Large swaths of the country are gearing up for — or already weathering — a major heat wave this week, with cities including Chicago, Detroit, New York and Washington, D.C. bracing for what could be their highest temperatures so far this summer, according to the Weather Channel. As the mercury rises, the American Red Cross warned people in affected areas to stay hydrated, seek shelter in cool places and keep an eye on people and animals who could be affected by the heat.

Here’s what else you need to know to stay safe in a heat wave, according to Dr. Laura Burke, an emergency medicine physician at Beth Israel Deaconess Medical Center in Boston.

Prevention is the best medicine

Burke recommends limiting strenuous outdoor activity and taking frequent breaks if it can’t be avoided, staying indoors or in a cool area, drinking plenty of water and refraining from leaving children and pets in hot cars, even for a short time.

“It’s important that people drink regularly — definitely when thirsty, but not just when thirsty —if they are in extreme heat,” Burke says. Fluid requirements vary from person to person, but she says two glasses of water per hour is a good rule of thumb if you’re out in high temperatures.

Know the signs of heat stroke

Heat stroke and heat exhaustion are two of the most common heat-related illnesses, and Burke says both come with non-specific symptoms including nausea, weakness, heavy sweating and headache. But more severe heat stroke, which occurs when body temperatures rise above 104 degrees and can be fatal or result in permanent organ damage, comes with a distinct hallmark. “Any kind of neurologic symptoms [such as confusion or trouble speaking and thinking clearly] are really the defining sign of heat stroke,” she says.

Since someone suffering these symptoms may not notice them independently, Burke says it’s important to keep a close eye on others if you’re outside during a heat wave, and to bring them to a medical professional if they start to show signs of confusion or neurological distress. Even if the issue hasn’t reached that point, “if you’re starting to feel unwell while in the heat, you should stay hydrated and get into a cooler space,” Burke says. “If the symptoms persist or get more severe, that would be an indication to seek medical attention.”

Know who’s at risk

Everyone should be cautious in extreme heat, but Burke says certain populations are particularly susceptible to heat-related illness. These groups include the elderly, who may have more health issues to begin with, as well as decreased thirst mechanisms, mobility and cognitive function; young children, who don’t sweat as much as adults, have higher metabolic rates and may not be able to control their environment; pregnant women, who need more fluids to stay hydrated; and anyone with a preexisting health condition.

“Hospitalizations go up during times of heat waves, and it’s not just for things like heat stroke,” Burke cautions. “Other medical conditions can be exacerbated as well. It’s important that people who have baseline vulnerabilities have others around them to help them in times of extreme heat.”

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Write to Jamie Ducharme at jamie.ducharme@time.com.