When the thermometer reads 24 degrees Celsius but it feels more like 30 degrees, you’re likely feeling the humidity. The combined effect of warm temperatures and humidity affects how hot people feel. Whether you are working outside in the summer heat or simply enjoying the outdoors, being mindful of the humidity is critical when temperatures can reach extremes. Understanding humidity and taking the necessary precautions can help prevent the risk of your body overheating and experiencing heat-related illnesses.
A humidex is used as a measure of perceived heat that results from the combined effect of excessive humidity (moisture in the air) and high temperature. Environment Canada uses humidex ratings to inform the general public when conditions of heat and humidity are possibly uncomfortable, and potentially dangerous. The body attempts to maintain a constant internal temperature of 37 degrees Celsius at all times and in hot weather sweat is produced, which cools the body when it evaporates. As the humidity in the air increases, sweat does not evaporate as readily, and stops entirely when the relative humidity reaches about 90 per cent. Under these circumstances, the body temperature rises and may
cause illness.
The development of heat related illnesses depends on many factors in addition to air temperature and humidity. Wind speed or air movement, workload, radiant heat sources, and a person’s physical condition are important. Whether or not a person is acclimatized to working in a hot environment is also important. New workers should acclimatize to working in hot conditions by slowly increasing the duration and level of heat exposure. It can take six to seven days for the body to fully adapt or acclimatize to a new thermal environment. Rushing this process can lead to heat-related illnesses.
Heat-related illnesses
- Heat rash, or prickly heat, occurs when blocked sweat glands become inflamed. This painful rash reduces the body’s ability to sweat and to tolerate heat.
- Heat cramps are painful spasms of the muscles. The muscles used in doing the work are most susceptible. The spasms are caused by the failure of the body to replace its lost body salts and usually occur after heavy sweating.
- Heat exhaustion results when the body loses large amounts of fluid by sweating during work in hot environments. The skin becomes cool and clammy. Symptoms include profuse sweating, weakness, dizziness, nausea, and headaches.
- Heat stroke is the most serious condition and requires immediate attention. The body temperature becomes very high (even exceeding 41 degrees Celsius). Complete or partial loss of consciousness is possible. Sweating is not a good warning sign of heat stress as there are two types of heat stroke:
- Classical where there is little or no sweating (usually occurs in children, persons who are chronically ill, and
the elderly) - Exertional where body temperature rises because of strenuous exercise or work and sweating is usually present.
How to determine the humidex
During the summer, radio and television weather broadcasts often include the humidex readings for their broadcast areas. Environment Canada also posts the relative humidity for all of Canada on its website.
If you know the temperature and relative humidity, you can determine the humidex using a humidex from temperature table such as the one found in the Humidex Rating and Work fact sheet. For example, if the temperature is 30 degrees Celsius and the relative humidity is 70 per cent, the humidex rating is 41.This level is considered a level of great discomfort and exertion should be avoided.
Tips for avoiding overheating
- Monitor environmental conditions and allow self-limiting of exposure when necessary. If this is not possible, adjust work/rest cycles accordingly.
- Avoid sun exposure. Take frequent breaks in a cool or well-ventilated area to get out of the sun and heat.
- Don’t be afraid to sweat. Sweating
is the body’s most effective cooling mechanism. - Take time to acclimatize to new working conditions and temperatures.
- Stay hydrated. Drink plenty of water in hot weather conditions, on average, one litre every hour.
- If possible, postpone strenuous work to a cooler time of day.
- Wear light clothing.
- Consider the use of cooling vests.
Workers should be trained to recognize the signs and symptoms of heat stress and how to avoid them. An emergency action plan that includes procedures for providing affected workers with first aid and medical care should be in place.
The Canadian Centre for Occupational Health and Safety (CCOHS) promotes the total well-being – physical, psychosocial and mental health – of workers in Canada by providing information, training, education and management systems and solutions that support health and safety programs and the prevention of injury and illness.
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