The effects of different thermal environments on performance of manual handling tasks.

Many people work in uncomfortable thermal environments where the need to perform manual handling tasks is a fundamental requirement. A review of the research into the effects of performing manual handling in these environments revealed design limitations and gaps in knowledge. Current industry guidance merely states that 'extremes of heat and humidity should be avoided'. The purpose of this thesis was to study participants' physiological and subjective responses while lifting in hot, warm and cold environments. Three studies were conducted using the psychophysical approach where participants self-selected the load in a floor to knuckle-height lift. For the first study 12 males (mean +/- sd), age 25.2 +/- 6 yrs, mass 74.9 +/-11.9 kg, stature 1.73 +/- 0.1m were recruited and acclimated over five days (1 hr sessions) in an environmental chamber at 38&deg;C, 70% relative humidity (RH). They completed 15x35-min trials on consecutive weekdays in five environments: thermoneutral, 21 &deg;C, 45% RH (17&deg;C WBGT); warm-dry, 30&deg;C, 25% RH (22&deg;C WBGT); warm-humid, 30&deg;C, 65% RH (27&deg;C WBGT); hot-dry, 39&deg;C, 22% RH (27&deg;C WBGT); hot-humid, 38&deg;C, 70% RH (34&deg;C WBGT) and three lift frequencies: 1, 4.3 and 6.7 lifts.min -1. Heart rate and aural temperature were significantly higher and maximum acceptable weight of lift (MAWL) significantly lower in the hot-humid environment compared to all others. Ratings of perceived exertion (RPE) were significantly higher in hot-humid compared to both warm-dry and thermoneutral. Although participants reduced workloads in the heat, they did not compensate adequately. There were no significant differences in response between two environments with the same WBGT (27&deg;C).Secondly, 12 males, age 26 +/- 5.6 yrs, mass 75.1 +/- 9.2kg, stature 1.77 +/- 0.1m were recruited. They completed 15x35-min trials in five environments: thermoneutral, 16&deg;C, 65% RH; 10&deg;C, 55% RH; 5&deg;C, 45% RH; 0&deg;C, 55% RH (standard ensemble); 0&deg;C, 55% RH (enhanced ensemble) and the same three lift frequencies. MAWL significantly decreased at higher lift frequencies. Mean aural temperature was significantly lower at 0&deg;C (standard) compared to thermoneutral. Mean MAWLs were higher than in the heat suggesting that participants increased activity to keep warm possibly placing them at greater risk of musculoskeletal injury. In all environments below thermoneutral the mean end aural temperature was <36.2&deg;C when lifting at 1 lift.min[-1].Finally, 10 males, age 28.4 +/- 5.1 yrs, mass 79.5 +/- 13.1 kg, stature 1.8 +/- 0.1 m were recruited to assess the effects of face-cooling on physiological strain and perceived exertion while lifting at 6.7 lifts.min[-1] in 30&deg;C, 65% RH (27&deg;C WBGT). Face-cooling significantly reduced local skin temperature and heart rate. There were no other significant differences. Face-cooling seems to be limited to mediating RPE (encompassing thermal strain) independent of core temperature which might continue to rise. The following recommendations are suggested for inclusion in future industry guidance. Workers should not regulate their own workloads in uncomfortable environments. RH does not impose additional strain in air temperatures up to 30&deg;C. At ~39&deg;C care must be taken when RH exceeds 25%. Attention must be paid to workers' clothing ensembles in temperatures below 16&deg;C so that they provide adequate insulation. Finally, face-cooling should not be thought of as a protective mechanism against heat stress.