Explaining four of the most common threats in the outdoors.

Unlikely bear and beaver attacks may get most of the attention, but it's the utterly predictable threats that pose the greatest risk. Spend enough time playing outside, and there's a solid chance that you or your buddy might suffer from heat stroke or hypothermia. These are the our most dangerous threats in the outdoors and what to do or how to prevent them.

Heat Stroke

In many sports, heat is treated as a nuisance instead of a threat. Runners race on 110-plus days, and cycling races are often run no matter the temperature. That's a dangerous mistake. Even the most experienced, record-breaking athletes can succumb to heat stroke, as ultrarunner Michael Popov did when he died in Death Valley this past August.Part of the problem lies in differentiating heat stroke, which can kill, from its milder cousin, heat exhaustion. The issue is that the symptoms aren't all that different to the untrained eye, says Dr. Douglas Casa, chief operating officer of the Korey Stringer Institute at the University of Connecticut. In fact, the majority of symptoms are shared: decreased performance or weakness, collapse, muscle cramps, nausea, and headache being the most obvious.The difference is that heat stroke victims experience central nervous system dysfunction—irrational behavior, irritability, emotional instability—and a rectal temperature greater than 104 degrees, something that is understandably difficult to measure in the field. Compounding the difficulty, heat stroke can strike when it isn't entirely expected, like on milder days, if athletes don't get enough water or are out of shape.

What Happens: You're creating more heat than you can dissipate. "Your core temperature gets so high that your cells are damaged," says Casa. "Organs like the kidney, brain, and liver start to malfunction. If the body is above 105 degrees for more than 30 minutes, that's when you run the risk of permanent complications or death."

Signs and Symptoms: While symptoms like irrational behavior can be a tip-off, "rectal temperature is really the only way to be sure that the reason they're having CMS dysfunction is due to heat stroke and not due to some other reason," says Casa.

Prevention: Hydrate properly and don't exercise in serious heat if you're not properly acclimated.

Treatment: "Cool the person as quickly as possible, and as soon as possible, usually with whole-body ice immersion," Casa says. "If that's not possible, use rotating cold, wet towels to cover as much of the body as possible."

The Myths: You stop sweating. If you're hydrated, you're safe. Ice baths are bad.

The Reality: Nope, heat stroke victims keep sweating. It can occur before you have time to become dehydrated. And ice is good, says Casa. "There used to be people thinking that cold water immersion as actually not good because you'd shiver or have peripheral vasoconstriction," he says. "They're wrong."

The Bottom Line: You've got 30 minutes to get your temperature under control. Do whatever it takes to make that happen. Cool first and get to the hospital later.

Lightning Strike

It sounds like an outrageous way to go, but lightning kills with regularity. In fact, over the last 30 years, lightning injuries have trailed behind only flash floods and tornadoes as the leading cause of storm-related deaths in the U.S. Worried about hurricanes, super-volcanoes, or earthquakes? Lightning kills more people each year than all of those threats combined, according to the National Weather Service.The good news? Ninety percent of lightning strike victims survive, though your chances depend on the kind of strike. Direct hits are, predictably, pretty fatal. But contact (when the victim is touching an object that was struck), side splash (where the current jumps to the victim from a nearby object), and ground strikes (where the current travels through the ground) are survivable.

What Happens: "You're in a channel," says Dr. Katie Walsh, an expert on lightning and director of East Carolina University's athletic training program. "A charge comes up from the ground and down from the sky at the same time. Anything that's in its path it will go through. Typically, your heart stops. Then you stop breathing. If your heart doesn't stop, you feel this big giant buzz." And if you survive, that doesn't necessarily mean you're in the clear. "It's almost like the concussions where people don't ever really feel 100 percent again—your brain changes or you have neurological tingling or don't sleep well," says Walsh. "Your whole life is changed. The majority struggle. They're short-circuited."

Signs and Symptoms: Generally the symptoms of a lightning strike are pretty self-evident but they can be misleading. "We have found people naked and dead because they were sweating," says Walsh. "They were hiking, lightning hit them and blew their clothing off. People think it's an assault. It's not."

Prevention: Avoid water, open spaces, high ground, and solitary trees. Be proactive: "Don't just check the weather, talk to a local who can say, be up by noon and back by two," says Walsh. "A local will know that, say, around three every day a storm rolls in." If you're trapped in the storm, make your way to safety as quickly as possible. Your best bet is a building or car. Short of that, squatting in a densely forested area or hiding deep in a cave is best.

Treatment: If the victim's heart has stopped, perform CPR. If not, get them to the hospital.

The Myth: Strike victims are dangerous to handle. If lightning hits the ground nearby, you'll be safe. If you're dry, you're good.

The Reality: Strike victims are perfectly safe to touch. When lightning hits the ground, you're not safe; it spreads out along the surface and can injure you if you're close enough. And just because you're dry (say, in a tent) doesn't mean you're safe. You need to be in a real building, with real plumbing, or a car.

The Bottom Line: Plan ahead, and "if you can hear thunder, you should be running," says Walsh.

Hyponatremia

It used to be that you weren't supposed to drink when you ran. Then, you were supposed to drink at every opportunity. Now, experts say you should drink only when thirsty. What gives? Also known as water intoxication, hyponatremia became well known after a woman competing in a "Hold your Wee for a Wii" contest run by a radio station died after consuming way too much water in a very short period of time. But while it might seem like over-hydration is something only Darwin Award competitors need worry about, it's a real threat for a certain subset of outdoor enthusiast: the recreational athlete, says Dr. Douglas Casa, chief operating officer of the Korey Stringer Institute at the University of Connecticut."Elite athletes aren't the ones getting hyponatremia," he says. "An elite marathoner doesn't have the time to over drink. You see it in the recreational person who stops at every aid station."

What Happens: You're drinking too much water over too long a period of time. And for some reason, your brain thinks you're dehydrated and stops urine production. You keep sweating, but it's not enough. Soon, your tissues become bloated, then the brain swells and pushes against the skull, cutting off blood supply. If it affects the parts of the brain that control breathing, you're a goner.

Signs and Symptoms: Somewhat similar to heat stroke. Headache and nausea, vomiting, altered mental status, bloating.

Prevention: If you're not thirsty, don't hydrate. "For recreational athletes, drink to your thirst," says Casa. "Not beyond it. And for elite athletes, develop a hydration plan."

Treatment: Drink less water, eat salty foods, consume oral hypertonic saline.

The Myth: It's impossible to over-drink because you'll pee it out. Water intoxication occurs only during exercise.

The Reality: You can over-drink. Hyponatremia can occur hours after exercise if lost sodium is not made up for.

The Bottom Line: If you're not thirsty, don't drink.

Hypothermia

Hypothermia has a twisted history. For one, the literature comes with baggage: unthinkably inhumane tests on prisoners during the Holocaust ground a fair bit of what doctors and scientists know. Additionally, the fear of freezing to death is so linked to mountaineering and Arctic exploration that people often don't take the risks seriously.So how big a risk is hypothermia for an outdoor athlete? Not a major one, provided that you don't run certain risk factors. According to Dr. Thomas Cappaert, an expert in cold weather safety at Central Michigan University, only one to three incidents of hypothermia result from every 1,000 exposures to cold weather. If you're exercising on land with a fair bit of clothing on, you're probably OK: you'll feel cold and turn back before you get into serious trouble.Unless you don't. Outdoor athletes have a tendency to ignore their perception of cold, says Cappaert. "If you want to go hiking, you accept that it's cold and ignore the psychic impulse," he says. "When you do that repeatedly, the impulse becomes weaker. And you ignore the symptoms until you cannot react."

What Happens: Your body is cooling faster than it can produce heat. Soon you're dropping below the temperature from which the body can recover. Shivering and your other defenses cannot handle the change. As the body continues to cool, organs shut down.

Signs and Symptoms: Watch out for the umbles: "You start to stumble, mumble, bumble, and grumble," says Cappaert.

Prevention: Be prepared for the weather to change, carry extra layers and food, and listen to your body.

Treatment: "Keep moving and seek out some sort of shelter," Cappaert says. If you're with a buddy, try to share body heat by huddling together. And if you have food, eat it: metabolizing fuel generates heat and "fills your carbohydrate stores so you have the energy to keep shivering." Once inside, use passive rewarming and focus on the trunk if you have a hot bottle. If you have frostbite, wait to get out of the cold before slowly warming the tissue under hot water: refreezing thawed-out fingers and toes can cause extra damage.

The Myth: Booze can help warm up a hypothermic person.

The Reality: Alcohol causes blood vessels to dilate, increasing heat loss.

The Bottom Line: Hypothermia isn't a joke. Dress warm and listen to your body.