Evaluation of Heat Stress and Strain in Electric Utility Workers
- Author(s): Brown, Eric Nicholas
- Advisor(s): Jackson, Richard
- et al.
Heat stress is a known occupational hazard. The purpose of this dissertation was to evaluate industrial hygiene heat exposures in electric utility line workers during work in regions with high ambient temperatures (75-115°F) and low relative humidity (8-37%).
Relationships between accepted heat stress and heat strain variables were examined. New variables, namely a modified version of the Physiological Strain Index, and the differences from baseline for the oral temperature and heart rate were examined as well.
The hypotheses tested in this study are as follows:
(1) Certain tasks and job classes had higher levels of heat stress and heat strain than others;
(2) The measured and derived independent heat stress variables, such as Wet Bulb Globe Temperature levels, metabolic effort levels, exposure levels differences from applicable guidelines and standards, and personal factors (age, weight, body mass index, acclimatization), influence or predict measured and derived dependent heat strain variables (heart rate, body temperature, and Physiological
(3) The test variable of the modified Physiological Strain Index offered improvement over the unmodified version;
(4) The test variables of the difference in heart rate from baseline measurements, and the difference in body temperature from baseline were significant improvements over using the heart rate or body temperatures alone; and
(5) The level of ambient temperature increase from global warming had an impact on occupational heat stress exposures.
The design of the study was a cross-sectional study. Eighty nine subjects, age 18-64, and 428 individual tasks of one to four hour length were evaluated in two populations.
Resultant data were analyzed using univariate comparisons and mixed effects multiple linear and logistic regressions.
Results indicated that workers conducting certain tasks (digging) and working as certain job classes (groundmen and journeyman linemen) represented exposures to higher heat stress levels than others. Results also showed that workers performing certain tasks (digging) and classes (Groundmen and apprentice linemen) exhibited
higher levels of heat strain. Personal variables (age, past shift work) were significantly predictive of increased heat strain. Race was not correlated to heat strain. The analysis of acclimatization status as a variable was removed from this study due to dissimilar populations. Heat exposure standards (ACGIH and NIOSH) appeared to be conservative and protective to the worker, however, this population (utility workers) is a very healthy population and shouldn't necessarily represent the general working population.
California OSHA's use of 85°F as a trigger temperature did not correlate well with increased heat strain metrics. The 95°F set-point correlated better. Oral temperatures, per se, were not correlated to exposure, but the differences in oral temperature from baseline were better heat strain indicators. Heart rate increases were strong indications of heat strain, but the differences in heart rate from baseline were better indicators, especially for tasks with less metabolic load variance. Heart rate set-points of 110 bpm and 120 bpm correlated well with increased exposures. The modified Physiological Strain Index was not an improvement over the non-modified one.
And applying forecasted global warming by the end of the century to the population in this study would increase worker exposures from 33% to 100% of the population being over the recommended limits.