Comparison of constant vs. intermittent forced-air ventilation under body armour and the impact on heat strain, thermal perception and thermal comfort

We investigated the impact of wearing a forced-air-ventilation system built-into body armour (13kg) on heat strain and thermal perceptions when 18 volunteers (1 female) exercised (60 minutes at 1.07 L.min^-1oxygen consumption) then rested (60 minutes) in 40°C, 20% RH air. We hypothesized that: heat strain would be reduced with constant air-ventilation (AV) compared to a no-ventilation control (CON); intermittent air-ventilation, either 1 minute on/off (AV1) or 2 minutes on/off (AV2), would reduce heat strain similarly to AV; AV1 & AV2 would better reduce perceptual strain compared to CON and AV. Compared to CON, during work, none of the AV conditions influenced the rise of rectal temperature (0.93°C to 0.93°C), but mean(SD) rate of heat storage (S) during the last 40 minutes of exercise was reduced from 271(89)W during CON to 227(73)W during AV (P<0.01), with that for AV1 and AV2 being intermediate and not different to these. During the last 40 minutes of rest, the mean rate of fall of S was enhanced from CON at 74(79)W to AV at 139(96)W (P<0.05), with AV1 and AV2 being intermediate to both. Heart rate (HR) was lower in AV compared to CON for the last 40 minutes of recovery (P<0.005), with HR for AV1 and AV2 being intermediate. After two hours of exercise and recovery, AV resulted in less than half the amount of additional heat stored than CON (P<0.05), with AV1 and AV2 being intermediate, but closer to AV than CON. Compared to CON, thermal comfort was improved in AV2 after 20 minutes of work (P<0.05), and in AV and AV1 after 40 minutes of work (P<0.01). During rest, after 20 minutes AV was more thermally comfortable than CON (P<0.05), whilst comfort in AV1 and AV2 were intermediate to AV and CON. Thermal sensation measures indicated that participants felt less hot compared to CON in AV by 20 minutes of work (P<0.05), and in AV1 by 40 minutes of work (P<0.01). During rest, the participants felt less hot compared to CON after 20 minutes in AV (P<0.01), after 40 minutes in AV1 (P<0.01), and bythe end of rest in AV2 (P<0.001) .Less sweat was produced, but more evaporated in AV compared to CON (P<0.01), resulting in 70% of sweat produced in AV evaporating, but only 53% in CON, AV1 and AV2 being intermediate. Participants preferred all AV compared to CON, with the most preferred being AV2 (P<0.01). We conclude that constant AV reduces heat strain, and to a greater extent during recovery than work, with perceptual benefits of AV during work and rest. During work, the perceptual benefits of intermittent AV (AV2 and then AV1) were obvious, whereas during rest constant AV produced better perceptual benefits.