Health Bulletin - Physical Hazards Fact sheet - Heat Strain Project
The aim of this project is to increase awareness of the hazards of heat strain, and increase the level of control measures used.
This Fact sheet is intended for use as a resource for inspectors visiting places of work where there is a risk of a heat strain condition developing as opposed to a simple comfort issue.
Heat Stress and Heat Strain
Generally speaking, strain can be looked at as being uncompensated stress. In the context of heat, strain occurs when a person is no longer able to physiologically maintain their core body temperature at its normal operating temperature of 37°C and the core temperature rises above this figure. Until such time as there is a risk of heat strain, the stress amounts to a simple comfort issue, and the Fact sheet on "thermal comfort" may prove helpful in this instance.
The human body does not maintain itself at the same temperature throughout, however the interior of the brain, the heart and the abdominal organs are maintained at a constant temperature (the so-called "core body temperature"), which fluctuates very slightly around 37°C. Maintenance of the core body temperature is essential for normal function of the vital organs.
What are some types of heat strain?
Heat syncope (fainting) - results from a drop in blood pressure during prolonged standing in heat, or a sudden change from sitting to standing.
Heat rashes - the most common heat rash is prickly heat (miliaria rubra) which usually occurs in areas where clothing is restrictive and gives rise to a prickling sensation, especially when sweating.
Heat cramps - fairly common among individuals who work vigorously in the heat. These are spasms in the voluntary muscles that occur following a reduction in the concentration of sodium chloride in the blood below a certain critical level.
Heat exhaustion - this is a more serious heat disorder that is linked to depletion of body fluids (dehydration) and electrolytes, and arises because of:
- increased dilation of blood vessels with depleted capacity of circulation to meet the demands for heat loss to the environment, exercise and digestive activities, and/or
- decreased blood volume due to dehydration, poling of fluid in the lower body, dilation of blood vessels or lack of salt (electrolyte imbalance)
Heat stroke - this is the least common, but most severe degree of heat strain, with a high mortality rate, especially if treatment is not given immediately. It causes a major disruption of the central nervous system and is characterised by convulsions, mania or coma, dilated pupils, a core body temperature of 41°C or above and usually a hot dry skin. Heat stroke may happen suddenly, or there may be warning signs such as irritability, dizziness or mental confusion.
Acclimatisation
Acclimatisation is the process by which people adapt themselves to be able to safely and comfortably work in a hot environment. This may take from 7 to 15 days and depends upon fitness, age, gender and race. After a person becomes acclimatised, he or she will feel thirst whenever the body needs more fluid, and as a result, the person will drink more. Where it known that a particular job will take a significant period of time, a managed process of acclimatisation should be carried out to minimise the likelihood of a person suffering heat strain.
How does the body go about managing heat?
It does this in two ways:
- Increasing the blood flow to the skin
- Sweating allows the body to benefit from the cooling effect of evaporation which is increased by air movement. The loss of salt due to sweating reduces with acclimatisation, as the sweat glands learn to conserve salt.
What factors impact upon a person to determine whether or not they feel hot?
Five key factors are involved:
- Air temperature - cold air has a direct cooling effect on a person
- Humidity - hot air has a higher moisture content that cool air, so humidity is more of an issue in hot environments than cold
- Air movement - increases evaporation of moisture from the skin, and hence cooling
- Physical activity - increases the generation of heat in the body, ad this heat must be allowed to dissipate, or heat strain will occur
- Clothing - restricts the dissipation of heat from the body - light weight cotton clothing, of a light colour is recommended in hot environments.
What personal factors affect the ability to handle or manage heat?
- Weight - overweight people are more at risk in both hot and cold environments due to an imbalance in heat transfer
- Health - several medical conditions increase the risk of harm to people working in a hot environment, such as heart disease, high blood pressure, respiratory disease, diabetes and Raynaud's disease
- Level of fitness - a physically fit person will acclimatise better and generally cope with both heat and cold stress better than an unfit person
- Age - As a person reaches middle age, lifestyle health issues start to emerge - these can make people more susceptible to harm caused by extreme hot or cold environments
- Use of prescribed substances - these can adversely affect people working in extreme hot or cold environments, and include antidepressants, morphine, and psychotropics
- Use of non-prescribed substances - these can adversely affect people working in extreme hot or cold environments, and include alcohol, cannabis, amphetamines, anaesthetics, cocaine and morphine
How do we respond to heat?
A person may respond to heat in one of the following ways:
- Thermal comfort - not feeling conscious of being hot or cold, or
- Thermal discomfort - feeling too hot, and even very uncomfortably so, but the body's temperature controls are working and there is low risk of harm, or
- Heat Strain - harm or serious harm can occur as a result of working in a hot environment
Examples of workplaces where heat strain is a risk.
- Outside in summer
- In boiler houses
- In spray drying plants
- In paper mills
- In foundries
Control of work environments causing heat strain
It is important that work activities carried out in extreme heat are controlled using the hierarchy of controls. An employer must consider each of the controls in the order listed.
Elimination: can the work be carried out without the heat source present? This might be achievable by changing the process to one that does not require heat, or in the event of a product that produces heat during its manufacture, consider purchasing the product rather than manufacturing it. It might also be achievable by carrying out the work when the heat is not a problem.
Isolation: involves separating the cold environment from the worker in space or in time. This could involve rescheduling the work for a period of time when excessive heat is not present. It might also involve enclosing a hot process so it will not effect the workers, or mechanising the process so workers do not need to go near it.
Minimisation: involves putting steps in place that minimise the effects of the exposure. This will usually involve the provision of personal protective equipment. It could also involve administrative changes, such as sharing the work so that individual workers spend less time at risk.
Minimisation of risk
Minimisation can be achieved by putting in place environmental or administrative controls, or by modifying a process. Examples of each are provided below:
Environmental - ventilation, air-conditioning, shielding
Administrative - restricting access, planning work, perform maintenance at shut-down, provision of appropriate clothing that might include cool vests, air vests or heat transfer suits, introduce work/rest regimes, allow people to acclimatise
Process modification - modify so that less heat is required, reduce the heat to the minimum level necessary
Medical Controls
It is important that people working in hot environments drink 100 - 150 mls every 15 to 20 minutes. Water or 50/50 water/fruit juice is adequate and should be freely available, served at room temperature. Before working in extreme heat, it is best to over-hydrate, to the point where they need to urinate slightly more often than usual. It is also important that workers maintain fitness during and between periods of working in hot environments.
Healthy eating should be promoted. Fatty foods should be avoided, and it is not usually necessary to add extra salt to the diet, except for unacclimatised workers, where additional salt is best taken in the form of a salty liquid such as beef soup. Added salt is not necessary for short exposure times, and salt tablets are not recommended, because it is possible to overcompensate for lost salt.
Pre-employment and regular on-going medical examinations can be considered to ensure that workers will be sufficiently fit and have no pre-existing medical conditions. Further information on pre-employment health assessments, as well as a health questionnaire are available in the Department's publication Guidelines for the management of work in extremes of temperature.
Lastly, personal hygiene is most important to reduce the risk of prickly heat. A shower and fresh clothing is advisable as soon as work has finished in the hot environment, and, in some situations, at the midday break as well.
Monitoring
When the minimisation option is selected, the employer has a duty to monitor both the employees' exposure to the hazard and the employees' health in relation to the hazard.
Monitoring exposure to extreme heat
The most appropriate way to monitor the exposure to extreme heat is by using an indicative method based on the WBGT method. This method takes account of the air temperature, relative humidity, radiant heat, air velocity, work activity and the clothing being worn. Further information is available in the Department's publication Guidelines for the management of work in extremes of temperature.
Monitoring health in relation to heat
Ideally the health of a worker who is exposed to extremes of heat should be monitored in such a way as to reflect, as closely as possible, the core body temperature. Where this is impracticable, or where the temperatures are less severe, simply measuring the heart rate may be acceptable. The heart rate will naturally increase with exertion, however when a person is affected by the extremes of heat, the heart rate will take longer to return to normal levels. Whatever method is used, data should be recorded by data capture, and retained to satisfy the health information requirements of the legislation. Measurement of the heart rate will also identify any cardiac abnormalities, such as arrhythmia and bradycardia.
General health surveillance can be achieved by excluding significant medical illness, reviewing body mass indices (BMIs) and assessing general fitness levels.
Emergency procedures
In every workplace where workers are likely to be exposed to extreme heat, the emergency procedures must include provisions for dealing with heat-related emergencies. For example, there should be an adequate first aid response available within the workplace.
Case study - Heat strain
- A volunteer fireman, wearing protective clothing for an extended period, collapsed during an exercise involving burning a house, putting out the fire, and re-igniting it. He was evacuated to hospital where he responded to fluid replacement treatment.
Issued by the Department of Labour, New Zealand
http://www.osh.govt.nz
No. 23 - January 2007
