCS risk in female divers are based on bubble formation as outcome measures with inconsistent results. Women had fewer circulating bubbles after open-sea SCUBA dives than men (5). Others have found no difference in bubble incidence, duration, latency, and time to peak grade after prolonged dry (6) and wet (14) hyperbaric exposure compared to men. INVITED COMMENTARY