Published in July 2005 in The Ottawa Citizen

With summer approaching, an understanding of summer heat upon the human body can help prevent heat-related illnesses.

The body has four means of dissipating heat: conduction, evaporation, radiation and convection.

Conduction is the transmission of heat through a substance like blood, water or other tissues. The muscles, warm from exercise, can dissipate heat directly to the skin surface. Blood can absorb great quantities of heat from the muscles and other tissues. It will return to the heart and then circulate to the small blood vessels in the skin. During exercise, the blood vessels dilate to allow greater quantities of blood to transfer heat to the skin surface.

The skin will radiate heat into the surrounding air and environment just like a space-heater. Sweat on the skin surface can absorb the heat and evaporate to reduce body temperature.

As the air warms around the body, it will rise. Cooler air moves in to replace it and absorbs body heat. This cycle is called convection and explains why fans help cool us.

Each mechanism works best within a specific temperature range. At temperatures less than 20ºC, radiation, convection and conduction will dissipate most generated body heat. Above 20ºC, evaporation of sweat is the primary means of heat dissipation.

Children do not sweat as much as adults and produce more heat for the same level of activity. They need to generate greater levels of heat before they do sweat.

Overweight individuals do not dissipate heat as well compared those of normal weight. The elderly have a decreased thirst response, and a reduced ability to circulate blood to the skin surface. Their blood vessels do not dilate as well as younger adults.

Certain medications can contribute to the risk of heat illness.

As temperature and humidity increase, evaporation becomes less effective. On a hot city day, core body temperatures increase because of radiant heat  from the sun’s and hot concrete surfaces.

Evaporation accounts for 85 percent of heat loss during vigourous exercise (a 70 kilogram athlete can lose one to two litres of sweat per hour). Failure to replace water and salt loss further compromises conduction and evaporation.

Adapting to the effects of heat during exercise over a specific time is termed acclimatization. This allows one to adapt to the increased demand to dissipate heat. Fluid replacement is essential for this process to work.

The five types of heat-related illness from mild to severe are; heat swelling (edema), heat cramps, fainting from heat (heat syncope), heat exhaustion and heat stroke.

Heat edema occurs in people who have not undergone acclimatization. Fluid leaks out into the tissues of the feet especially when standing for prolonged periods. Leg elevation reverses this process.

Heat cramps are painful abdominal, arm or leg muscle spasms occurring when too much salt and water is lost. This is a warning sign of pending heat exhaustion. Drinking water, juice or sport drinks and eating salty foods will relieve the cramps.

If there is no cool-down period after exercise, fainting is a risk. Blood pressure can drop when quickly transferring from a sitting to standing position. Dehydration worsens heat syncope. Lying flat with legs elevated rapidly reverses this condition.

Heat exhaustion occurs with excessive sweating in a hot humid environment. Body fluid volume is lost. The core body temperature increases from 38ºC to 40.5ºC. Symptoms include profuse sweating, fatigue, headache, dizziness, visual disturbances, lack of appetite, nausea, vomiting, vertigo, chills, muscle weakness, rapid heart rate (tachycardia), low blood pressure (hypotension) and skin flushing.

The person must be moved to a cool area. Applying cool water-soaked cloths helps. Elevate the legs. Those who are alert need one litre of oral fluid replacement per hour for two to three hours. Disoriented or unresponsive people require emergency treatment. All need a thorough medical evaluation at the hospital.

Heat Stroke is the most severe form of heat-related illness. Body temperature exceeds 40.5ºC and leads to multi-organ damage and failure. Altered mental status is a critical determinant of heat stroke. This medical emergency needs prompt evaluation and treatment.

Preventing heat-related illness is straightforward.

• Stay in air conditioning if possible.
• Drink lots of water before, during and after any outdoor activity.
• Avoid drinks with caffeine or alcohol. They will increase fluid loss via urination.
• Increase the amount of time you spend outdoors every day little by little.
• Take frequent rest breaks while outdoors on hot days.
• Avoid direct sunlight and stay in the shade when possible.
• Wear light-colored, loose-fitting, open-weave clothes.
• Avoid activities that require helmet use.
• Try scheduling activities or workouts early in the morning or late evening. Avoid heavy outdoor activity between 10 a.m. and 6 p.m.

Enjoy your summer.