Thermal comfort

This page explains explain what ‘thermal comfort’ in the workplace means and how employers and occupational health specialists can measure it and maintain it.

We have separate advice for employers and workers on basic steps they can take to manage workplace temperature.

What is thermal comfort?

Thermal comfort means taking into account a range of environmental, work-related and personal factors when deciding what makes a comfortable workplace temperature.

There are six basic factors that will help you decide if temperatures are uncomfortable.

Why is thermal comfort important?

People feeling uncomfortably hot or cold are more likely to behave unsafely. Their ability to make decisions and/or perform manual tasks deteriorates. For example:

• people may take short cuts to get out of cold environments
• workers might not wear personal protective equipment (PPE) properly in hot environments, increasing the risks
• a worker’s ability to concentrate may start to drop off, which increases the risk of errors

As an employer, you should be aware of these risks and make sure the underlying reasons for these unsafe behaviours are understood and actively discouraged and prevented.

Adapting to workplace temperature

People adapt their behaviour to cope with their thermal environment, for example:

• drinking more hot or cold drinks
• adding or removing clothing
• subconsciously changing their posture
• choice of heating or moving to or away from cooling/heat sources

Where possible, you should manage workplace temperatures to allow the chance for workers to adapt.

In some instances the environment within which people work, or the need to wear a uniform or PPE etc, means they cannot adapt to their environment.

It is therefore important to have:

• adequate ventilation or air conditioning
• windows that can be easily opened or closed

Six factors indicating thermal comfort

Air temperature alone is not a valid or accurate indicator of a comfortable workplace temperature or heat stress. It should always be considered in relation to other environmental and personal factors.

Taking all these factors into account will help you decide if your workplace has a comfortable temperature.

• Air temperature
• Radiant temperature
• Air movement and speed
• Humidity
• Clothing and PPE insulation
• Work rate and metabolic heat

Air temperature

The temperature of the air surrounding the body is usually given in degrees Celsius (°C). It can be measured using thermometers but, on its own, this will not give an indication of thermal comfort.

You should provide a minimum working temperature in workrooms – usually at least 16°C or 13°C for strenuous work.

There's no law for maximum working temperature, or when it's too hot to work. This is because in many indoor workplaces high temperatures are not seasonal but created by work activity, for example in bakeries or foundries.

Radiant temperature

Thermal radiation is the heat that radiates from a warm object.

Radiant temperature will have a greater influence than air temperature on how hot the workplace will feel.

Examples of radiant heat sources include:

• the sun
• electric fires
• ovens
• cookers
• dryers
• hot surfaces and machinery
• very hot, molten materials such as metals or glass

Air movement and speed

The speed of air moving across a worker may help cool them if the air is cooler than the environment.

This is an important factor in thermal comfort, for example:

• still or stagnant air in artificially heated indoor environments may cause people to feel stuffy. It may also lead to a build-up in odour
• moving air in warm or humid conditions can increase heat loss through convection without any change in air temperature
• physical activity also increases air movement, so it can be corrected to account for a person's work
• small air movements in cool or cold environments may be perceived as a draught as people are particularly sensitive to these movements

Humidity

Humidity in indoor environments can vary greatly. It may depend on whether there are drying processes (in paper mills, laundries etc) where steam is given off.

High-humidity environments have a lot of vapour in the air, which prevents sweat evaporating. In hot environments, humidity is important because less sweat evaporates when humidity is high (over 80%). Sweat evaporating is the main method of heat reduction.

When workers are wearing certain personal protective equipment (PPE), it may increase the effects of humidity.

Clothing/PPE insulation

A comfortable working temperature very much depends on the insulating effect of clothing.

When workers are wearing non-breathable, vapour-impermeable personal protective equipment (PPE), for example asbestos or chemical protection suits, the humidity inside it increases because the sweat cannot evaporate.

Wearing too much clothing or PPE may be a primary cause of heat stress even if the environment is not considered warm or hot.

In cold conditions, if clothing does not provide enough insulation, the wearer may be at risk from cold stress and suffer injuries such as frostbite or hypothermia.

Make sure people can add or remove layers of clothing to help control their thermal comfort. Take account of situations where workers are required to wear a specific uniform or PPE.

It is important to identify how workers’ clothing contributes to thermal comfort or discomfort. By periodically evaluating the level of protection provided by existing PPE and evaluating newer types of equipment, you may be able to improve the level of thermal comfort.

Work rate and metabolic heat

The impact of work rate on thermal comfort is critical because physical activity creates more body heat.

Always take a person's physical characteristics into account when considering their thermal comfort.

Factors such as their size and weight, age and fitness level can all have an impact on how they feel, even if other factors such as air temperature, humidity and rate of air movement are all constant.

Measuring thermal comfort

A simple way of estimating the level of thermal comfort in your workplace is to ask your workers or their safety representatives (such as unions or employee associations) if they are satisfied with the thermal environment.

You can use the workplace temperature checklist to help you identify whether there may be a risk of thermal discomfort to your workers.

Please note that this basic checklist does not replace a suitable and sufficient risk assessment, taking account of thermal comfort.

Assessing thermal comfort

Once you have identified a problem using the checklist, in most instances the guidance on this website will be sufficient to enable you to improve thermal comfort in your workplace.

If you need to take further action in measuring thermal comfort, you should refer to the relevant British Standards.

If thermal comfort is an issue in your workplace you may need to consider it as part of your risk assessment process.

There are six basic factors affecting thermal comfort. Think of how they may be affecting your workers and how you can resolve the ones having the largest impact.

If the environment is affected by seasonal factors you may need to reassess the risk at different times of year. For example, consider scheduling maintenance work to a cooler time of the day.

Is heat stress a problem in your workplace?

Unless someone collapses from heat exhaustion, the possible health effects may not be obvious. This means you must identify whether those workers who may be at risk from heat stress are suffering from any heat-related illnesses.

You can use information you will already have to identify people who may be at risk, for example:

• look for patterns in absenteeism, types of illnesses and their frequency of occurrence, worker complaints etc. Take particular note of where people work, their job, how experienced they are, whether any illnesses are recurring etc
• read through any RIDDOR reports and any internal accident or injury reports. Are there any patterns to the nature of reportable accidents or injuries? Could any repeated accidents be attributed to the effects of heat, eg fatigue, loss of concentration etc?
• speak to other companies that are involved in the same business as your organisation, contact industry federations or associations etc
• carry out observations of your workplace and record any findings. Our heat stress check listmay help you assess the risk

Measuring heat stress

Measuring heat stress can be complex – you may need help from an occupational health professional to measure it in your workplace. This may involve:

• measuring the heat stress an individual is exposed to (eg by using a technique such as the wet bulb globe temperature index, see BS EN 27243)
• estimating metabolic rate (see BS 8966)
• measuring heart rate
• estimating clothing insulation values (see BS ISO 9920)
• if other alternatives cannot be implemented, you could consider physiological monitoring (see ISO 9886)

British Standards are available from BSI Group.

Control measures to make workplace temperatures more comfortable

There are a number of ways that you can make temperatures more comfortable in the workplace.

Control the environment

• Replace hot air with cold, or replace cold air with hot, as required
• Humidify or dehumidify the air as required
• Increase air movement by ventilation or air conditioning
• Reduce draughts by directing any ventilation or air movement so it doesn't blow directly onto workers, for example using baffles

Separate the source of heat or cold from workers

• Use barriers that shield or insulate the work area or restrict access
• Redesign jobs to remove workers from the area

Control the task

• Reduce the amount of time workers are exposed to hot or cold conditions
• Control the amount of work and rate of work people are expected to do
• Introduce mechanical aids (eg lifting aids or power tools) for physically demanding jobs in warm and hot environments or when workers are wearing a lot of clothing

Control the clothing

• Make sure workers are not wearing more than is needed
• If uniforms are worn, choose designs or materials that improve the thermal comfort of clothing
• Relax dress codes so workers can adapt their clothing where possible
• Multiple layers of clothing enable workers to make reasonable adjustments to their clothing based on their individual needs

Allow workers to adapt their behaviour

• Where possible, remove all restrictions that may prevent workers from making minor adjustments to their clothing or work rate
• Provide warm-up or cool-down areas
• Provide personal heaters or fans
• Allow workers to adjust thermostats or open windows as appropriate

Monitor the employee

• Provide appropriate supervision and training
• Obtain medical advice from an occupational health professional for workers who are pregnant, have an illness or disability, or are on certain medication, for example in very hot weather
• Your risk assessment should already cover risks to pregnant workers, but once your worker has informed you in writing they are pregnant, you must complete an individual risk assessment and make any necessary changes to support them

Applying the control measures in these webpages should ensure the welfare of those with a hormonal imbalance due to the menopause or thyroid imbalance.

Administrative controls

Administrative controls can include:

• scheduling 'hot' work for cooler times of the day
• having flexible hours to help avoid the worst effects of working in high temperatures

You can also use more permanent controls, for example:

• emergency procedures for responding to extreme heat
• having competent first aiders who can recognise and manage heat-related illnesses and ensure you have the right first-aid equipment

Engineering controls

These should be the first choice to reduce or eliminate the hazard. Although the initial cost of engineering controls may seem high, this is often offset by the resulting improvements to production.

Any practical solution to controlling thermal comfort is likely to require a combination of different options developed in consultation with workers with workers and their representatives.

Heating

Many types of heating systems are available:

• hot air-based heating systems
• water-based central heating systems using radiators
• combined heat and ventilation systems using air conditioning systems
• electrical heating systems using electrical heaters
• under-floor heating systems using either electrical coils or heated fluids
• overhead heating systems

Most of these systems are useful. However, the beneficial effects may be restricted in some situations to the immediate locality of the heat source.

Air movement and speed

There are many methods for increasing air movement, such as fans of various sizes, but these may cause draught or noise problems.

Large-diameter ceiling fans can provide air movement that is effective over a wide area. Large exhaust fans, mounted in roofs and walls, are useful for removing heated air and drawing in cooler air from outside.

Air conditioning

This can range from small units that lower the air temperature but do not control humidity levels or air movement, to large units that can cope with extreme conditions as well as humidity and air movement.

When air conditioning systems are used, take care to ensure uniform air distribution throughout the workplace, otherwise some workers may complain of feeling cold while others are feeling hot.

Air conditioning units should be operated as per the manufacturer's instructions.

Evaporative cooling

Evaporative coolers produce a moderate reduction in air temperature and increase humidity. They operate by passing hot air over water-saturated pads and the water evaporation effect reduces the air temperature.

Thermal insulation

There are many different types of thermal insulation materials, eg loose fills, rock wool and insulation boards. The material acts as a barrier, which slows heat flow in the summer and heat loss in the winter.

However, it is only effective where there is a temperature difference between the inside and the outside of the building or between two areas inside a building.