Imagine a comfortably warm place; there is no wind and no noticeable humidity. You are sitting, calm, and relaxed. You are likely not perspiring (sweating) in this scenario. Your body is "idling" in terms of maintaining your body temperature. The heat produced by your metabolism keeps your core at 37 degrees Celsius with any excess being radiated off your skin into the surrounding air.
Now think about variables which can individually change this energy balance between metabolic heat production and body temperature.
• environmental temperature hotter than your body temperature?
• effect of hot lights or the sun?
• vigorous activity of some kind?
• high humidity?
Each variable causes a net gain of body heat.
• If environmental temperature is hotter than body temperature, your body will take on heat, or at least have a harder time losing excess heat.
• Humidity affects evaporation of sweat. As sweat evaporates from the skin, it takes away a significant amount of heat. If the air is humid, sweat is unable to evaporate and there is a reduction of heat loss so body temperature will rise.
• Physical activity increases metabolic heat production and the body needs to get rid of the excess heat.
How does your body feel as it overheats? Finally, let's put these all together and imagine doing strenuous work or exercise outdoors on a hot, humid, possibly sunny day. These conditions can overtax the body's ability to maintain heat balance. The result can be heat-related illness.
Elephants have big ears. You may think they are for great sense of hearing. Elephants do have good hearing but it is not the function of their large outer ears. The ears are richly supplied with blood vessels and are in fact radiators to help the elephant to get rid of excess body heat. The ears help increase the surface area to volume ratio of these large animals to make it easier to lose body heat.
In this post, we will consider how we maintain a steady, or "normal" body temperature and what happens if that temperature gets too high — heat-related illness. Scientists tell us that climate change will result in hotter summer temperatures for many places in the world. Therefore it is even more important to know the causes and symptoms of heat-related illness as well as variables which make some people more susceptible than others. We'll also look at how to protect against overheating.
Healthy human internal body temperature is 37 degrees Celsius or 98.6 degrees Fahrenheit. Normal variation can be about one degree Celsius, depending on the time of the day, level of physical activity or emotional state. For example, body temperature often decreases slightly at night while we are sleeping. Bodily functions depend on blood circulation and biochemical reactions that occur best at about 37 degrees. A bigger than one degree change in body temperature can be a sign of illness and affects those biochemical reactions.
Our bodies produce energy through a variety of biochemical processes which are collectively called metabolism. This energy is used for all life processes and to produce physical activity. Physical activity (work or exercise) converts about three-quarters of the energy used to heat with only one quarter into actual motion. Some of the heat produced is used to maintain our body temperature. When we are active, the body usually generates more heat than it needs to keep its steady temperature and therefore has to get rid of excess heat so that it doesn't cause body chemistry to go awry.
Our bodies automatically attempt to keep a steady internal body temperature. We have a "thermostat" in our brains that respond to changes in body heat. As internal temperature rises, heart rate increases and more blood is pumped to the skin. Sweat production is increased drawing water from the blood and pushing it out through pores onto the skin where it can evaporate. Sweat evaporating from our skin has a significant cooling effect except in very humid conditions. If it's not extremely hot, extra blood near the skin surface loses heat by radiation and convection to the air. As we heat up the body is trying to increase heat loss to keep that steady temperature. We call this the "heat balance" -- heat gained must equal heat loss. If these innate mechanisms can't keep up, heat illness results.
The heat balance can be difficult to maintain. The more strenuous the activity, the more heat is generated, and the more your body has to get rid of heat. If the environmental temperature is warmer than skin temperature, heat will attempt to go into the body. Hot, humid environments make it harder for the body to get rid of excess heat because less heat is lost from the body as the ambient temperature increases. With higher humidity, the cooling effect of sweat evaporating is lessened. In hot dry conditions, little heat is lost to radiation and convection while sweat production can't keep up and the skin is dry and hot -- internal temperature rises.
As you heat up, surface blood vessels dilate to allow more blood toward your body surface for cooling. The result is less blood available to serve your muscles, brain and other internal organs. And prolonged sweating draws a lot of water from the bloodstream, further reducing its capacity to deliver nutrients, clear out wastes, lubricate joints and cool you. You can easily sweat out one litre of water during an hour of heavy work in hot weather, 3/4 litre in less strenuous work.
If your body continues to lose fluid, you are likely to experience increasingly severe symptoms of heat illness -- general discomfort, loss of coordination and stamina, weakness, poor concentration, irritability, muscle pain and cramping, fatigue, blurry vision, headache, dizziness, nausea, confusion and unconsciousness. These symptoms and even mild effects of heat stress also increase your chance of suffering an accidental injury.
As the temperature or heat burden increases, people may feel:
The single best way to reduce your heat stress risks during physical activity is to steadily replenish the water you lose as sweat. Drinking small amounts frequently, such as about a cup (250 mL) every 15 minutes, is more effective than large amounts less often.
Relying on thirst as a signal to drink is dangerous. Most people do not feel thirsty until their fluid loss reaches two percent of body weight and is already affecting them.
The risk of heat-related illness varies from person to person. A person's general health influences how well the person copes with heat.
Those with extra weight often have trouble in hot situations as the body has difficulty maintaining a good heat balance. Age (particularly for people about 45 years and older), poor general health, and a low level of fitness will make people more susceptible to feeling the extremes of heat.
Medical conditions can also increase how susceptible the body is to excess heat. People with heart disease, high blood pressure, respiratory disease and uncontrolled diabetes may need to take special precautions. Heat has been shown to worsen the symptoms of Multiple Sclerosis and is known as heat sensitivity. In addition, people with skin diseases and rashes may be more susceptible to heat. Other factors include circulatory system capacity, sweat production and the ability to regulate electrolyte balance.
Substances -- both prescription or otherwise -- can also have an impact on how people react to heat.
The National Institute for Occupational Safety and Health (NIOSH) reports that several studies comparing the heat tolerances of men and women have concluded that women are less heat tolerant than men. While this difference seems to diminish when such comparisons take into account cardiovascular fitness, body size, and acclimatization, women tend to have a lower sweat rate than men of equal fitness, size and acclimatization. This lower sweat rate means that there can be an increase in body temperature.
Symptoms of heat exhaustion may start suddenly, and include:
Heat Stroke symptoms include those above and also:
If you or someone else has heat exhaustion, treat symptoms in the following ways:
Call 911 if:
There are many things you can do to prevent heat-related illnesses. Babies, children, and elderly people are more sensitive to heat and require extra attention. You also are at greater risk if you are ill or obese, or have heart disease. People who exercise or work outside or in a hot setting also are at risk of heat exhaustion and heatstroke.
Don’t go outside when the temperature and heat index are high. If possible, stay indoors in air-conditioned areas. If you must go outside, take the following precautions:
Certain medicines can put you in danger of heatstroke. They affect the way your body reacts to heat. Talk to your doctor if you take any of these or if you have an ongoing health problem. Your doctor can help you manage the heat with your condition. These medicines include:
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