True or false? 10 health myths, facts and half-truths

When it comes to the facts about staying healthy, misinformation is rampant. We've all heard conventional wisdom on everyday issues - like how to fight a common cold and how to get the best abs. Some doctors even perpetuate it! But is all of it really true?

If you're not sure what's fact, fiction or a little of both, this guide to health myths will help you sort it all out.

1. Reading in dim light is bad for your eyes

False. It won’t damage your eyes, it's just uncomfortable, says Marguerite McDonald, M.D., clinical professor of ophthalmology at NYU Langone Medical Center. If you read in low light, the extra effort your eye muscles make to pull your vision into sharper focus can cause a headache, she explains. “You can read more comfortably, faster and longer with good lighting.”

2. You'll get arthritis from cracking your knuckles

False. A lot people crack their knuckles to loosen up stiffness in the fingers, but it doesn't cause arthritis. If you already have it, though, cracking your knuckles can “place a minor stress on the joints,” says David Katz, M.D., director of the Yale University Prevention Research Center. Because that stress could potentially make osteoarthritis a bit worse, Dr. Katz advises that you think twice before cracking your knuckles.

3. If you drop food on the floor, you can eat it if grab it in 5 seconds (or less)

Maybe. It depends on the floor, says Katz. “In general, our gastrointestinal tracts can tolerate many of the germs in the environment and kill them,” he says. “But if your floor is contaminated with Salmonella, it would be a bad idea!” Researchers at Clemson University did a series of experiments in which they dropped traces of Salmonella on wood, tile and carpeted floors. They found that the bacteria can survive in high enough concentrations for up to four weeks and be easily transferred onto food that was dropped on the floor. So the five-second rule should probably be broken -- a few seconds is enough time to attract some nasty bacteria to your food. Really, it’s enough to make you sick.

4. Sugar isn't any better for you than high-fructose corn syrup

True. Your body basically can’t tell the difference so it processes both sweeteners the same way, says Leslie Bonci, M.P.H., R.D., director of sports nutrition at the University of Pittsburgh Medical Center and author of Walk Your Butt Off. The problem is that high fructose corn syrup is so ubiquitous that you may be eating and drinking it all the time without even noticing. "It’s used in a lot of foods because it is cheap and blends well,” Bonci says, so it’s easy to ingest vast quantities of it from sodas, fruit drinks, candies and other processed foods. Your best bet is to read the package label: If you see high fructose corn syrup high up on the ingredients list, put the item back on the shelf.

5. Taking vitamin C can prevent a cold

False. There’s no evidence that vitamin C can prevent colds, says Katz. But research from Basel, Switzerland, suggests that taking a combination of 1,000 mg of vitamin C and 10 mg of zinc while you have a cold can reduce congestion by up to 27 percent. This vitamin combo can also shorten the length of your cold, which usually lasts for about a week.

6. Ringing in your ears means you're losing you hearing

True. “The lion’s share of people who have tinnitus [ringing in the ears] have some hearing loss,” according to Dwight Jones, M.D., professor and chairman of otolaryngology-head and neck surgery at the University of Nebraska Medical Center. The hair cells in in the inner ear amplify low-level sound, so if they die off, it usually results in hearing loss and tinnitus, explains Dr. Jones.

Unfortunately, there isn’t an effective way to treat tinnitus although you can use a fan or white-noise machine to mask the ringing in your ears while you’re sleeping or trying to concentrate. Ringing in the ears can also be a sign that wax is impacted in the outer ear canal, but that's a much less common cause.

7. Green mucus indicates a sinus infection

False. “Ear, nose, and throat doctors don’t put a lot of stock in the color of the mucus because it doesn’t always mean anything,” says Jones. You can get green nasal discharge from mucus that’s been sitting in the nasal cavities or from adenoids in the back of your nose. Better indications of a sinus infection are facial pain and pressure, a fever, headache or streaks of blood in the mucus.

8. Brushing your teeth after drinking coffee or red wine prevents stains on your teeth

Maybe…but at a price. Both drinks are fairly acidic, which means that brushing your teeth right after drinking one of them can scratch the enamel of your teeth, explains Gigi Meinecke, D.M.D., a dentist in Potomac, Maryland, and a spokesperson for the Academy of General Dentistry. “Abrading the protective enamel of the tooth can make it thin out and become more susceptible to decay. The abrasions may also make your teeth more sensitive to temperature and sometimes sweets.” A better approach: Rinse your mouth with water after drinking one of these beverages and wait at least 30 minutes before you brush.

9. You should use hydrogen peroxide to clean cuts and wounds

True. “Hydrogen peroxide helps remove dead tissue from a wound so the body’s immune system can heal what’s left," says Michael Carius, M.D., chairman of the department of emergency medicine at Norwalk Hospital in Conn. (But it doesn't make the wound heal any faster). To treat a cut or scrape, wash the wound with soap and water, dab it with a cotton swab dipped in hydrogen peroxide, then apply an antibacterial ointment, followed by a bandage, to help the wound stay clean as it heals. Repeat once or twice a day.

10. Crunches give you six-pack abs

False. “Crunches work mostly your upper abdominal muscles,” explains Jennifer Cohen, a fitness expert in Los Angeles and author of No Gym Required. To tone and strengthen your entire core, which includes that six packs -- it’s more effective to do planks and side planks. You’ll also need to burn body fat through aerobic exercise and by improving your eating habits, she adds.