As Americans come out to commemorate the nation’s 250th birthday on Saturday, the temperatures across huge swaths of the nation will be anything but celebratory. An intense heat wave is settling over the East and Central U.S., exposing millions to temperatures above the seasonal average and putting them at risk of heat-related illnesses.

Up and down the East Coast and the Central U.S., the mercury could surpass 100 degrees Fahrenheit (38 degrees Celsius), with some places, such as New York City, potentially feeling as hot as 115 degrees F (46 degrees C) during the day. The simplest way to stay safe is to remain indoors in air-conditioned spaces with easy access to cold water—both for drinking, and for showering or wetting towels to cool your body. But that won’t work for everyone.

Most public health messaging during a heat wave focuses on two heat-related illnesses: heat exhaustion and heat stroke. And for good reason: heat stroke, the more serious condition, can be fatal without immediate medical attention. It can occur when the body’s internal temperature rises above 104 degrees F (40 degrees C) or by extreme dehydration.

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But there is another condition you need to be aware of: hyponatremia, which translates to low sodium in the blood and is often linked to drinking too much water. Hyponatremia is barely mentioned in public health messages to do with heat: I’d never heard of it until I was diagnosed with it.

In hyponatremia, your body’s salt-water balance, crucial for the proper function of cells, is thrown off. When salt levels in your blood take a plunge, your cells swell up. In its early stages, hyponatremia is often confused with heat exhaustion, prompting people to drink more water in a bid to cool down. But that just makes the swelling worse, rapidly causing other body parts to swell—including the brain, which can cause a loss of consciousness, seizures and in some cases, death.

Here are three science tips you can use to stay safe in the heat this Independence Day weekend.

Know the signs of heat illness

Heat exhaustion is the first step toward heat stroke; it is caused by overheating and dehydration—specifically, the loss of salt and water through sweat. People with heat exhaustion typically feel dizzy, start sweating profusely, have a fast pulse and can feel sick.

Heat stroke is the more dangerous condition. If heat exhaustion is left untreated, it can spiral into heat stroke, a potentially fatal condition. People with heat stroke can feel confused and agitated and be unable to speak properly. Some breathe heavily, notice their heart rate going into overdrive and can even lose consciousness. Unlike in the earlier stage of this illness, they might not be sweating at all. If untreated, the extreme heat inside the body causes cells to essentially “cook,” leading to organ failure, brain swelling and often a heart attack.

Another heat-related condition (although it isn’t always to do with heat!) is hyponatremia. It can show early symptoms such as muscle cramps, nausea, headache and tiredness. If untreated or, as in many cases, made worse by the instinct to drink more water to try to alleviate these heat exhaustionlike symptoms, the signs become starker: People can suffer seizures, feel confused or have hallucinations, or fall into a coma. Within just hours of developing the condition, hyponatremia can cause the brain to swell so much that it herniates, pushing the swollen brain stem into the spinal cord—this is often fatal.

Do drink water—but not too much

The U.S. Centers for Disease Control and Prevention (CDC) recommends that people drink water to stave off dehydration. It states that for people working outside or otherwise exerting themselves in the heat, they should drink eight ounces (a cup) of water every 15 to 20 minutes. In an eight-hour workday, that would equate to drinking at least 1.5 gallons, or almost six liters, of water. The CDC also warns that thirst is a poor marker of dehydration, but some physiologists and wilderness medicine experts argue that that advice isn’t always helpful because it can lead to overhydration.

Importantly, the CDC warns that drinking more than 48 ounces of liquid in an hour could cause your salt levels to dip to dangerously low levels. Remember that liquid can mean water, fruit juice, tea, soda or any number of other drinks.

The CDC also notes that to be truly hydrated, you need to eat regularly in the heat to replenish salt levels. And for those wondering if a sports drink such as Gatorade might be good enough to hydrate you while also protecting you from hyponatremia, you are incorrect. Off-the-shelf sports drinks typically contain less than 20 percent of the recommended amount of salt an adult should consume in a day. That’s just not salty enough to raise your blood salt levels. Eating a one-ounce serving of salted pretzels is a better bet.

In extreme cases of hyponatremia, doctors feed patients extremely salty solutions, such as a half-cup of broth made with four standard beef bouillon cubes. That amount of salt is much more effective in raising your salt levels back to where they need to be.

Cool down your body

An effective way to prevent heat illness is to stay inside with your windows and blinds closed—light means heat—in an adequately air-conditioned space. Depending on where you live, your city may provide cooling stations where you can access air conditioning even if you don’t have it installed at home. The National Center for Healthy Housing has a state directory to help you find a local cooling center.

If you don’t have air conditioning but you do have fans, these can provide a cooling breeze under certain conditions. If the air outside your home is cooler than it is inside, position the fan by an open window and point it inward. If possible, place yourself in front of the fan, and to really cool yourself down, wear wet clothes while you do so.

Sometimes, it is more effective to turn the fan outward at night, when temperatures naturally drop. This enables the fan to push hot air out the room and pull cooler air in.

If you do go outside, wear a hat, loose clothing and sunscreen—and reapply regularly. Stay in the shade as much as possible, and bring a handheld fan if you can. Bring a cold, wet face towel or an ice pack to put on your neck or pulse points to help your body stay cooler in the heat. Avoiding alcohol and caffeine can also help your body maintain its internal temperature regulation.

Some research suggests that soaking your feet in a bath of ice water can help cool your body’s core. Taking regular cool showers and baths, wetting clothes or towels with cold water and putting your pillowcases in the fridge or freezer can all help further cool your body down and hopefully let you get a good night’s sleep.