When is it too hot to work? And what to do about it.

Whenever I’ve delivered a health and safety course such as the NEBOSH General Certificate, the one issue that raises the most lively debate is temperature, specifically warm temperatures. Here are some of the facts and figures around warm temperature limits and practical steps that can be taken to deal with the working situation when the mercury rises.

The human body operates within a narrow core temperature band between 36.8°C and 37.2°C. The range represents the balance between the heat exchange with the thermal external environment, and the internal generation of heat by metabolic processes and clothing.

The combined effect of external thermal environment and internal metabolic heat production stresses the human body. The responses of the cardiovascular system (increased heart rate), the thermoregulatory system (sweating), the respiratory system (panting), and the renal and endocrine systems constitute thermal strain.

When the body’s physiological responses to the stress are unable to adequately regulate the core temperature, heat illness or heat strain can occur. Excessive fluid loss can occur during a work shift, in a hot and humid environment, due to increased sweating, which is made worse by a lack of rehydration.

The Workplace (Health, Safety and Welfare) Regulations 1992 state that:

During working hours, the temperature in all workplaces inside buildings shall be reasonable.

What is reasonable depends on the nature of the workplace, for example: expectation in an office will be different than in a warehouse.

The ACoP for these regulations states that the:

Temperature in workrooms should normally be at least 16oC unless much of the work involves severe physical effort in which case the temperature should be at least 13oC. These temperatures may not, however, ensure reasonable comfort, depending on other factors such as air movement and relative humidity.

The HSE considers that:

An acceptable zone of thermal comfort for most people in the UK lies roughly between 13°C (56°F) and 30°C (86°F), with acceptable temperatures for more strenuous work activities concentrated towards the bottom end of the range, and more sedentary activities towards the higher end.

Heating and cooling systems should be installed, maintained and used so that harmful fumes, gases or vapours do not injure or offend workers, or others who may be affected.

Thermometers should be readily available to enable persons at work to measure temperatures throughout the workplace, but don’t need to be provided in each workroom.

Thermal comfort is affected by more than just the room temperature. Whether or not a person feels too hot or too cold depends on a combination of:

  • Environmental factors, including the air temperature, radiant temperature, relative humidity and air velocity.
  • Personal factors, such as the level of physical activity and the amount of clothing being worn

Health Effects


Heat stroke

Heat stroke is the most serious of the heat illnesses. It is a state of thermoregulatory failure which should be treated as a medical emergency.

Heat stroke is imminent when the core body temperature exceeds 40oC. It is characterised by:

  • hot, dry skin
  • rapidly rising body temperature
  • collapse
  • loss of consciousness
  • convulsions.

Without prompt medical attention, including removal of the victim to a cool area and suitable methods for reduction of the increasing body temperature, heat stroke will be fatal.

Heat exhaustion

Heat exhaustion is usually less severe than heat stroke, although it can escalate to heat stroke. It most often occurs in persons whose total blood volume has been reduced due to dehydration, but can also be associated with inadequate salt intake even when fluid intake is adequate. Heat exhaustion is generally characterised by:

  • clammy, moist skin
  • weakness or extreme fatigue
  • nausea
  • headache
  • no excessive increase in body temperature
  • low blood pressure with a weak pulse.

Lying down in a cool place and drinking cool (10-15oC), slightly salted water (0.1% Sodium Chloride) or an electrolyte supplement, will usually result in rapid recovery of the victim of heat exhaustion, but a physician should be consulted prior to resumption of work. Salt-depletion heat exhaustion may require further medical treatment under supervision. Without prompt treatment, collapse may occur.

Heat syncope (fainting)

Exposure of fluid-deficient persons to hot environmental conditions can cause a major shift in the body’s remaining blood supply to the skin vessels in an attempt to dissipate the heat load. The resulting reduction in blood supply to the brain leads to a faint.

Heat cramps

Heat cramps, are characterised by painful muscular spasms. They occur most commonly during the heat exposure, but can occur sometime after heat exposure. Resting in a cool place and drinking 250 ml of saline solution (0.9% Sodium Chloride) will alleviate the cramps rapidly. Maintaining a balanced electrolyte intake, with meals is important.

Prickly heat (heat rash)

Heat rashes occur as a result of continued exposure to humid heat. The skin remains continuously wet from sweat that hasn’t evaporated, resulting in blocked glands, itchy skin and reduced sweating. Keeping the skin clean and as cool and dry as possible to allow the skin to recover is generally the most successful treatment.

Chronic effects

There is no conclusive evidence of chronic long-term effects, such as a higher incidence of kidney stones.

Tips for avoiding heat illness

  • Replace lost fluids (drink more water, juice, sports drinks or other non-alcoholic drinks). Drinks of 100-200ml water at frequent intervals will be adequate to reduce fluid loss if sweating.
  • Have rest breaks in a cool place.
  • Minimise caffeine, carbonated drinks, alcohol and tobacco use.
  • Do not take salt tablets other than on a doctor’s advice.
  • Inform the employer if aware of any underlying health condition that may increase the risk of heat illness.
  • Maintain a healthy lifestyle (healthy diet and regular exercise).
  • Wear cool clothing, a wide brimmed hat and use sunscreen.
  • Take a break if feeling tired, dizzy or weak or having trouble concentrating.

Predisposing factors

The effects and severity of heat strain on individuals depends on the physiological capacity of the individual, as influenced by the following personal factors:


As people get older, the personal factors of general good health and level of physical fitness are more important than simply age itself. The physical condition of the person is more significant than age related debilitations.


In groups matched for cardiorespiratory fitness levels there was no difference in the tolerance levels of males and females.

General health

Some medical conditions can contribute to the risk of developing heat related illness. Examples include past episodes of heat related illness, chronic cardiovascular, disease, diabetes and skin disorders. If a person has experienced heat related illness in the past, they are at higher risk for developing it again.

Excess body fat affects the body’s ability to cool itself, as it is a good thermal insulator and the excess bulk requires the body to exert more energy during activity. This can be an advantage in cold stress but is a major disadvantage during heat stress.

State of hydration

Workers typically only replace one half of the water they are losing as sweat (a phenomenon known as ‘voluntary dehydration’), unless they are ‘programme drinking’, i.e. stopping typically every 15 minutes to drink 250 ml of water. Waiting an hour or more and then attempting to drink a litre of cold water, when very thermally stressed, is likely to lead to nausea, vomiting or headache.

Alcohol and caffeine

Alcohol and caffeine are diuretics, which increase urine output and therefore fluid loss, They can significantly contribute to dehydration.


Nicotine is a vasoconstrictor (tightens blood vessels), which means it impedes the skin’s blood vessels from widening and letting heated blood reach the surface to release heat.


Some common prescription and over the counter medicines (for example: diuretics, tranquilisers, antidepressants and antihistamines) affect the body’s ability to regulate temperature.


Acclimatisation is a physiological adaptation, which occurs with repeated exposure to hot environments. The heart rate decreases, sweating increases, sweat becomes more dilute, and body temperature will be lower. The ability to acclimatise varies among workers. Generally, individuals in good physical condition acclimatise more rapidly than those in poor condition.

Protective clothing and other protective equipment

Protective clothing may impede sweat evaporation and heat loss and significantly increase the risk of heat illness. It is important to ensure that sweat can be evaporated off the skin to obtain the maximum cooling effect. This is the reason that slightly loose fitting clothing, which allows for good ventilation and air circulation, is recommended.

Respirators may restrict breathing, while air-supplied respirators may alter the temperature of air breathed. The loading (how much air is breathed in) of the RPE depends on whether the worker is active or sedentary. The need to work in hot environments should be considered when assessing a worker’s ability to wear respiratory protection.

Work situations likely to lead to thermal discomfort

Workers may be exposed to high heat and humidity, or extremely cold working conditions due to weather conditions and the need to work outside, for example: construction work in the summer and winter. Many manufacturing and supply chain environments also have the potential to cause thermal discomfort.

Practical control measures


Engineering controls

Engineering controls can be an effective way of reducing heat stress and preventing or minimising occurrence of heat illness. Examples include:

  • Increasing air movement using fans.
  • Installing shade structures to reduce radiant heat from the sun.
  • Installing shields or barriers to reduce radiant heat from sources such as furnaces or hot vessels.
  • Removing heated air or steam from hot processes using local exhaust ventilation.
  • Insulating indoor workplaces.
  • Installing air conditioners or coolers to reduce air temperature.
  • Locating hot processes away from people.
  • Insulating /enclosing hot processes or plant.
  • Using mechanical aids to reduce physical exertion.

Management controls

Heat stress can be reduced by attention to the way work is organised, for example:

  • Rescheduling work so the hot tasks are performed during the cooler part of the day, or in cooler times of the year.
  • Reducing the time an individual spends doing the hot tasks, for example: by job or task rotation.
  • Arranging for more workers to do the job.
  • Making sure there is easy access to cool drinking water.
  • Providing additional rest breaks in cool, shaded areas.
  • Providing training and information to enable workers to:
    • identify hazards
    • recognise symptoms of heat stress and heat illness
    • understand how to avoid heat illness.

Clothing and personal protective equipment (PPE)

Suitable clothing and PPE may help to further reduce the risk of heat illness, for example:

  • broad brimmed hat
  • appropriate protective clothing to cover workers at least between elbow and knee, however long sleeves and trousers provide the best protection
  • sunscreen
  • sunglasses.