Fluid losses and hydration status of industrial workers under thermal stress working extended shifts.

AIMS: To assess whether workers under significant thermal stress necessarily dehydrated during their exposure and whether "involuntary dehydration" was inevitable, as supported by ISO 9866 and other authorities. Other objectives were to quantify sweat rates against recommended occupational limits, to develop a dehydration protocol to assist with managing heat exposures, and to understand the role of meal breaks on extended shifts in terms of fluid replacement. METHODS: A field investigation to examine the fluid consumption, sweat rates, and changes in the hydration state of industrial workers on extended (10, 12, and 12.5 hour) shifts under significant levels of thermal stress (wet bulb globe temperature (WBGT) >28 degrees C) was conducted on 39 male underground miners. Urinary specific gravity was measured before, during, and at the completion of the working shift. Environmental conditions were measured hourly during the shift. Fluid replacement was measured during the working periods and during the meal breaks. RESULTS: Average environmental conditions were severe (WBGT 30.9 degrees C (SD 2.0 degrees C), range 25.7-35.2 degrees C). Fluid intake averaged 0.8 l/h during exposure (SD 0.3 l/h, range 0.3-1.5 l/h). Average urinary specific gravity at start, mid, and end of shift was 1.0251, 1.0248, and 1.0254 respectively; the differences between start and mid shift, mid and end shift, and start and end shift were not significant. However, a majority of workers were coming to work in a moderately hypohydrated state (average urinary specific gravity 1.024 (SD 0.0059)). A combined dehydration and heat illness protocol was developed. Urinary specific gravity limits of 1.022 for start of shift and 1.030 for end of shift were selected; workers exceeding these values were not allowed into the workplace (if the start of shift limit was exceeded) or were retested prior to their next working shift (if the end of shift limit was exceeded). A target of 1.015 as a euhydrated state for start of shift was adopted for workforce education. CONCLUSIONS: This study found that "involuntary dehydration" did not occur in well informed workers, which has implications for heat stress standards that do not make provision for full fluid replacement during heat exposure. Fluid replacement during meal breaks was not significantly increased above fluid replacement rates during work time, with implications for the duration and spacing of meal breaks on long shifts. Testing of urinary specific gravity was found to be a good indication of hydration status and a practical method of improving workforce awareness and understanding of this important risk factor. Approximately 10 000 dehydration tests have been conducted under the dehydration protocol in a workforce of 2000 persons exposed to thermal stress and has proved practical and reliable.

Fatigue in industrial workers under thermal stress on extended shift lengths.

A field investigation to examine the fatigue levels in industrial workers working extended (10, 12 and 12.5 h) shifts under significant levels of thermal stress was conducted on 45 male underground miners. Studies were conducted both before and after a major change to the working-in-heat protocol used at the operation. Prior to the change, shortened (6 h) shifts had been used when thermal conditions exceeded certain values. This reduced shift length was removed and replaced with other protocols. Heart rates were continuously monitored, and a cycle ergometer was also used to assess cardiovascular fatigue over the shift. Average heart rates, as well as highest 10 and 30 min averages, and heart rate durations within various bands were analysed. No worker reported heat illness during the study. Results showed that removing the shortened shift did not increase the fatigue levels. Workers did experience fatigue, but this occurred in the first half of the shift. Evidence was found that these workers practised self-pacing.