Standby divers in warm to hot environments suffer heat stress and hypohydration

Standby divers must be fully dressed in the appropriate ensemble during military and commercial diving operations. These garments are often fully encapsulating and may result in heat stress and hypohydration when worn in warm environments. We examined the physiologic responses to heat in subjects wearing a Viking HD drysuit during 1 hour of exposure to dry-bulb temperatures of 33°C, 36°C, 39°C, and 42°C. Euhydrated subjects donned a heavy rubber drysuit and chemical protective gloves over a lightweight base layer. The drysuit was appropriate for contaminated water diving with integrated boots and neck dam. Heart rate (HR), core (Tc), and skin (Tsk) temperatures were monitored. Eight subjects (4 males) aged 27 ± 5 y completed all study conditions. HR and Tc increased over time (<i>p</i> < 0.001). Seated HR peaked at 138 ± 17 bpm in the 42°C and at 114 ± 13 bpm in the 39°C condition. Peak Tc was 37.0 ± 0.2, 37.1 ± 0.2, 37.4 ± 0.3, and 37.9 ± 0.5°C in the 33°C, 36°C, 39°C, and 42°C conditions, respectively, and differed between all conditions (<i>p</i> < 0.001) except 33°C and 36°C (<i>p</i> = 0.60). Sweat rates progressively increased in the warmer conditions and corresponded with a -0.20 ± 0.10, -0.40 ± 0.19, -0.69 ± 0.46, and -0.99 ± 0.55% change in body mass. Even in the absence of radiant heating, significant hypohydration and heat stress occurs in standby divers after 30 min of exposure to 42°C and after 40 min at 39°C. Awareness of the conditions and rotation of standby divers could increase mission safety in these hot environments.