Extreme heat is serious business—it’s the leading cause of weather-related deaths in the United States each year. An “excessive heat warning” means that conditions will become dangerously hot within the next 12 to 24 hours. So if you see this warning pop up in your weather app, you’ll want to be prepared.

Most of this preparation seems like common sense. Ensure you have ample supplies for that day’s activities and avoid strenuous work outdoors if possible. Keep track of loved ones, pay extra attention to your little ones, and don’t forget the pets! Stay cool and stay hydrated.

But within these common-sense rules, you’ll find confusion in unexpected places—especially when it comes to staying hydrated. Many people think that staying hydrated means chugging water like a storm drain even when you’re not thirsty.

In practice, this belief can lead to dangerous electrolyte imbalances, particularly when combined with warm-weather activities that induce a lot of sweating. Unsurprisingly, people hospitalized during a heat wave often have clinically low sodium levels due to sweat loss and overhydration.

So, hot-weather hydration plans actually need to be a bit more calculated than common sense may imply. I’ll lay out that plan in the second half of this article and share practical tips for preventing, identifying, and treating heat-related illnesses. First, we’ll cover the fundamentals of heat alerts so you understand what various alerts mean and how to be prepared.