The aim of this project is to increase awareness of the hazards of cold 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 cold strain condition developing as opposed to a simple comfort issue.

Cold Stress and Cold Strain

Generally speaking, strain can be looked at as being uncompensated stress. In the context of cold, strain occurs when a person is no longer able to physiologically maintain their core body temperature and is therefore at risk of developing hypothermia. Until such time as there is a risk of cold 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 factors make people feel cold?

Five key factors are involved:

• Air temperature - cold air has a direct cooling effect on a person
• Humidity - cold air has a lower moisture content that warm air, so humidity is not a factor in cold environments except that mist, rain or wet clothing lowers the insulating characteristics
• Air movement - provides extra chill to people in a cold situation, with the so-called "wind chill factor" making a person feel considerably colder than if there was no wind
• Physical activity - increases the generation of heat in the body, and can help keep a person warm
• Clothing - restricts the dissipation of heat from the body

How does the body go about raising its core temperature?

It does this in two ways:

• Reducing the blood flow to the skin - a strategy that makes a person more susceptible to cold injuries such as frostbite
• Shivering, which is an involuntary muscle activity that increases the production of metabolic heat

Moderating factors?

• 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 cold 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 cold environments, and include antidepressants, tranquilisers, morphine, insulin, anaesthetics and antithyroids
• Use of non-prescribed substances - these can adversely affect people working in extreme hot or cold environments, and include alcohol, cannabis, and morphine

How do we respond to cold?

A person may respond to cold in one of the following ways:

• Thermal comfort - not feeling conscious of being hot or cold, or
• Thermal discomfort - feeling too cold, and even very uncomfortably so, but the body's temperature controls are working and there is low risk of harm, or
• Cold Strain - harm or serious harm can occur as a result of working in a cold environment

Examples of workplaces where cold strain is a risk:

• Outside in winter, and/or at altitude
• In wet conditions
• Inside walk-in freezers

Health effects of cold environments

As the temperature drops, sensitivity and dexterity of the fingers declines. At lower temperatures still, deeper muscles are affected, reducing muscle strength and stiffening joints. Cold-related discomfort also affects mental alertness, and for this reason, accidents are more likely to occur under cold working conditions.

Cold injuries can be divided into those that affect the body's extremities and those that affect the body's core.

Cold injuries to the extremities - these occur because of a reduced blood supply to the extremities, occurring in an effort to conserve the core body temperature. They include frostnip, frostbite and immersion foot or trenchfoot.

Frostnip - the mildest form of cold injury, affecting the toes, fingers, cheeks, nose and earlobes causing the skin to turn white. Frostnip is easily prevented by wearing warm clothing and treated by simple warming.

Frostbite - is caused by exposure to extreme cold or cold objects. It occurs when the temperature of tissue falls below freezing point. Blood circulation may cease and blood vessels can be ruptured and irreversibly damaged. In milder cases, the symptoms include a patchy inflammation of the skin, accompanied by slight pain. In more severe cases, there is often tissue damage without pain, or there could be blistering accompanied by a burning or prickling sensation. Frostbitten skin is susceptible to infection and gangrene. Treatment involving a slow return to normal temperature is important.

Immersion foot or trenchfoot - occurs in individuals whose feet have been wet and cold for extended periods of time (days or weeks), but not frozen. Injury occurs to nerve and muscle tissue. The symptoms are numbness, swelling, and in some cases, superficial gangrene.

The effects of cold on the body's core - In moderately cold environments, the body's core temperature does not usually fall more than 1°C below its normal level of about 37°C. However, in extremely cold environments, without proper protective clothing, the body's normal thermal regulatory mechanisms are unable to maintain this core temperature. A worker's core temperature should never be allowed to fall below 36°C. The oral equivalent is of the order of 35.5°C. The first signs of the body's core temperature dropping are a sensation of cold, followed by pain in exposed parts of the body. The most dangerous situations occur when the body is immersed in cold water, as cold water is a better conductor of heat than air. As the exposure time increases, or the temperature continues to drop, increasing numbness develops, and the pain decreases. This is followed by weakness and drowsiness, which usually begin as the core temperature drops below 33°C. This condition is known as "hypothermia". Additional symptoms include a cessation of early symptoms, slurred speech, diminished consciousness, dilated pupils, irrational behaviour - sometimes a person will even begin to discard clothing - and an unawareness of the danger to life that exists. When the temperature reaches approximately 26 - 27°C, unconsciousness occurs.

Factors affecting sensitivity to cold

1. Gender - the rate of cooling of the torso is slower in women, but that of the extremities is faster. Women have less capacity for metabolic heat production, by either exercise or shivering, and so should be regarded as being at a greater risk of cold injury.
2. Other factors - other factors that increase the risk of cold injury include increasing age, circulatory disease, fatigue, certain drugs, alcohol, Raynaud's disease, previous cold injury and exposure to other workplace hazards.

Wind Chill

At any temperature, a person will feel colder as air movement increases. The combined effect of air movement and temperature is called the "equivalent chill temperature" (ECT), or "wind chill temperature". The ECT is the temperature that would, in still air, produce the same cooling effect on exposed flesh as the given combination of air movement and temperature. The ECT (or wind chill) temperature is often denoted T_ECT, for example -12°C_ECT.

Control of work environments causing cold strain

It is important that work activities in the extreme cold are controlled using the hierarchy of controls. An employer must consider each of the controls in the order listed.

Elimination: involves eliminating the cold environment. The only way that the cold environment can be eliminated is to change the temperature to remove the risk. This may require changing the location of the work, so that it is conducted in a different environment.

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 cold will not be an issue, or moving the work away from the cold environment. It may also be possible to enclose the cold process.

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 workers spend less time at risk, or introducing a schedule of working and warming up periods, or a combination of the two. The Department of Labour publication Guidelines for the Management of Work in Extremes of Temperature provides a table to assist with the design of a suitable schedule. It may also be possible to mechanise a process so that workers do not need to go near it except for setup and maintenance purposes.

Minimisation of risk

Where continuous work will be carried out below 0°C_ECT, heated warming shelters should be provided. Metal handles and bars should be covered with thermal insulation, and tools, controls and equipment should be designed so that they can be operated without having to remove gloves or other personal protective equipment. Personal protective equipment should be worn in all environments where work takes places at temperatures below 4°C_ECT and should be selected to suit the temperature, the nature of the work, and the level of physical activity. Multiple layers are preferable to a single thick layer, as air trapped between the layers forms a good insulating barrier. In wet conditions, the outer layers should be water repellent, and in windy conditions, windproof. Where fine work needs to be conducted for greater than 10 to 20 minutes at temperatures below 16°C, special precautions need to be taken to keep hands warm e.g. providing warm air blowers, insulated handles on tools etc.

If a worker cannot be adequately protected from the effects of cold, then work must be suspended, or work regimes modified, to remove the risk of harm.

It is important to control the level of work activity of a person working in extreme cold. Obviously it is important that the level of activity is sufficient to generate metabolic heat, but if that level of activity is too high, they will produce sweat, and be at a greater risk of cold strain when the work activity is reduced. For this reason, a worker in these circumstances must be able to change into dry clothing before resuming activities in the cold environment.

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 cold

1. A suitable thermometer should be in use in every workplace where the atmospheric temperature may drop below 16°C, to monitor any further temperature changes.
2. In workplaces where the temperature is below freezing (0°C_ECT), and/or the speed of air movement is greater than 2m/s, the air temperature should be monitored at least every 4 hours.

Monitoring health in relation to cold

It is important that workers know the symptoms and signs of cold stress so they can recognise them in themselves and others. Frostnip on the face and ears can best be identified by a regular buddy lookout system, while fingers and toes can be monitored by the worker him/herself. Divers, for example, are at risk of lowered core body temperatures, and workers at risk of this should have their core body temperatures monitored, with the results retained to satisfy health information requirements of the legislation.

Emergency procedures

In every workplace where workers are likely to be exposed to cold working conditions, the emergency procedures must include provisions for dealing with cold-related emergencies. For example, there should be an adequate first aid response available within the workplace.

Alcohol

Alcohol is still regarded by some people as being an appropriate treatment regime for people suffering from exposure to cold conditions. Use of alcohol, particularly when a person is suffering exposure, can kill. The reason for this is that alcohol dilates the blood vessels and increases the blood flow to the surface. As blood flows near to the skin, it will cool, and when the cooled blood reaches the internal organs, the core body temperature will drop further. NEVER give alcohol to a person showing signs of exposure to cold.

Case studies - Cold strain

1. A port worker suffered frostbite to his ring finger (which had been frozen to the bone) and to his big toe while unloading fish from a fishing vessel's freezer. The worker was a loaned employee brought in to assist because poor weather had delayed the unloading process earlier in the day. He had not been provided with appropriate personal protective equipment; the boots he was wearing were gumboots that were too small and it appeared that the gloves he was wearing were either damp, damaged or both. In addition, he had never been advised of the hazards he would encounter while doing this work.

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Issued by the Department of Labour, New Zealand
http://www.osh.dol.govt.nz

No. 22 - January 2007