True. Even when temperatures are not extremely high, humidity, sun exposure, exercise, and inadequate water intake can each increase the risk and help trigger heat exhaustion. For example, if the humidity is 80 percent and you're out in direct sunlight, 85 degrees F (29 degrees C) feels like nearly 110 F (43 degrees C) . Exercising or drinking too little water can turn up the heat even further.

People who aren't used to the heat are particularly vulnerable to heat illness. Others at risk include young children and people over age 65 or so; those who are overweight, out of shape, or dehydrated; people with neurologic or heart disease; and those taking certain medications that affect the body's ability to cope.

To stay safe: Acclimate to warmer temperatures before summer sightseeing or outdoor exercise by getting out for at least 30 minutes a day for two weeks or more. When it's very hot or humid, limit your activities, seek shade, drink plenty of fluids, and consider staying indoors.

Mild heat-related symptoms, such as fatigue, swelling of the hands and feet, or prickly heat (an irritating rash caused by blocked sweat pores), are a sign you need to cool off to avoid more serious trouble. Seek immediate help if you develop any of these symptoms of heat exhaustion or heat stroke during or after hot or humid weather:

• Confusion, lethargy, or agitation.

• Intense muscle aches, fever, or nausea

Convulsions or even a fleeting loss of consciousness

False. Unless you're consuming plenty of vitamin D, you need a little sunlight to stimulate production of that vitamin and, in turn, keep your bones healthy. Moreover, while too much sun increases the risk of skin cancer, the vitamin D produced by just a little sunlight may help prevent cancers of the breast, colon, pancreas, and prostate. High intake or blood levels of the vitamin may also reduce the risk of developing both types of diabetes, multiple sclerosis, and falls in older people (possibly by helping to strengthen their muscles), and may boost immunity against respiratory and skin wound infections. Indeed, one analysis estimated that nearly 24,000 people in the U.S. may actually die annually from an insufficient supply of vitamin D.

Moreover, sunlight itself may help you maintain a sunny disposition by helping to set the body's internal clock, which regulates its daily, or circadian, biological rhythm. Anything that disrupts that rhythm—such as winter, shift work, sleep disorders, or certain drugs or diseases—can alter production of brain chemicals that control mood. Appropriate exposure to light can restore the rhythm and help lift your spirits.

However, you need not—and should not—expose your skin for long to get enough vitamin D or reset your clock. "You can't use the real and possible health benefits of sunlight as an excuse for getting a tan or spending any extended period of time out in the sun unprotected," says Martin Weinstock, M.D., chairman of the American Cancer Society's Skin Cancer Advisory Group.

For example, evidence suggests that white people under age 60 or so who live in a line roughly linking Boston, Chicago, and southern Oregon can typically get all the vitamin D they need for the year by exposing their unprotected skin—hands and forearms and either face or lower legs—to the midday sun (from about 11 a.m. to 3 p.m.) for just five minutes a day, three days a week, during the spring, summer, and fall. (People who live farther north, have dark skin, or are overweight or over 60 need to spend a little more time outdoors or show a little more skin; those who live farther south need even less time or skin exposure, some of which can come during the winter.) And consuming lots of vitamin D from foods (notably fatty fish or fortified milk) or supplements lowers sun requirements further still.

You can get the mood-boosting benefits of sunlight without exposing any skin because the clock setting occurs mainly through the eyes. However, the exposure needs to occur in the morning, not at midday. For the average person, spending about an hour outdoors soon after waking (without sunglasses) or rearranging your home or work environment to maximize morning light should be enough. Individuals with severe complaints, such as those with seasonal affective disorder (SAD), or winter depression, should talk to their doctor about light-box therapy, which usually involves sitting in front of special lights for a while in the morning.

True—in some cases. You can get sunburned right through fabrics that let pinpoints of light shine through when they're held up to strong illumination, particularly if they're light-colored. Clothes that are wet or tight also let in more sunlight because they tend to stretch.

Shade and clouds are also unreliable shields. For example, if half the sky is visible through a leafy tree canopy, the tree protects you only as much as a very weak sunscreen with a sun protective factor (SPF), or potency, of 2. Under a beach umbrella, you still get 40 to 50 percent of the sun's radiation, since it reflects off the sand and sky. Even on overcast days, 10 to 50 percent of the sun's radiation penetrates the clouds.

To stay safe: Consider boosting the sun-shielding abilities of your clothes by washing them with a laundry detergent that contains brighteners, which absorb ultraviolet radiation. But for prolonged or intense sun exposure, use sunscreen and don't rely just on clothing or shade. To protect your clothes, let the sunscreen soak into your skin for about half an hour before getting dressed.

False. Even with scrupulously applied sunscreen, some radiation still slips through. Eventually, the cumulative effects will start to burn your skin if you stay outdoors too long. Reapplying the screen at that point won't extend the amount of time you're protected.

However, reapplying sunscreen long before you start to redden will help ensure that the protection actually lasts as long and works as well as it should. In fact, using enough sunscreen and reapplying it often are probably more important than a product's SPF. Researchers at New York University found that SPF-15 and SPF-30 sunscreens worked equally well to keep skiers from burning as long as the skiers consistently reapplied them.

To stay safe: Reapply sunscreen with an SPF of at least 30 on all exposed skin every two hours as well as after you go swimming or sweat profusely. Use a good handful (one-quarter of a 4-ounce bottle) each time.

False. The darkness of the glasses has nothing to do with their protectiveness against the main types of ultraviolet radiation, UVA and UVB. Instead, a chemical in or on the lenses does the job.

To stay safe: Choose sunglasses labeled as meeting American National Standards Institute (ANSI) requirements for general purpose use, which block at least 95 percent of UVB and 60 percent of UVA radiation. While that's enough for most people, sunglasses labeled as absorbing 99 or 100 percent of UVA and UVB may be best for those who have cataracts or take drugs that increase the risk of sun-induced skin and eye damage. Use sunglasses even if you wear contact lenses with UV protection, which don't cover the entire eye.

False. Products containing deet are safe if used properly. Canadian researchers combed through all the reports of adverse reactions to deet over the past half-century. Their review, published in the August 2003 Canadian Medical Association Journal, found that serious side effects in adults were confined mostly to people who ingested the chemical. In children, they found a few reports linking skin use of deet to seizures. But all occurred before 1992, and none definitively ruled out other causes.

The risks of catching a dangerous disease, such as West Nile or Lyme disease, from an infected tick or mosquito are far more worrisome than the very slight risks from deet, which is still the best way to prevent those illnesses.

To stay safe: To minimize any possible risks from deet, choose a product with the lowest concentration that works for you and your children. (Higher concentrations don't work any better, though they last slightly longer.) Use no more than the label directs, don't apply it near the nose and mouth or on broken skin, and don't apply it under clothes, which increases skin absorption. When you come home, wash the repellent off. The American Academy of Pediatrics now says deet concentrations up to 30 percent are safe on children over 2 months of age. Young children shouldn't apply the product themselves, and parents should keep it off kids' hands.

False. Particularly in drier climates, ticks are much more likely to lurk in the moist environment of a tree or log than on the forest floor. Ticks also cling to leafy plants waiting to hitchhike on a passing animal—or human—host.

To stay safe: When hiking in tick-infested areas, avoid brush, stay on the path or in areas covered by small plants and leaf litter, and don't lean on trees or sit on logs. Wear long pants and tuck them into your socks, and apply a deet-based repellent to your skin or an insecticide containing permethrin to your clothing (though not on your skin). After hiking or gathering firewood, check your body, your hair, and your pets for ticks.

True. For one thing, only some ticks carry the disease, even in regions where it's prevalent: The average tick-infection rate in such areas is around 30 to 60 percent. Moreover, you have up to 30 hours to remove an infected tick that's embedded in your skin before it transmits the virus.

To stay safe: Despite the small risk, people living in Lyme-endemic areas should watch for symptoms of the disease. Seek prompt medical attention if you develop a bull's-eye-shaped, solid, blotchy, or expanding rash, which appears in most cases.

Even without the rash, see a doctor if you develop any of these other symptoms a week or two after being in Lyme country in the spring or summer: headache, chills and fever, sore muscles or joints, or fatigue. Left untreated, the disease may progress over weeks and months to more-severe symptoms, including extreme fatigue, a stiff, aching neck, tingling in the hands and feet, and facial paralysis.

False. Home remedies to kill or make bugs "let go" are, at best, ineffective and can even be dangerous. By the time a chigger bite is visible and starts to itch, the bug itself is typically long gone. More worrisome: Applying irritants to an infected tick can cause it to expel more of the virus into your blood.

To stay safe: The best way to remove an embedded tick is to grasp it with tweezers close to the skin and slowly but firmly pull it off. Chiggers, or redbugs, are nearly impossible to see, but a thorough scrub in a hot soapy shower will rid you of most pests. Swab bites with alcohol or an antiseptic to kill any possible survivors. Lotions with anesthetics or itch relievers or creams with 1 percent hydrocortisone can relieve itching. If intense itching persists, try an over-the-counter topical antihistamine containing diphenhydramine (Benadryl, Caladryl and generic). If that doesn't help, oral diphenhydramine (Benedryl Allergy, Genahist and generic) or loratadine (Alavert, Claritin and generic) usually will.

False. Very few people react to their first sting, but in the roughly 20 percent of people who are hypersensitive to the venom, each subsequent sting by the same type of insect—or in some cases by another insect—intensifies the reaction. Most hypersensitive persons just experience more pain and swelling, but 1 to 2 percent have potentially deadly systemic reactions. You won't know whether you're in that allergic minority until you're stung again.

If you're not allergic, you'll experience only a mild, localized reaction that's easily treated with soap, water, and ice. If pain and irritation persist, over-the-counter pain relievers and the products recommended above for chiggers can provide relief.

To stay safe: See a doctor if you experience either major swelling at the sting site�an entire limb is swollen, for example�or systemic symptoms such as hives, fever, painful joints, or swollen glands. Get immediate medical help if you experience any of these signs of a potentially life-threatening allergic reaction: nausea, abdominal cramps, or vomiting; hoarseness or swelling in the throat or face; chest pain or rapid heartbeat; difficulty breathing; faintness; or confusion. If you're allergic to insect stings, ask your doctor to prescribe an epinephrine-injection kit (EpiPen and generic), and carry it with you outdoors. In addition, a series of allergy shots against insect venom can reduce the risk of serious reactions.

True. The old advice was to gently scrape out the stinger and attached venom sac with a fingernail, knife blade, or credit card to avoid squeezing the sac and injecting more venom. But a 1996 study showed that speed is the key factor. Sting victims were left with welts of about the same size regardless of how they removed the stinger, but welt size increased by 25 percent when removal time jumped from 2 to 8 seconds.

To stay safe: Remove the stinger immediately with whatever's handy—including your fingernails—and don't waste time seeking a tool.

False. That notion stems from the days when people often made mayonnaise and salad dressings from scratch, with raw eggs, which can harbor salmonella bacteria. Commercially prepared products use pasteurized eggs. And the acidity of their vinegar, salt, and citric juice kill some types of bacteria. In fact, a review article published in the Journal of Food Protection in 2000 found no case of food-borne illness linked directly with commercial mayonnaise or dressing. Far more likely to make you sick: unwashed fruits and vegetables and undercooked meats.

To stay safe:

• Wash fruits and vegetables thoroughly, including those you peel.
• Keep your cooler as cold as possible by leaving it in the shade covered with a blanket or, at the beach, partially burying it in sand.
• Pack perishables in watertight containers to avoid contamination from melting ice, and put leftovers back on ice as soon as possible.
• Wash your hands and all surfaces and cutting tools that the food will touch before preparing or eating it. If there's no water, bring wet washrags or towelettes.
• Don't let raw meats touch the same surfaces as other food. Brownness is a poor indicator of doneness, so bring a meat thermometer if you grill. Cook poultry to an internal temperature of 165 degrees F; ground beef, lamb, and pork to 160 F; beef and lamb steaks and roasts to 145 to 160 F; and fish to 145 F.