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This presentation is based on content presented at the Exploration Safety Roadshow held in August 2009 It is made available for non-commercial use (e.g. toolbox meetings) subject to the condition that the PowerPoint file is not altered without permission from Resources Safety Supporting resources, such as brochures and posters, are available from Resources Safety For resources, information or clarification, please contact: or visit

Toolbox presentation: Heat Stress

Exploration Safety Roadshow 2009
Outline Thermoregulation Heat gain, storage and loss Heat strain and related illnesses Risk factors for heat strain Risk assessment and control We can extend from your experience to be able to identify situations and people at risks and more importantly manage such risks. It’s a good idea to tie in this experience with some of the physiology and theory behind heat stress.

Normal body temperature
Thermoregulation Normal body temperature Optimal conditions for cellular reactions in the human body include a core body temperature of approximately 37°C

Thermoregulation Heat sensors in the skin and body – transmit “information” to hypothalamus in the brain, which directs an increase in heart rate, vasodilatation and sweating Sweat loss may be as high as 1.5 litres per hour Evaporation of one litre of sweat can be equivalent of 670 watts of energy – highly effective NEXT SLIDE HEAT GAIN STORAGE AND LOSS Throughout the presentation we will see the intimate links between vasodilation, relaxation of peripheral blood vessels, sweating, blood volume, and heart rate as they relate to heat strain A core body temperature between 36°C and 39°C can be maintained despite external conditions that may range between 12°C to 60°C using a number of physiological processes that are collectively known as thermoregulation. Thermoregulation is orchestrated by the hypothalamus in response to information transmitted from heat sensors throughout the skin and body. The response to rising core body temperatures includes an increase in heart rate and the activation of heat reducing mechanisms such as vasodilation and sweating. An increase in core body temperature results in activation of the sympathetic nervous system and the subsequent vasodilation of the capillary network just under the surface of the skin. Vasodilation can be defined as the relaxation of the muscular wall of a blood vessel resulting in an increase in the vessel’s diameter (lumen). In the process of vasodilation blood flow to the peripheral blood vessels may be increased by up to 30%, increasing the rate of heat exchange from the blood to the skin. Blood, having been warmed at the core, arrives at dilated capillaries in the outer shell to exchange heat to the skin and surrounding environment through the process of convection and radiation.The increase of blood flow to the skin during vasodilation results in an increase in heart rate in order to maintain cardiac output. The process of sweating is initiated by the hypothalamus and the subsequent activation of the sympathetic nervous system resulting in the release of acetylcholine on the cells of the sweat glands. Acetylcholine stimulates the transport of sodium and chlorine ions out of the cells followed by the movement of water down its concentration gradient into the sweat gland and onto the surface of skin as sweat. As the body becomes acclimatised to a hot environment, the concentrations of solutes in sweat decreases and the volume increases[. Daily water loss through sweating may be as high as 1.5 litres per hour and even 3 litres per hour over short periods in extreme conditions. The effectiveness of heat loss through the evaporation of sweat has been estimated to the equivalent of 12 times the basal heat production or 800 calories of body heat per hour, or a loss of 675 watts of energy for each litre of evaporated sweat.

Heat gain, storage and loss
Exploration Safety Roadshow 2009 Heat gain, storage and loss Heat inputs Heat production – from metabolic activity or work intensity Heat storage – due to insulation of the “inner core” Heat gain – from external heat sources (radiation and convection) In temperatures below 25°C, “…increase in core temperature is proportional to work intensity and relatively independent of environmental conditions”. Beyond these temperatures the external environment has an increasing influence in raising core body temperature.

Exploration Safety Roadshow 2009 Heat gain, storage and loss Physics of heat transfer Conduction – transfer of heat between two materials from high to low heat energy areas Convection – bulk transfer of heated matter from warm low density regions via a moving fluid (gas or liquid) to cooler more dense areas Radiation – process of heat transfer over distance between surfaces (particularly at the infrared wavelength) Evaporation – warmer molecules gain sufficient energy to leave the liquid surface and enter the gaseous phase. Remaining molecules have less average kinetic energy, resulting in decrease in temperature of liquid Physics of heat transfer Transfer of heat between the body and the surrounding environment occurs through Conduction Convection Radiation Evaporation. Conduction involves the transfer of heat at the atomic level between two materials (solids or still fluids) from high to low heat energy areas. Convection involves the bulk transfer of heated matter from warm low density regions from a moving fluid (gas or liquid) to cooler more dense areas. Radiation is the process of heat transfer in the electromagnetic spectrum (particularly at the infrared wavelength) between surfaces. Radiant heat transfer is independent of the intervening air temperature. Evaporation occurs when a liquid is heated to the point of phase change to become vapour. Warmer molecules gain sufficient kinetic energy to leave the surface and enter the gaseous phase. The remaining molecules have less average kinetic energy resulting in a decrease in the temperature of the liquid. Evaporation increases with rising temperature but decreases with increasing saturation of the surrounding air or the concentration of solutes in the liquid

Exploration Safety Roadshow 2009 Heat gain, storage and loss Thermal balance Body must balance the heat transferred into the body, heat generated in the body and heat dissipated to the environment NEXT SECTION Heat strain and heat related illnesses Radiation and convection to the environment account for up to 75% of heat loss when the body is at rest. However, as heat loss through radiation and convection remain relatively constant during exercise, heat loss through evaporation must increase if thermal balance in the body is to be maintained. This underlies the importance of evaporation in the control of heat stress and the contribution of humidity in restricting heat losses. According to the equation the body can be seen to be in thermal balance when the heat generation bought about by metabolic processes [M] (less the energy that actually produces mechanical power [W]) is balanced by respiratory tract heat exchange through convection [C res] and evaporation [E res]) and combined with skin heat exchange through convection [C], evaporation, conduction [K] and radiation [R] and the storage of heat [S] accumulating in the body

Heat strain and heat related illness
Exploration Safety Roadshow 2009 Heat strain and heat related illness Heat stress and heat strain Heat stress – sum of environmental influences (air temperature, radiant heat, humidity, air velocity) that, when coupled with metabolic heat generation and effects of clothing, may result in heat strain Heat strain – physiological response to heat stress on the body Heat related illness may occur when the total heat load exceeds the body’s capacity to maintain a constant temperature resulting in an increase in core body temperature above the optimal 37ºC. In terms of occupational exposure limits, the World Health Organisation recommends that core temperature does not exceed 38C for sustained work in hot conditions

Exploration Safety Roadshow 2009 Heat strain and heat related illness Heat illness Heat oedema – mild form of heat illness resulting in pooling of fluid in legs brought about by transient peripheral vasodilation Heat rash – skin that has been persistently wetted by sweat may develop a rash characterised by raised lumps that may be intensely itchy. Bumps caused by blocked sweat glands, which subsequently burst, causing a stinging sensation Heat fatigue – additional blood is diverted to skin as result of peripheral vasodilatation, reducing output to the brain and internal organs, and resulting in fatigue and reduction in strength Heat illnesses Heat Oedema: a mild form of heat illness resulting in pooling of fluid in the legs brought about by transient peripheral vasodilation Heat rash: skin that has been persistently wetted by sweat may develop a rash characterised by raised lumps that may be intensely itchy. The bumps are caused by blocked sweat glands which subsequently burst causing a stinging sensation. Heat fatigue: impaired sensorimotor and mental alertness due to a lack of acclimatisation. Additional blood is diverted to the skin as result of peripheral vasodilation reducing output to the brain and internal organs, resulting in fatigue and reduction in strength.

Exploration Safety Roadshow 2009 Heat strain and heat related illness Heat illness (continued) Heat cramps – painful involuntary muscle spasms resulting from electrolyte dilution arising from hard work in hot environments, heavy sweating and excessive water intake Heat syncope – dizziness or fainting brought about by lowered blood pressure arising from vasodilatation and pooling of body fluids into legs and resultant lack of blood flow to brain Heat exhaustion – mild form of shock with symptoms including extreme weakness or fatigue, uncoordinated action giddiness, nausea, headache and a weak rapid pulse Heat Cramps: painful involuntary muscle spasms resulting from electrolyte dilution (particularly sodium) arising from hard work in hot environments, heavy sweating and excessive water intake. Heat Syncope: dizziness or fainting brought about by lowered blood pressure arising from vasodilation and pooling of body fluids into the legs and resultant lack of blood flow to the brain. The risk of syncope is exacerbated by lack of acclimatisation and dehydration Heat exhaustion is a mild form of shock and includes symptoms such as extreme weakness or fatigue, uncoordinated actions giddiness, nausea, headache and a weak rapid pulse. Move the victim to a cooler environment immediately. Shade is better than sun, air conditioning is better than outside, etc. The cooler the better. Remove the victim's clothing to encourage heat loss. If the victim is conscious and able to follow commands, he or she can drink fluids to rehydrate.

Exploration Safety Roadshow 2009 Heat strain and heat related illness Heat illness (continued) Heat stroke – body’s thermoregulatory system has failed to prevent core body temperatures rising to critical levels above 40°C Symptoms include: lack of sweating and hot dry skin confusion irrational behavior loss of consciousness convulsions Heat stroke may result in permanent damage to the brain and other vital organs; death may occur Heat Stroke: Heat stroke is the most serious of heat related illnesses. In this condition, the body’s thermoregulatory system has failed to prevent core body temperatures rising to critical levels above 40°C. Symptoms include: lack of sweating and hot dry skin; confusion; irrational behaviour; loss of consciousness; convulsions. Heat stroke may result in permanent damage to the brain and other vital organs; death may occur

Exploration Safety Roadshow 2009 Heat strain and heat related illness Factors leading to heat strain Increase in core body temperature Lack of acclimatisation Lack of fitness and/or the presence of medical conditions Type and amount of clothing Dehydration Increase in core temperature: the thermoregulatory system may be unable to maintain a stable core temperature in circumstances such as carrying out heavy work in a hot humid environment, leading to an increase in core body temperature. Bethea and Parsons[i] suggest that “…metabolic rate is a major contributor to heat stress even when environmental conditions would suggest that the worker is not at risk. Dehydration: as discussed, one of the body’s principle methods of cooling is through sweating, or rather, through the evaporation of sweat. A considerable amount of fluid may be lost through sweating. I In extreme cases, total fluid losses may equate to a loss of up to 11% of body weight. Symptoms of dehydration include thirst, fatigue, irritability and dry mouth. Dehydration is a significant contributing factor for several of the acute heat illnesses listed below (heat cramps, heat syncope, heat exhaustion and heat stroke). Lack of fitness / medical conditions: Poor physical fitness reduces the ability of the individual to acclimatize to hot environments. For example, poor cardiovascular health results in reduced blood flow from the core to the skin and an associated reduction in heat exchanges. Similarly, medical conditions such as hypertension or diabetes may decrease heat tolerance or increase the risk of dehydration and subsequent heat related illness. Some prescription medicines (ie some antidepressants; diuretics) may increase an individual’s susceptibility to heat illness. Clothing levels: Clothing that does not allow the free circulation of air across the surface of the body can restrict the amount of evaporation and associated cooling that takes place. Some protective clothing (ie water and air impermeable; insulating) greatly increases the risk of heat illness occurring.

Exploration Safety Roadshow 2009 Heat strain and heat related illness Acclimatisation Reduction in heat rate Reduction in core body temperature Increase in sweat rate Decrease in the electrolyte content of sweat Increase in blood plasma volume NEXT SECTION Risk factors for heat strain Acclimatisation: acclimatisation is the human body’s adaptation to repeated exposure to hot environments over time, resulting in a reduction of the risk of heat stress. The physiological indicators of acclimatisation include: A reduction in heat rate A reduction in core body temperature An increase in sweat rate A decrease in the electrolyte content of sweat An increase in blood volume The risk of heat stress increases with the lack of acclimatisation. Full acclimatization to a hot environment may take from one to three weeks. Gradual and increasing exposure to the hot environment can be used to reduce the risk of heat strain.

Risk factors for heat strain
Environmental risk factors High temperature and/or humidity Reduced air movement Working near radiant heat sources Contact with conductive heat sources

Exploration Safety Roadshow 2009 Risk factors for heat strain Individual risk factors Age (especially greater than 60 years old) Low level of physical fitness Medical conditions (diabetes, cardiovascular disease) Some medications Drug and alcohol use Lack of acclimatisation Dehydration Age, fitness, medical conditions and/or medications/ drug and alcohol use………

Workplace risk factors High frequency, duration or intensity of physical activity Requirement for use of personal protective equipment and clothing (may increase humidity levels and prevent air flow across the skin)

Exploration Safety Roadshow 2009 Risk factors for heat strain Indices of heat strain Wet Bulb Globe Temperature Index Thermal Work Limit NEXT SECTION Risk assessment and control Indices of Heat Stress Heat stress indices use environmental and behavioural parameters in order to model the thermal strain experienced by the body. According to Brake (2002) in excess of 50 heat stress indices have been developed since the 1920’s. The most commonly measured environmental variables include: Air temperature Relative humidity Air velocity Radiant heat A further two variables included in heat stress indices relate to individual behaviour: Clothing levels Metabolic rate (referring to the heat generated by the body’s activity) 5.11 Goal of heat stress indices Heat stress indices seek to integrate environmental conditions and personal variables into a single numerical value that predicts the heat strain on the individual, and to provide recommended limits on environmental or physiological conditions that reduce the risk of heat related illness occurring[ii]. Such recommendations generally seek to: limit core body temperature to °C; limit core body temperature rise to 1°C; limit heat storage in the body to w.hr.m-2 limit specific aspects of heart rate (peak; average; recovery patterns) Wet bulb globe temperature index The WBGT index was developed in the 1950s by Yaglou and Minard (1957) to assist the US military reduce heat stress casualties by defining limit values for training regimes. The index integrates the contribution of air temperature, humidity and radiant heat and is calculated using one of two equations that adjust for the presence or absence of direct sunlight: WBGTout = 0.7 T nwb Tg Tdb WBGTin = 0.7 T nwb Tg Where nwb = natural wet bulb temperature; g = Globe temperature ; db = dry bulb air temperature The ACGIH screening criterial for heat stress exposure uses WBGT measurements to recommend work-rest schedules according to the acclimatization of the individual and the metabolic demands of the work being carried out (The values in table 1 below assume a 5 day, 40 hour working week; hourly time weighted averages can be be applied on an hourly basis where the workload varies). Environmental variables obtained for the calculation of the WBGT index can also be used to carry out a basic thermal risk assessment (as recommended in the Australian Institute of Occupational Hygiene publication “Heat stress standard & documentation developed for use in the Australian environment”) and where required, used to calculate second tier heat strain risk assessments (such as the required sweat rate). Heart Rate Heart rate has been indicated as a measure of heat strain; corrections have been developed for acclimatized persons[xi]. Vogt et al. suggest that thermal strain can be determined from the difference between recovery heat rate (in the hot environment) and resting heart rate in the neutral environment. 5.13 Critique of heat stress indices Brake (2002) suggests that most heat stress indices have significant shortcomings that include one or more of the following problems: Poor correlation between some indices and development of heat stress Over estimation of metabolic rate Failure to consider the contribution of wind speed or overestimating the contribution of air movement induced over the body from the work (movement) being carried out. Failure to consider the contribution of clothing Poor incorporation of radiant heat Difficulty in establishing the degree and impact of acclimatisation Overly complex and difficult to use. Brake further argues that heat stress indices are overly conservative in order to account for such uncertainties. However, indices of heat stress such as the WBGT and heart rate provide useful screening tools as part of preliminary risk assessments

Risk assessment and control
Exploration Safety Roadshow 2009 Risk assessment and control Heat stress guidance Australian Institute of Occupational Hygienists Heat stress standard & documentation for use in the Australian environment This document provides a structured process for decision making in the heat stress risk assessment process. A tiered approach to the assessment is recommended, commencing with a “basic thermal risk assessment” moving onto a more detailed analysis of environmental and work related variables and finally, where indicated, physiological monitoring

Heat stress risk assessments should consider: individual characteristics nature of the work environmental conditions under which work is to be carried out

Heat stress controls Managing the risk of heat stress should consider the hierarchy of controls (elimination, substitution, engineering, administration, personal protective clothing and equipment) Controls can be directed toward: work environment the task being carried out individuals themselves

Engineering controls Ventilation – fans, blowers, chillers Airconditioning – crib room, 4WD Insulation or shielding – tents, shade

Exploration Safety Roadshow 2009 Risk assessment and control Administrative controls Training and education Employment assessment and monitoring Setting patterns of work Acclimatisation schedules Encouraging self pacing of work Maintenance of hydration -

Exploration Safety Roadshow 2009 Risk assessment and control Training Ensure workers are fully aware of the underlying mechanisms that allow heat strain to occur Recognize the symptoms of heat illness Understand and implement the correct responses to heat stress conditions (including emergency first aid) Training ensure workers are fully aware of the underlying mechanisms that allow heat strain to occur and to recognize the symptoms of heat illness and to understand and implement the correct responses to heat stress conditions (including emergency first aid).

Exploration Safety Roadshow 2009 Risk assessment and control Assessment and monitoring Medical surveillance may be required — determination of general fitness, presence of medical conditions and use of medications that may predispose employee to heat strain Persons at risk of heat stress can be monitored at workplace for signs of heat illness and to ensure work-rest and hydration regimes are followed Monitoring may also include assessment of: recovery heart rate times oral temperatures end-of-shift weight loss (to determine level of dehydration) Employee assessment and monitoring An acclimatization program should be implemented that ensures workers are acclimatized through progressive exposure to the hot workplace; workers should also be encouraged to increase their physical fitness. Appendix 2 sets out an example of an acclimatization schedule.

Exploration Safety Roadshow 2009 Risk assessment and control Work patterns Shifts can be adjusted so that physical work occurs in cooler periods of the day Self pacing Regular breaks and work-rest schedules Provision of cool rest or recovery areas Provision of relief workers

Exploration Safety Roadshow 2009 Risk assessment and control Acclimatisation schedule Un-acclimatised: 50% exposure on day 1, increasing by 10% per day (i.e. full work regime by day 6) Acclimatised but returning to work after more than 9 days off: 80% exposure on day 1; 90% on day 2 then full return to work The US Department of Labor proposes the following acclimatisation schedules for hot workplaces (where WBGT = Day 1 50% exposure Day % exposure Day 3 70% exposure Day % exposure Day 5 90% exposure Day % exposure For acclimatised workers returning to work after leave of more than 9 days a reduced schedule applies: Day percent exposure Day percent exposure Day percent exposure

Hydration Goal is to restrict fluid loss to < 3% Absorption rate through digestive tract about 1 litre/hour (can lose up 1.5 litre/hour) Encourage drinking cool fluids 250 ml every 15 minutes Electrolyte replacement (especially non-acclimatised workers) Avoid caffeine, alcohol, milk, carbonated drinks, fruit juices

Urine chart

Exploration Safety Roadshow 2009 Risk assessment and control PPE – cool vests NEXT SECTION: REVIEW phase change

Review Thermoregulation Heat gain, storage and loss Heat strain and related illnesses Risk factors for heat strain Risk assessment and control We can extend from your experience to be able to identify situations and people at risks and more importantly manage such risks. It’s a good idea to tie in this experience with some of the physiology and theory behind heat stress.

Major points to consider
Exploration Safety Roadshow 2009 Major points to consider Environment – temperature, humidity, wind speed Task – physical requirements, pace of work, clothing Acclimatisation – new to site or return to site Hydration – availability Individual – training, self regulation, buddy systems

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