High altitude, dry air, and hot summer temperatures multiply the effects of the sun. Below the rim, the canyon’s rocky walls hold heat and reflect the sun’s rays. Extreme inner canyon heat catches many people unprepared, and heat-related conditions are common. Protect yourself from sunburn by wearing a wide-brimmed hat, UV lenses, sunscreen or long sleeves, and lip balm. Above all, drink plenty of water.

Because the hot, dry air quickly dries perspiration, hikers may not realize how much moisture they are losing from sweat—up to two quarts of water every hour. Rangers along the Bright Angel Trail treat as many as 20 cases of heat exhaustion daily during summer months. Symptoms are pallor, nausea, headache, cramps, and cool, moist skin.

Left untreated, heat exhaustion can lead to heatstroke, a life-threatening emergency. The face becomes flushed and the skin dry. The pulse is weak and rapid. The body’s ability to regulate temperature is overwhelmed, and body temperature goes up, leading to mental confusion and eventual unconsciousness.

Hyponatremia (water intoxication) can look like heat exhaustion, with cramping, clamminess, headache, and nausea. But this serious condition is the result of drinking too much water without replacing electrolytes, leading to low concentrations of sodium in the blood. Like heatstroke, hyponatremia can lead to altered mental states and rapid pulse, and it too can be deadly.

To prevent heat-related illnesses, drink plenty of water and balance water intake by eating salty snacks and/or using electrolyte replacement. Keep cool, resting in the shade and hiking within your abilities. If your hiking companion’s mental state is altered, get immediate help. If you suspect heatstroke, cool the victim immediately by pouring water on his or her skin, clothing, and hair.