Safety 2008 - ASSE - American Society of Safety Engineers

Figure 1Figure 1Heat Stress ChartAmong the heat stress indexes that have beenproposed (Bingham et a., 2001), the wet bulb globetemperature (WBGT) is the current index supportedby the American Conference of GovernmentalIndustrial Hygienists (ACGIH, 2006). The ACGIHevaluation scheme was assessed to determine itsapplicability under the work conditions in Qatar.The normal daytime conditions in Qatar during theperiod of monitoring were a wet bulb temperatureof 82.4 °F (28 °C) and a globe temperature of 104 °F(40 °C) taken in the shade. These conditions correspondto a calculated WBGT value without directexposure to sun of 89 °F (31.6 °C). In addition:•Clothing was assumed to allow air movement.No WBGT adjustment was made for clothing type.•The heavy activities category was selected,which corresponded to the type of activities performedby a significant proportion of staff, includingconstruction workers involved in several majorexpansion projects who were the primary concernfor exposure to heat stress.•Acclimatization to heat was assumed.Under these conditions, the ACGIH screening criteriafor 25% work, 75% rest was exceeded and additionalpersonal evaluation of the task would havebeen required. Since it was anticipated that thesewould be the prevailing conditions for the months ofJuly and August in Qatar and the required personalmonitoring was not practical on a large scale, otherindexes were considered for use. In an effort to provideflexibility and ease of measurement the heatstress index was selected as the monitoring criteria.Specifically, the historical air temperature observedduring the month of August for the period of 1962 to2003 in Qatar was 95 °F (35 °C) and 118.4 °F (48 °C) forthe average and average maximum (CAA). These valueswere obtained by averaging the reported dailyaverage and high temperatures, respectively. Theaverage and average maximum relative humiditywere 57% and 80%, respectively, calculated using asimilar methodology for the same timeframe. If noteffectively controlled, such conditions have led toheat-related disorders among workers (AIHA, 1975).Data collected during the survey provided severalinteresting observations that were useful to defininginteractions of heat stress parameters; identifying keypatterns of heat stress conditions; and describing dayand night extreme heat conditions. Specifically, theconditions observed during August 2006 were typicaland resulted from the combined effect of high relativehumidity brought in by the east wind and elevated airtemperature. A total of 660 hours of monitoring datawas collected using heat stress monitors that recordeddry bulb temperature, natural wet bulb temperature,relative humidity, globe temperature and heat index at1-minute intervals. A nearby monitoring station(RLIC, 2006) recorded the wind conditions. The effectof providing shade under high relative humidity wasevaluated by comparing data from shaded and directsunlight heat stress monitor locations.Note. Adapted from U.S. NOAA National Weather Service Heat Index.32 PROFESSIONAL SAFETY AUGUST 2008 www.asse.orgDaily PatternsFor the monitoring period (Aug. 6 to Sept. 18), thedaily averages of dry bulb temperature and relativehumidity were 94.5 °F (34.7 ºC) and 61%, respectively,corresponding to a heat index of 45, obtained asthe average of all the daily values used to createFigure 2. The average heat index parameters areillustrated throughout a 1-day cycle in Figure 2. Theparameters shown were obtained by averagingreadings for the corresponding 3-hour portion of theday over the monitoring period. Figure 3 shows asample daily pattern observed in September 2006.A heat index level of 54 was selected as the point atwhich all work should be stopped, as it is associatedwith extreme danger and potential heat stroke in theheat index chart (Figure 1). In addition, a heat indexlevel of 50 was defined as the point at which all ele-