Environmental factors and heatstroke

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1 Environmental factors and heatstroke N. J. Shanks * and G. Papworth * RAS Gas Co. Ltd, Doha, Qatar; and Ministry of Health, Abu Dhabi, UAE The objective of this study was to determine the extent of the influence of temperature and humidity on the number of heatstroke presentations. Three hundred and forty-five labourers presented to the Accident and Emergency Hospital in Abu Dhabi with heatstroke during a 3 month summer period. There was no significant predictive association between the maximum daily temperature and/or humidity and the presentation of heatstroke. There was no significant association with the maximum temperature on the previous day, day of the week or temperature trend. The largest statistical correlation was between the maximum temperature and humidity and the log of the number of cases. It is possible that there are other significant explanatory variables that we have not included in the model. Key words: Arabian Gulf; heat illness; heatstroke. Received 26 April 2000; revised 17 July 2000; accepted 23 August 2000 Introduction The United Arab Emirates (UAE), one of the seven Gulf Cooperative Council countries, has a very hot and humid climate, with summer (June, July and August) temperatures exceeding 50 C along with relative humidity of 100%. Such a climate creates a potentially dangerous environment to those working outside. Heat stress is an occupational health issue [1]. With high heat stress levels, mental confusion can develop and predispose workers to perform unsafe acts: the risk of accidents increases with extremes of temperature [2]. Exertional heatstroke often affects young, fit, healthy individuals engaged in the military [3 5], sports and leisure activities, and during the Hadj pilgrimage to Mecca in the summer months [6]. We felt it would be of interest to investigate whether two easily observed environmental factors maximum daily temperature and humidity were responsible for presentation of cases of heatstroke, with a view to the potential use of these parameters in a predictive manner. Temperature is regulated by autonomic and behavioural thermoregulatory processes that modify the rate of heat loss and production by shivering, sweating, variations in basal metabolism and peripheral vasomotor tone. These regulatory processes aim to maintain the body temperature within a restricted range, the body Correspondence to: Nigel J. Shanks, Chief Medical Officer, RAS Gas Co. Ltd, Doha, Qatar. Tel: ; fax: ; njshanks@rasgas.com.qa core. The range of normal internal temperatures is estimated to be between 36 and 38 C, and the limits of body temperature for efficient thermoregulation at between 35 and 40 C [7]. Heat illness is a spectrum of disorders including heat syncope, heat exhaustion, heat cramps, classic and exertional heatstroke. The general population is at increased risk of heat illness during heatwaves [8]. Certain host factors that affect thermoregulation through heat loss mechanisms (e.g. fatigue, tiredness, dehydration, failure to acclimatize, skin disorders) or those that predispose to heat production (e.g. sustained exercise, obesity, low physical fitness, febrile illness) all contribute towards the development of hyperthermia. In particular, fluid and electrolyte losses or inadequate intake commonly contribute to heat illness. Susceptibility to malignant hyperthermia should be tested for in cases of heatstroke. There are two main factors affecting the body temperature when working in the heat. Metabolic factors, where the heat generated by the workload must be dissipated, and environmental factors including air temperature, radiant temperature, air speed and humidity. The body core temperature is controlled by four different physical methods: convection, radiation, conduction and evaporation. There are several stages of heat illness, ranging from heat cramps to heat exhaustion and finally to heatstroke. Heat-related illnesses cause 240 deaths annually in the USA [9]. Patients presenting to emergency departments with heat-related illnesses increased significantly 5 days before the first heat-related death [10]. It has been Occup. Med. Vol. 51 No. 1, pp , 2001 Copyright Society of Occupational Medicine. Printed in Great Britain. All rights reserved /01

2 46 Occup. Med. Vol. 51, 2001 recommended that real-time emergency department computer automated databanks be constructed to improve public health response to heatwaves. Heat illness may be prevented by identifying those at risk and using appropriate hydration, taking into consideration acclimatization and environmental conditions. Sunstroke was an important cause of death among troops in India in the nineteenth century [11], but if protected by a wicker helmet and well covered, no one need fear sunstroke [12]. Soldiers are probably more at risk than marathon runners; the practice of making them run in battle order promotes the risk [13]. There has been a reduction in heat casualties in the British Army, attributable to commanders following the wet bulb globe temperature (WBGT) guidelines [14]. Guidelines for preventing heat injury among military personnel are not directly applicable to civilian personnel. Military guidelines call for large volumes of prophylactic water consumption and restriction of physical activity, depending on WBGT [15]. The most effective means of reducing the body temperature is by the use of cool mist spray and fanning across the patient s body to promote rapid evaporation [16]. The effects of heatstroke are reversible and, with the proper precautions, entirely preventable [17]. We felt it would be of interest to investigate whether two easily observed environmental factors, maximum daily temperature and humidity, were responsible for presentation of cases of heatstroke, with a view to the potential use of these parameters in a predictive manner. Heat illness among labourers in the Gulf has previously been unreported. Material and methods The Central Hospital, Abu Dhabi, is the main accident and emergency hospital for the city of Abu Dhabi and the surrounding area. It provides care to new cases per year. All types of emergency cases are seen and a carefully maintained daily diagnostic log is maintained for future analysis. The population at risk is difficult to determine. The population of Abu Dhabi Medical District in 1998 was reported as being [18], of whom were male and female. Twenty-six per cent of the population is under the age of 15. The excess males are expatriate labour in a diversity of occupations. In 1995, the distribution of workers by sector was agriculture 7.1%, manufacturing 10.4%, construction 16.4%, trade and services 18.2%, transport and communication 9.3%, domestic service 11%, government services 13.9% [19]. For this prospective study, during the summer months of June, July and August 1998, a separate daily record was maintained of the time of presentation and number of patients seen with heatstroke. Heatstroke was the accepted diagnosis in those patients presenting with features of classical heat illness associated with a tympanic temperature >40 C. Those suffering from minor heat illness were excluded from the study. The highest and lowest recorded daily temperature and humidity were obtained from the published meteorology office data. In addition, recordings of highest daily temperatures were obtained from a large construction site, where it is necessary to have accurate temperature readings. A statistical analysis was performed to determine whether there was any association between maximum daily temperature and humidity and the number of patients presenting with heatstroke. The data were analysed using the SAS System for Windows version 6.12 on a personal computer. Log linear modelling was performed as the dependent variable (the number of patients) is a count. Counts have a Poisson distribution. Results Figure 1 shows the maximum daily temperature (in degrees centigrade) at the two sites, the maximum humidity (in per cent) and the number of patients seen with heatstroke. Figure 2 shows the number of patients seen with heatstroke plotted against the maximum daily temperature. The maximum temperature recorded from the construction site in the city tended to be higher. This is thought to reflect the location of the meteorology recording location at the airport. Three hundred and forty-five patients were seen and treated for heatstroke. All were fit, healthy males who were acclimatized to the heat. These were labourers, from Egypt or Pakistan, and all had been in the UAE for a minimum of 6 months. All patients, on presentation, were initially treated with 2 l of normal saline by i.v. infusion. Only one patient required admission due to additional pathology cerebral malaria. Routine biochemistry was abandoned, for cost-effectiveness reasons, after the first 50 patients revealed no abnormalities. Using the Poisson distribution and the log link function, the GENMOD (generalized linear model) procedure gave the final model: log(number of patients) = (temperature) (humidity) (1) The results are given in Tables 1 and 2. An additional analysis was performed using an interaction term (Tables 3 and 4). This was based on the premise that the effects of humidity and temperature are not independent. Statistically, this model gives a better fit in terms of likelihood ratio. However, the model including the interaction term gives negative values for the

3 N. J. Shanks and G. Papworth: Environmental factors and heatstroke 47 Figure 1. Maximum daily temperatures ( C), maximum humidity (%) and number of patients seen with heat stroke. Figure 2. Number of patients seen with heat stroke plotted against the maximum daily temperature. parameter estimates for temperature and humidity. This is clearly counterintuitive. Of concern is the large value of the scaled deviance in the final fitted model ( and 89 d.f.; P < ). The scaled deviance has an approximate χ 2 distribution with the residual degrees of freedom and a very small P value for this scaled deviance. This indicates a relatively poor overall fit for the model. Discussion The outstanding finding in this study was that there was no significant predictive association between the conventional maximum temperature and/or humidity and the number of patients presenting to the emergency department with heatstroke. The statistical correlation between the same day maximum temperature and

4 48 Occup. Med. Vol. 51, 2001 Table 1. Model using temperature and humidity: criteria for assessing goodness of fit Criterion d.f. Value Value/d.f. Deviance Scaled deviance Pearson χ Scaled Pearson χ Log likelihood Table 3. Model using temperature, humidity and an interaction term: criteria for assessing goodness of fit Criterion d.f. Value Value/d.f. Deviance Scaled deviance Pearson χ Scaled Pearson χ Log likelihood Table 2. Model using temperature and humidity: analysis of parameter estimates Parameter d.f. Estimate SE χ 2 Pr > χ 2 Intercept Temperature Humidity Scale Table 4. Model using temperature, humidity and an interaction term: analysis of parameter estimates Parameter d.f. Estimate SE χ 2 Pr > χ 2 Intercept Temperature Humidity Interaction term Scale humidity and the number of cases is given in equation (1). There was no significant association with the maximum temperature on the previous day, day of the week or temperature trend. The maximum humidity alone is a poor predictor in the model. The maximum temperature alone is a better predictor and gives a direct, positive correlation. The combination of temperature and humidity gives the best results. In the data set there are many observed values of zero counts (sampling zeros). The larger the sample size, the more stable the parameter estimates. It is possible with small sample sizes that the sampling zeros are overrepresented. It is also possible that there are other significant explanatory variables which we have not included in the model. The value of the demonstrated correlation is limited by the size of the residuals when use of the model in a predictive manner is considered. From a statistical viewpoint, this study and model might be improved by repetition with a greater number of centres providing geographic diversity and a longer period of study providing more cases. The other issue is the parameters that were recorded. The WBGT was not measured. This was unfortunate as the WBGT index is a method for determining how long a person can work in a hot environment. Other thermal stress indices, such as the Effective Temperature Index and Heat Stress Index, were also not measured. More recently, an easy, quick and accurate assessment of heat stress as a potential risk has been developed. The Modified Discomfort Index (MDI), compiled from ambient and wet bulb temperature, is easier to calculate and use than the WBGT. This is an attractive alternative to the WBGT index in assessing heat stress [20]. A physiological strain index to evaluate heat stress has also been developed [21]. Other potential confounding variables include the specific work environment temperature, air velocity and estimates of the actual workload, its duration and work/ rest schedules. Physical work elevates the metabolic rate, and this can add times more heat to the body than radiation and convection combined can remove [22]. It is possible that recognition of the early symptoms of heat stress might lead to prevention of its natural progression by fluid intake. We did not study the extent of fluid replacement in the workplace or compare the patients with unaffected peers. Recording patient temperature using an infrared ear thermometer is subject to error [23]. A true otoscopic technique was not used, thus the temperature of the mid-aural canal is likely to have been recorded. Errors may occur when the outer canal temperature is affected by evaporative head cooling [24]. Temperature measurements are closely connected with clinical condition, and rectal readings are rarely required. The three variables (insulation, permeability, ventilation) associated with clothing, which greatly alter thermal balance, were not studied. However, clothing was not considered a relevant factor as all patients wore thin, light, loose-fitting cotton garments. Certainly, no patients were wearing personal protective clothing or equipment (PPE), such as reflective items or ice/water-cooled garments, which have an important part to play in the prevention of heat illness in industry. PPE that could have aggravated heat stress (e.g. jackets, face masks, etc.) were not encountered.

5 N. J. Shanks and G. Papworth: Environmental factors and heatstroke 49 Sodium loss in sweat varies greatly and is not significantly related to the sweat rate. Acclimatization results in a significant decrease in sweat sodium and an increase in the sweat rate in the summer compared with the winter [25]. Biochemical abnormalities were not detected in this study. It has been suggested that a heat shock protein, which is synthesized after heat stress, could be useful as a marker of recent thermal stress [26]. Conclusion This study and model might be improved by repetition with a greater number of centres providing geographic diversity, and a longer period of study providing more cases. Acknowledgement The authors are most grateful to Dr Stephen Jones, Exxon- Mobil, for his support and thoughtful reviews of various drafts. References 1. Dukes-Dubos FN. Hazards of heat exposure. A review. Scand J Work Environ Health 1981; 7: Ramsey JD. Effect of the workplace thermal condition on safe work behavior. J Safety Res 1983; 14: Bricknell MC. Heat illness in Cyprus. J R Army Med Corps 1994; 140: Dickinson JG. Heat illness in the services. J R Army Med Corps 1994; 140: Kark JA, Burr PQ, Wenger CB, Gastaldo E, Gardner JW. Exertional heat illness in Marine Corps recruit training. Aviat Space Environ Med 1996; 67: Yaqub B, Al Harthi, S Al Orainey. Heat stroke at the Mekkah Pilgrimage: clinical characteristics and course of 30 patients. Q J Med 1986; 59: Stolwijk JA. Responses to the thermal environment. Fed Proc 1977; 36: Richards DA. Deaths in a heat wave. Aust NZ J Med 1995; 25: Barrow MW, Clark KA. Heat-related illnesses. Am Fam Physician 1998; 58: Rydman RJ, Rumoro DP, Silva JC, Hogan TM, Kampe LM. The rate and risk of heat related illness in hospital emergency departments during the 1995 Chicago heat disaster. J Med Syst 1999; 23: Anon. Disease among the European troops in India. Lancet 1858; ii: National Army Museum Archives. Letters from the Crimea and India by Major GG Clowes, 12 November 1858; ( : 6). Letters from Major Robert Poore, 15 April 1858 ( : 85). 13. Porter AM. Heat illness and soldiers. Mil Med 1993; 158: Bricknell MC. Setting heat stress limits for acclimatised soldiers exercising in heat. J R Army Med Corps 1997; 143: Cooper JK. Preventing heat injury: military versus civilian perspective. Mil Med 1997; 162: Scott J. Heat-related illnesses. When are they a true emergency? Postgrad Med 1989; 85: Eichner ER. Treatment of suspected heat illness. Int J Sports Med 1998; 19(Suppl. 2): S150 S Preventive Medicine Department. Annual Report, Abu Dhabi: Ministry of Health. 19. Abu Dhabi Chamber of Commerce and Industry. Abu Dhabi in Figures. Abu Dhabi. 20. Moran DS, Pandolf KB. Wet bulb globe temperature (WBGT) to what extent is GT essential? Aviat Space Environ Med 1999; 70: Moran DS, Shitzer A, Pandolf KB. A physiological strain index to evaluate heat stress. Am J Physiol 1998; 275: Bernard TE. Thermal stress. In: Plog BA, Niland J, Quinlan PJ, eds. Fundamentals of Industrial Hygiene. National Safety Council, 1996; Chapter 12, p Brengelmann GL. Dilemma of body temperature measurement. In: Shiraki K, Yousef MK, eds. Man in Stressful Environments: Thermal and Work Physiology. Springfield, IL: Charles C. Thomas, 1987; Ash CJ, Brenglemann GL. Rectal temperature in marathon runners. Ann Emerg Med 1997; 29: Bates G, Gazey C, Cena K. Factors affecting heat illness when working in conditions of thermal stress. J Hum Ergol (Tokyo) 1996; 25: Bratton SL, Jardine DS, Mirkes PE. Constitutive synthesis of heat shock protein (72 kd) in human peripheral blood mononuclear cells: implications for use as a clinical test of recent thermal stress. Int J Hypertherm 1997; 13: