Health Bulletin - Physical Hazards Fact sheet - Thermal Comfort Project
The aim of this project is to increase awareness of thermal comfort, and increase the level of control measures used.
Thermal comfort is a condition of the mind, which expresses satisfaction with the thermal environment.
Often health and safety inspectors are asked:
how hot is too hot to work? or
how cold is too cold to work?
These are questions that cannot be answered without further information, because the potential for heat or cold stress is affected by many different factors, environmental, occupational and personal.
Environmental factors
Air temperature
Obviously air temperature is a very significant factor in terms of thermal comfort. All other things being equal, the colder the air temperature, the greater the rate of heat loss. Air speed creates a cooling effect on the surface of the skin, with the degree of cooling being proportional to the air speed.
Humidity
Humidity is an important factor too. Workers tend to find higher air temperatures more acceptable when they are accompanied by lower humidities, but care should be taken not to dry the air too much. When the air is particularly dry, the mucous membrane in the throat becomes desiccated, forms into lumps, and loses the ability to get rid of dust particles.
Radiant Heat
Radiant Heat is both an environmental and an occupational factor, and refers to heat that is emitted from a hot object. Radiant heat is considered environmental when the radiant heat source is the sun, and occupational when it is directly related to the workplace.
Occupational factors
The intensity with which work is conducted affects one's thermal comfort. People doing essentially sedentary work, requiring little or no manual effort generate little body heat and so unless other factors act to keep them warm, they will tend to feel cold.
Some workplaces involve work in areas of particularly high intrinsic radiant heat. Examples include metal extrusion presses used to extrude lengths of aluminium. When one takes account of other factors, such as work intensity or protective clothing the capacity for discomfort is very high and heat stress becomes a risk.
Personal factors
Several personal factors are relevant to thermal comfort, including one's general state of health. Physical fitness, along with some medication, affects one's ability to adapt to small variations in the surrounding temperature, and hence can affect one's thermal comfort. One factor, over which we usually have the most direct and immediate control, is that of clothing, which we use to assist us to lose body heat when we are feeling warm, and to retain it when we are cold.
Thermal discomfort may not cause an immediate health and safety problem, but it will affect morale, and feelings of tiredness and irritability, this may lead to a lack of productivity.
Health and safety does become a direct concern where there is a risk of a person developing either heat or cold stress. Heat and cold stress conditions arise due to the body's inability to adapt to changes in the thermal environment.
Improving comfort during hot weather
In summer, the temperatures can become quite hot and this can happen quickly - often before people have the opportunity to acclimatise. Typically hot spells last for a few weeks. It may not always be practicable to provide air conditioning, particularly where fresh air requirements can be satisfied by natural ventilation. Although employers should be aware of the signs of heat stress conditions and take immediate action accordingly, the following strategies may be useful during these hot-spells to increase thermal comfort.
- Wear lighter-weight and preferably cotton clothing
- Wear lighter-coloured clothing - dark colours absorb heat; lighter colours reflect heat
- Keep fluid intake up - once a person has become thirsty that person has already started to become dehydrated
- avoid tea and coffee - they're diuretics (promote water loss by the kidney)
- avoid soft drinks with high sugar content - the sugar can make you thirstier
- drink plain water, or electrolyte drink where heat stress is a risk
- Ensure that an adequate source of fresh air is available - an office, for example, you need 10L of fresh air per person per second - see panel on Fresh Air, below
- Lower the humidity to no less than about 45% relative humidity with the use of dehumidifiers - a greater range of temperatures is comfortable in drier air
- Install blinds/curtains or fit reflective coatings to windows to reduce radiant heat from sunlight
- Place heat-generating machinery in a location that allows heat to escape from the workplace and vent if necessary to the external environment
- Increase air movement - this provides a cooling effect on the skin. This can be achieved using an oscillating fan
- Schedule activities requiring greater physical effort for the cooler times wherever possible
- Take shorter, more frequent rest breaks
- For the first 5 to 7 days of intense heat, assign lighter duties with longer rest periods
Heat Stress
| Stress Condition | Recognising it | What should you do? |
|---|---|---|
| Heat Cramps | Muscle pain following strenuous exercise. Sometimes accompanied by dilated pupils and weak pulse resulting from depletion of salt. | Move victim to a cool place to rest. Muscle massage may help relieve pain. |
| Heat Exhaustion | A condition resulting from dehydration and/or salt depletion or lack of blood circulation. Is usually accompanied by fatigue, nausea, headache, giddiness, clammy skin and a pale appearance. | Lie victim down in a cool place, loosen clothing, provide water and electrolyte for drinking and seek medical attention if victim vomits. |
| Fainting (Heat Syncope) |
Brief fainting, blurred vision, nauseated feeling and may vomit | Lie patient down in a cool environment and provide fluids. |
| Heat Stroke | The most severe form of heat stress resulting from loss of the body's ability to sweat. It is characterised by hot dry skin, dizziness, nausea, severe headache, confusion, delirium, loss of consciousness, convulsion and coma. Accompanied by a red face, and rapid strong pulse. | Get victim to hospital immediately. Any delay can be fatal. Cool victim to normal body temperature, using fans, sponging or air conditioners. DO NOT give fluids. |
A few tips to improve comfort during cold weather
As with hot weather in summer, unseasonably cold conditions can descend upon us at any time during the year, and often before people have the opportunity to acclimatise. Although these cold periods may only last a matter of days, it is important that workers are able to feel comfortable as they work. The strategies, that may be also be useful during the extended winter months, are mostly self-evident and may include the following:
- Wear heavier clothing, better able to insulate against the cold, such as wool
- Wear darker-coloured clothing - dark colours absorb heat; lighter colours reflect heat
- Consider wearing thermal underwear
- Have frequent hot drinks
- Lower the humidity to no less than about 40% relative humidity with the use of dehumidifiers - a greater range of temperatures is comfortable in drier air
- Install heating, ensuring that where any fuels are burned, flues are in place and working, or that adequate ventilation is provided
- Minimise draughts, while still ensuring that there is adequate fresh air available (see below)
Fresh Air
The Health and Safety in Employment Regulations, 1995, require employers to take all practicable steps to ensure that employees are provided with ventilation providing fresh or purified air. In order to establish how much fresh air is required, we use the standard NZS 4303:1990 Ventilation for Acceptable Indoor Air Quality. This standard requires between 8 and 18 Litres of fresh air to be provided per person per second in a commercial facility, depending upon the nature of the commercial facility. As a rule of thumb, 10L per person per second is quite adequate for most situations, including offices, dining rooms and conference rooms. It should be noted that fresh air ventilation is not intended for the removal of contaminants; where present, specific engineering controls should be used for their removal.
Conditions in which most people will feel comfortable
| Condition | Summer value | Winter value |
|---|---|---|
| Air temperature (C) | 19-24 | 18- 22 |
| 16-21 (active work) |
16-19 (active work) |
|
| Relative humidity | 40-70% | |
| Air speed | 0.1-0.2 m/s, no draughts up to 0.5 m/s (active work) |
|
| Radiant heat | No direct exposure to a radiant heat source | |
| Clothing | Light clothing | Winter clothing |
Issued by the Department of Labour, New Zealand
http://www.osh.govt.nz
No. 21 - January 2007
