OHS Canada Magazine

Beating the Heat

July 12, 2019
By Jean Lian

Doffing our heavy winter coats and throwing a sun hat over our heads cannot feel better after months of cool weather. But for people who work long hours outdoor during the summer months, it is important to take precautions and know when to retreat from the heat.

Heat stress occurs when body heat builds up due to exertion, environmental factors (like the temperature, humidity, air movement, radiation from the sun and proximity to hot surfaces or heat sources) and the clothing and equipment worn or used by a worker.

Jobs that carry the risk of heat stress include outdoor work in construction, road repair, forestry, agriculture and resource extraction. Indoor workers who are vulnerable to heat exposure include those operating in restaurant kitch­ens, foundries, steel mills, bakeries, laundries, smelters, glass factories and furnaces, according to the Centre for Ca­nadian Occupational Health and Safety (CCOHS) in Ham­ilton, Ontario.

The human body feels comfortable when the air temperature is between 20°C and 27°C and a humidex that ranges from 35 to 60 per cent. In very hot environments, the internal body temperature can increase several degrees above the normal 37°C, overwhelming the body’s natural cooling systems and leading to serious and possibly fatal conditions, the CCOHS states.

Certain physical conditions and personal risk factors can reduce one’s ability to withstand high temperatures and put an individual at heightened risk of heat stress. Being over­weight reduces the body’s efficiency at losing heat while the lack of physical fitness diminishes the body’s ability to cope with increased demands that heat puts on the body, says the Infrastructure Health and Safety Association in Mississauga, Ontario.

As the sweat glands become less efficient with age, workers aged 40 and above may not cope as well in hot envi­ronments. Pre-existing conditions like heart disease or high blood pressure can also heighten heat-stress risk as cooling the body requires the heart to pump blood to the skin.


Heat stress can pose secondary hazards like dropping tools while working at heights due to sweaty palms, or poor vision caused by the fogging of eyeglasses. It also puts workers at risk for illnesses like heat cramps, heat syncope, heat exhaustion and heat stroke. The following are the symptoms of heat stress:

  • Heat cramps: Sharp pains in the muscles due to the loss of salt from perspiration.
  • Heat syncope: Dizziness, light-headedness and feeling faint or loss of consciousness due to low blood pressure.
  • Heat exhaustion: This occurs when the body loses body water and salt through excessive sweating. Symptoms in­clude heavy sweating, weakness, dizziness, visual distur­bances, intense thirst, nausea, headache, vomiting, diar­rhea, muscle cramps, breathlessness, palpitations, tingling and numbness of the hands and feet.
  • Heat stroke and hyperpyrexia: This is the most serious of all heat illnesses that require immediate medical attention.

While heat stress poses a serious health risk, it can be prevented easily by taking the following precautions as recommended by CCOHS:

  • Acclimatize: Adapt your work and pace to the tempera­ture and how you feel, as it can take up to two weeks for a person to build up tolerance to working in hot conditions.
  • Take breaks: Take breaks to cool off in the shade or in an air-conditioned building or vehicle; reduce physical exer­tion if a shady or cool place is not readily available.
  • Keep cool: Stay out of the sun as much as possible and schedule physically demanding tasks for the early morning or late afternoon hours when the sun is less intense.
  • Stay hydrated: Drink one cup of water every 15 to 20 min­utes, regardless of whether you are thirsty or not.
  • Avoid alcohol and drugs: Consumption of these sub­stances can worsen the effects of heat illness.


Protective clothing may be the last line of defence, but one that cannot be overlooked. Wearing clothes suitable for hot conditions can help the body cool down.

A common misconception is that taking off the shirt helps to bring down the body’s temperature. According to a WorkSafeBC document on preventing occupational heat stress, wearing a layer of light, loose-fitting clothing main­tains the layer of air next to the skin and protects it from direct contact with the hotter air when the temperature in the environment is greater than the skin temperature, which is around 35 degrees Celsius.

Loose-fitting clothing made from fabrics like cotton and silk allows air to pass through and evaporate perspira­tion from the skin, facilitating the cool-down process. This is why people who live in hot desert climates cover them­selves in light and loose clothing from head to toe.

Light-coloured clothing reflects the heat better than dark-coloured ones, and large-brimmed hats provide shade for the face. If hard hats are worn, attaching a light-coloured piece of cloth to the back and side rim of the hat shields the neck from the sun


Employers can implement engineering and administrative controls to decrease the risk of heat-related illnesses. For engineering controls, Ontario’s Ministry of Labour recommends reducing the physical de­mands of work through mechanical assistance like hoists and lift-tables, controlling the heat at the source by using insulating and reflective barriers, lowering the temperature and humidity through air cooling, increasing air movement with fans and providing air-conditioned rest areas.

In terms of work-practices, employers should assess the demands of all jobs and implement monitoring and control strategies in workplaces with a high environmental tem­perature. Apart from assigning additional workers or slow­ing down the pace of work, they should also train workers to recognize the symptoms of heat stress and establish a “buddy system” since people are not likely to notice their own symptoms. Last but not least, provide trained first-aid providers and have an emergency response plan in place.

According to a heat-stress awareness guide by Occupa­tional Health Clinics for Ontario Workers Inc., a heat-stress prevention program need to establish the “triggers” to im­plement the plan. Criteria may include humidex reaching or exceeding 35 degrees Celsius, weather advisories issued by Environment Canada or the provincial agency and heat waves of three or more days with temperatures at or above 32 degrees Celsius. Generally, hot-weather plans should be in place from May 1 to September 30.

Jean Lian is editor of OHS Canada.