Will your cell phone kill you?
Illustration: Jillian Takami

Is my microwave emitting death rays? Is bird flu still a threat? I do yoga, but should I also be lifting weights? You don't need to waste another second wondering and worrying. We've got the definitive answers right here.
1. Could my cell phone kill me?
It seems unlikely. But if you use your mobile phone a lot, consider getting an earpiece or putting your caller on speaker so you can hold the phone away from your head. The biggest study yet, in which Danish researchers tracked 420,000 cell phone users for up to 21 years, found no cancer risk, but much of the data was collected when cell phones were more of a novelty than a primary form of communication. In a smaller recent Israeli study of 1,726 people, heavy cell phone use raised the risk for salivary gland tumors 50 percent on the side on which the subjects usually held the phone (though the risk overall is still vanishingly small). The biggest threat, however, has nothing to do with cancer: Driving while talking on a cell phone puts you in the same league as a drunk driver. You're four to five times more likely to have an accident.

2. Will vitamin D save my life? Should I really be taking four times the recommended daily dose?
A growing body of evidence strongly suggests that vitamin D in high doses not only helps keep bones strong but also reduces the risk of colon, ovarian, and breast cancers, and diseases such as diabetes and multiple sclerosis. And many of us don't get enough because of a lack of exposure to sunlight (the sun triggers D's production in the skin) or diets that omit good sources (fatty fish such as salmon, mackerel, and tuna, and fortified milk and cereal). While the official daily dose for people age 51 to 70 is 400 IUs, most experts agree that they should aim for 800 to 1,000 IUs of supplemental D a day. But if you're under 50 and you consume the recommended 200 IUs (the equivalent of two glasses of milk daily) and get 10 to 15 minutes of sun exposure—without sunscreen—a day, a 400 IU supplement should do you fine.

3. Is it okay to cleanse your body by fasting from time to time?
As long as you are in good health, a brief liquid fast or cleanse is fine. But don't expect wonders—other than a sense of personal accomplishment, perhaps: Any physiologist will tell you that properly functioning lungs, liver, kidneys, and intestines do a fantastic job of keeping your body free of impurities without the help of fasting. If you do pursue a fast, always make sure to drink enough fluids to avoid dehydration.

4. Can I trust my tap water?
Sure. Unless you're on a private well, tap water comes from municipal treatment plants that are carefully monitored and better regulated than bottled water. (Some popular brands like Aquafina and Dasani are just that: tap water.) Very strict federal rules now require extensive filtering of the water supply, but minuscule amounts of chemicals and pharmaceuticals may still turn up. If you want to ensure you're drinking the purest water possible, consider adding a filter to your tap. For information on filters, go to NRDC.org/waterfilters.

5. Is my microwave giving me cancer?
No. Microwaving doesn't alter food in any way that could make you sick. All a microwave does is spur the water molecules in your food to move, and the friction of those molecules heats up your meal. The ovens do generate a tiny magnetic field, but there's very little evidence that such a field poses a problem for humans. What's more, there's an easy way to avoid any potential harm—step back when the oven is on.

6. How long am I contagious when I have the flu or a cold?
As long as you have symptoms. Your ability to spread these viruses remains until the last sniffle, says Bill Schaffner, MD, a physician and infectious disease expert at Vanderbilt Medical Center in Nashville. And you're contagious 24 hours before you first show symptoms.

As a reminder, always consult your doctor for medical advice and treatment before starting any program.

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