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7 Medical Misconceptions

The HealthCentral Editorial Team Jun 5, 2012 (updated Jan 25, 2014)
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Urban legends and medical myths are pervasive in the U.S. and around the world, but some people still believe they are true. Here are some of the most pervasive.

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Electric fans cause death
Electric fans cause death
There is an urban legend in South Korea that leaving an electric fan on at night could lead to death by hypothermia, high levels of carbon dioxide and suffocation. None of these outcomes are backed by medical science, but The Atlantic suggests that there might be a placebo effect occuring in South Korea, where believing that a fan is harmful actually causes your body to react to it.
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Most body heat is lost through the head
Most body heat is lost through the head
The myth that we lose most of our body heat through our head has been around since the 1950s due to a flawed study by the US military. The study looked at volunteers who were exposed to very cold conditions. The head was the only portion of the body left uncovered, and therefore, most of the heat was lost through the head. 
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You have to wait an hour after eating to swim
You have to wait an hour after eating to swim
Another myth is that going swimming within an hour after eating will increase the risk of cramps and drowning. However, a 1989 study in Pediatrics showed that mixing alcohol with swimming could increase likelihood of drowning.
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Shaving causes hair to grow back thicker/coarser/darker
Shaving causes hair to grow back thicker/coarser/darker
Although hair does not actually grow back as thicker, coarser or darker, shaving can cause the end of the hair to be blunt, and feel rough, compared to the tapered edge of unshaven hair. This is only present as the hair is growing out.
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You have to drink 8 glasses of water a day to maintain good health
You have to drink 8 glasses of water a day to maintain good health
The amount of water someone should drink varies on the needs of individuals. Factors include, weight, activity level, clothing and heat and humidity levels. Also consuming other beverages, such as juice, tea or milk, or eating fruits and vegetables also keeps you hydrated as those things contain water.
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Sugar causes hyperactivity in kids
Sugar causes hyperactivity in kids
Several double-blind trials have shown that there is no difference in children's behavior if they are given sugar diets or sugar-free diets. They found this was even true for children with attention-deficit/hyperactivity disorder (ADHD).
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People only use 10 percent of their brain
People only use 10 percent of their brain
Scientists say that we use just about every part of our brain, and most is active at all times. The myth can be attributed Harvard psychologists in the 1890s who were studying a child prodigy.