Deadly 'wet bulb' temperatures are hitting the US this week. Learn the 3 stages of heat stroke, and how to intervene before it becomes fatal.

• Heat stroke can seriously damage your vital organs and may be life-threatening.
• It's important to drink water and seek shade as soon as you feel ill, before neurological symptoms begin.

An historic heat wave is putting millions of people across the US at risk for heat stroke this week.

Your body will issue several warning signs before you reach the point of heat stroke. If you start feeling lightheaded, thirsty, or generally unwell, getting out of the heat and drinking plenty of cool water is enough to bring most people back to a healthy baseline.

However, people who are pregnant, elderly, or very young have a higher risk of overheating. Certain medications and conditions, including diuretics and some psychiatric medications, also increase the risk of dehydration and heat stroke.

Even healthy people can die of heat stroke in "wet bulb" condition — days where the heat and humidity are both extremely high — especially if they exercise while under heat stress. Once your body's cooling mechanisms fail, heat stroke comes on hard and fast, and it can be lethal.

As heat waves become increasingly common and severe, it's important to know your risk. Here are some of the early signs of heat stroke and how to intervene.

Stage 1: Dehydration

The first stage of overheating is dehydration. As your body sweats to cool itself down, you lose vital fluids and electrolytes and may start to feel unwell.

You might feel weak, tired, and thirsty at this stage of heat illness, Cecilia Sorenson, an emergency medicine doctor and climate researcher at Columbia University, told Insider. Some experts call this early stage "heat fatigue."

If you attempt to exercise in high heat or wet bulb conditions, you may be more likely to experience severe cramping. Muscle spasms in the hands, calves, and feet are most common, but some people also experience cramps in their abdomen as a result of dehydration, Sorenson said.

It's important to get out of the sun and rehydrate as soon as you start feeling ill in the heat, Caleb Dresser, an emergency medicine physician at Beth Israel Deaconess Medical Center, told Insider.

"I think we've all felt too hot before," Dresser said. "Many times we will seek shade or seek air conditioning or reduce physical activity, and that's often enough to cool down provided we continue to hydrate."

Most people will recover from symptoms of heat fatigue within an hour of resting in a cool place and replenishing fluids, according to the Mayo Clinic. If symptoms persist, you should seek medical attention.

Stage 2: Heat exhaustion

If you don't stay on top of your rehydration, you will continue to overheat until you feel physically sick.

At this stage, you might have a headache and feel nauseous. You definitely feel tired and weak. Doctors call this stage of overheating "heat exhaustion."

When you're exhausted by the heat, your core body temperature is still close to normal, but the more serious effects of high temperature and loss of fluids have set in. This may include dizziness, fainting, fever, and shallow breathing, according to the Cleveland Clinic.

To others, you may appear pale and clammy. Your heart rate may be elevated as your body works harder to cool down. If you notice someone who is high-risk suffering from heat exhaustion, you may want to take them to the hospital, Dresser said.

For someone who is otherwise healthy, it is usually sufficient to have them drink cool water, cool the skin with a wet towel or spray bottle, and get to a fan or AC if possible.

Stage 3: Heat stroke

When the body runs out of fluids and electrolytes to sweat out, it loses the ability to cool itself down. The core body temperature continues to rise to potentially dangerous levels, but now there's no internal mechanism to stop it. This stage is called "heat stroke."

People with heat stroke may appear red and flushed. Their skin is hot and dry to the touch, as the body is no longer able to sweat. At this point, it is essential to get out of the heat.

Once your body temperature reaches 104 degrees Fahrenheit or higher, the change in symptoms can be dramatic, Dresser said. Someone who was feeling unwell for several hours might suddenly become confused or unaware of their surroundings.

Their condition may worsen in a matter of minutes, as the body has no way of regulating its internal temperature. Vital organs including the brain, kidneys and heart are at risk for permanent damage.

"If you suspect that someone around you has heat stroke, your first response should be to call 911 and get them to the emergency department," Dresser said. "This is a rapidly life-threatening condition in which minutes matter in terms of intervening to save their life."

Heat stroke is treated with the same level of urgency as a major heart attack or neurological stroke in emergency departments, Dresser said. In serious cases, several doctors, nurses, and EMTs will work together to cool the patient from inside and out with ice baths and IV fluids.