Last reviewed 14 June 2022

Complaints about hot, stuffy offices are a common feature of UK summers. As the frequency of summer heatwaves increases, employers need to consider the effect of the heat on employees and take the steps necessary to protect their health and welfare. Gordon Tranter looks at the issues of a maximum office temperature, the risks of working in hot conditions indoors and the actions that should be taken to prevent heat stress.

When the workplace is too hot

When the workplace gets too hot it is not just an issue about comfort. It reduces productivity and can also be a health and safety issue. In hot conditions people can suffer from a loss of concentration and increased tiredness, dizziness, fainting, or even heat cramps. There is an increase in the likelihood of accidents due to reduced concentration and slippery, sweaty palms.

Indoor environmental conditions substantially influence health and safety and productivity. The air temperature in the office has a strong effect on working performance. Although there is some disagreement on the temperature for optimal productivity, most studies have found it is between 21°C and 23°C. Generally, there is agreement that productivity starts to diminish when office temperatures rise above this.

An over hot workplace can impact the health and wellbeing of employees, with risks including dehydration and heatstroke. If a person’s blood temperature rises above 39°C, there is a risk of heat stroke or collapse. Above 41°C, delirium or confusion can occur and there is a risk that if blood temperature reaches this level it can prove fatal or cause irreparable organ damage.

Everyone is different. Some individuals can tolerate much higher workplace temperatures than others. Weight and, in particular, how much insulation (fat) there is in the body will impact on its ability to lose heat. Young people, pregnant women and the disabled tend to have a lower tolerance to high temperatures and there is some evidence of a gradual reduction in the effectiveness of the body to regulate its own temperature after the age of 60. In addition, medical factors and medication may affect an individual’s tolerance.

Is there a maximum indoor temperature?

In the UK there is no maximum temperature for workplaces inside buildings. However, the Workplace (Health, Safety and Welfare) Regulations 1992 require that the temperature in all workplaces inside buildings during working hours is “reasonable”. There is no consensus over what a reasonable temperature is. It depends on the type of work being done (manual, office, etc), the type of workplace (kitchen, air-conditioned office, etc) and the nature of the workforce.

The TUC has called for the introduction of a legally enforceable maximum indoor workplace temperature. It would require employers to act when the temperature inside reaches 24°C. It would mean that staff could be sent home and their employers prosecuted if temperatures at work hit 30°C (or 27°C for those engaged in physically demanding work).

On its website, the Health and Safety Executive (HSE) claims that a meaningful maximum temperature cannot be given due to the high temperatures found in, for example, glass works or foundries. The HSE points out that in such environments it is still possible to work safely provided appropriate controls are present. In addition, factors other than air temperature, ie radiant temperature, humidity and air velocity, become more significant and the interaction between them become more complex with rising temperatures.

Other factors influencing thermal comfort at work

Air temperature is not the only parameter affecting thermal comfort. Humidity, wind speed, radiant heat sources, clothing including personal protective equipment, etc also are likely to have an effect which an ordinary thermometer will not take into account.

Humidity can contribute to thermal discomfort and the risks to health caused by hot weather. High humidity environments have a lot of vapour in the air, which prevents the main method of heat reduction, the evaporation of sweat from the skin. This causes the body’s temperature to rise. In certain conditions (air temperatures above 35°C/95°F and high humidity) the body's normal temperature control system is unable to effectively regulate its internal temperature with serious risks to health.

Radiant temperature (ie heat radiated from hot objects such as hot surfaces, furnaces, ovens, kilns, dryers, molten metals and machinery, etc) has a greater influence than air temperature on the ability of persons to lose heat to the environment.

Air velocity — the speed of air moving across the employee — is an important factor in thermal comfort. It can help cool them if the air is cooler than the environment. The greater the air velocity, the greater the heat exchange between people in a space and the air around them. However, opening windows is only effective as a coolant if the temperature outside is lower than that inside.

Thermal comfort is very much dependent on the insulating effect of clothing on the wearer. This is particularly so where employees are not allowed to make adaptations to their clothing, ie they have to wear a specific uniform or need to use personal protective equipment (PPE). Wearing non-breathable vapour-impermeable PPE prevents the evaporation of sweat, producing a high humidity.

The metabolic heat produced by physical work in a hot environment has to be lost so that the worker does not overheat. If insufficient heat is lost, blood temperature will rise, and so does the risk of adverse health effects.

Risk assessment for hot weather

A risk assessment needs to be carried out to assess whether there is a risk of the exposure of indoor workers to high temperatures during the summer. This should identify any hazards and problems, identify those at risk, and indicate the precautions that need to be taken to ensure the health of the workers.

Aspects that should be considered during the assessment are as follows.

  • The working climate: including the air temperature, humidity, air movement and the presence of any heat sources.

  • Physical activity: the extent to which physical activity is likely to generate body heat. This should consider the amount of work and the time in which it has to be done, working hours and the time of day the work is carried out.

  • Clothing and PPE: particularly respiratory protective equipment, and the extent to which it is likely to contribute to overheating.

  • Vulnerable workers: identify susceptible employees such as the young, old, disabled, pregnant or those on medication, and any additional procedures or resources required.

How to control temperature and humidity

Employers are not legally obliged to provide air conditioning in workplaces. They are expected to provide reasonable temperatures and a sufficient quantity of fresh or purified air.

Ventilation should be used to combat high temperatures and high humidity. Suitable ventilation can take the form of air conditioning and, to a lesser extent, mobile air-conditioning units and fans. Blinds or curtains can be used to block out sunlight. Solar control window film can be used to reduce solar heat gain and can also combat intense glare. If employers use air conditioning, they need to ensure that the system is regularly serviced and checked to ensure it is working at optimal levels at all times. Badly functioning air conditioning can lead to increased rather than lower temperatures and can also dry out the work environment, causing throat irritation and problems for workers who wear contact lenses.

During periods of high summer-time temperature, workers should take regular breaks. In hot weather employers should consider arranging for workers to rotate their jobs.

An indoor heat stress prevention plan

An indoor heat stress prevention plan should be developed to handle indoor heat. It should include the following.

Hydration: employees should be encouraged to keep hydrated to avoid overheating and heat stress. Employers should provide cool water supplies in the workplace and encourage employees to drink regularly. They should advise employees to drink water rather than tea or coffee, both during working hours and at home.

Information: if it is necessary for workers have to work in high temperatures they should be educated about the symptoms of heat-related illnesses and the proactive measures they can take to protect themselves.

Clothing: wearing too much clothing may prevent the loss of heat from the body. This can occur when employers have a dress code in the workplace, eg a uniform to communicate a corporate image. In hot weather it is important to assess whether the clothing has an adverse effect on thermal comfort and whether there is a case for relaxing the dress code by allowing workers to wear more casual clothing or by supplying the workers with comfortable, lightweight clothing.

Personal protective equipment: PPE reduces the body’s ability to evaporate sweat. In warm/hot environments wearing PPE that is cumbersome and/or heavy may increase the heat being generated inside the body, particularly when there are high work rates. PPE that is non-breathable and vapour-impermeable prevents sweat evaporating. It is therefore important to assess the risk from wearing PPE in hot weather and whether the PPE is the most appropriate available. Can PPE made from a breathable fabric be used, or is there a new, more appropriate, version of the PPE available? Care must be taken to ensure that in the heat, employees do not unzip or remove their PPE just to cool down, thus exposing themselves to potential hazards and risks. The need for reducing the time working in the PPE should be considered.

Vulnerable workers: hot weather can make workers feel tired and less energetic, especially those who are young, older, pregnant or on medication. Employers may wish to give these workers more frequent rest breaks and reduce their hours. The Management of Health and Safety at Work Regulations 1999 requires employers to assess risks to pregnant women from extremes of heat as they tolerate heat less well. A person’s physical characteristics should always be borne in mind when considering their thermal comfort, as factors such as their size and weight, age, fitness level and sex can all have an impact on how they feel, even if other factors such as air temperature, humidity and air velocity are all constant.

Fasting: when Ramadan falls in the summer months Muslims who fast each day from sunrise to sunset may wish to use annual leave. Employers could consider holding meetings, etc in the mornings when energy levels are higher or if possible, consider a temporary change in working hours.

Manual work: the more physical the work, the greater the metabolic rate and the more heat is produced. This means that more heat has to be lost to prevent overheating. Workers carrying out manual work in hot conditions should have regular breaks and drink plenty of water. Consider whether you can shift working hours away from the hottest times of the day.

Sun glare: bright sunlight can make work in offices particularly very difficult due to the incessant sun glare on screens. This can be combatted by using blinds or solar control window film.

Note on first aid

When a heat wave is forecast, it is worth ensuring that your organisation’s first aiders are aware of the symptoms of heat stress and its treatment.


After a spell of high temperatures the arrangements for ensuring thermal comfort should be reviewed. Were the arrangements effective? What do the employees think? Have they expressed their satisfaction or have there been any complaints? Has there been any change in the frequency of unsafe actions, accidents, illnesses and absenteeism? Has productivity been maintained or reduced?

Heatwaves seem set to become a more common feature of future summers and, consequently, managers of offices and other indoor workplaces should have procedures in place ready to counter the effect of high temperatures.