Summer is a time for barbecues and other outdoor fun, but it’s also a time for sweltering heat. And experts say everyone, especially the elderly and very young, need to know how to limit the sometimes-deadly effects of high temperatures.

The ancient Greeks and Romans called the sultriest days of summer the “dog days.” The Old Farmer’s Almanac marks the time as 40 days from July 3 to Aug. 11, coinciding with the rising of the star Sirius, also called the Dog Star.

Looking at past years, the dog days have proved dangerous.

Heat warnings and advisories have been issued over a large swath of the country, with heat waves smothering the Northeast and shifting into the South and West. In Quebec, at least 70 people reportedly have died from a heat wave hitting eastern and central Canada.

More than 600 people die every year in the U.S. from heat-related illnesses that are preventable, according to the Centers for Disease Control and Prevention. The agency estimates more than 65,000 Americans visit an emergency room for acute heat illness each summer.

Dehydration can begin within just a few hours of extreme heat, so drinking extra fluids is important, especially when taking certain medications. Fatigue, headaches, muscle cramps, dizziness, sleepiness and dry mouth can all be signs of dehydration.

Dehydration causes the heart to work harder, putting it at risk. Hydration helps the heart more easily pump blood through the blood vessels to the muscles. And, it helps the muscles work efficiently.

A 2016 Environmental Protection Agency analysis of heat-related deaths said high temperatures could be a factor in many more deaths than officials realize – or count.

Heat exhaustion and heat stroke are serious medical emergencies that require treatment. Both can cause headaches, nausea or vomiting. Heat stroke also can cause a high fever, warm skin with no sweating and confusion or unconsciousness.

Heat stroke is not the same as a stroke. Stroke happens when a blood vessel to the brain either bursts or is blocked by a clot, causing a decrease in oxygen flow to the brain.

Beyond some of the obvious and sometimes extreme physical symptoms brought on by sweltering temperatures, a recent study showed it could affect how we think. And it doesn’t just affect the most vulnerable.