Cardiac strain among women workers in an industrial laundry

Occupational heat exposure standards define permissible thermal conditions on the basis of metabolic heat production. However, even when energy expenditure is low, upright posture and repetitive upper limb motion may influence thermoregulatory behaviour through increased cardiovascular stress. The present study was undertaken to examine the relationship between work activity, thermal exposure and cardiac strain among women laundry workers engaged in sedentary, repetitive work activity. Ambient temperatures, work activity and heart rate were recorded during complete work shifts for 11 women over three days in summer and three days in winter. Workstation temperatures were significantly higher in summer. Analysis of continuous recordings of heart rate with respect to work activity showed (i) parallel increases in pulse rate and dominant arm movement frequency; (ii) different patterns of heart rate fluctuations for the two work cycles, with peaks during particular sub-tasks and when arms were in elevated positions. Recommended limits for cardiac strain indices were surpassed in both seasons, although in summer they were exceeded significantly more frequently. The part of cardiac effort attributable to thermoregulatory adjustments was also higher in summer while the fraction reflecting metabolic needs did not change with the season. These findings demonstrate high levels of cardiac strain in this work situation and raise the question of redefining heat exposure standards to include the prevention of excessive cardiac strain resulting from cumulative effects of heat load and ergonomic stressors.