The U.K. is enduring a serious heat wave that has resulted in an estimated 540 to 760 deaths over nine days.

Yet the highest the temperature has risen is 90 degrees Fahrenheit. Most of the time, temperatures have hovered in the high 80s, which, while toasty, doesn't seem like it could kill hundreds of people.

Last month, for example, the temperature hit 117 degrees in Las Vegas during a heat wave that scorched the Southwest and parts of California. The heat was so intense that aviation officials in Phoenix grounded several planes because it was too hot to fly. Yet, across a vast region of the United States, only a few fatalities were recorded.

The same goes for the current heat wave hitting the Northeast. Temperatures have reached nearly 100 degrees in New York, yet it hasn't experienced the same kind of troubles the U.K. has.

So why does 89 degrees kill in the U.K. when it doesn't in the United States?

One reason is that air-conditioning is relatively rare in England. Only 0.5 percent of homes in the U.K. have air-conditioning, according to the BBC. Compare that to the United States, where an estimated 87 percent of households have an air-conditioning unit.

In fact, flipping on the AC accounts for as much as 15 percent of America's total energy consumption. While all of those air-conditioners might not be great for the Earth, they come in handy when the temperature starts climbing toward 100 degrees.

Temperatures approaching 90 degrees are unusual in the U.K. In London, the average high in July is only 73 degrees. As a result, many people don't take the proper precautions against extreme heat.

"Other countries are used to very hot summers, we are not ready for this," said Deputy Prime Minister Nick Clegg. "Thirty-five degrees [95 degrees Fahrenheit] is pretty brutal and you need to be careful, particularly if you're frail."

In London, 10 children, including a 4-week-old baby, were admitted to a single hospital over the last week to be treated for severe sunburns. Around 457,459 people flooded into the U.K.'s A&E (accident and emergency) wards during the first week of the heat wave — the second-highest total since the country began tracking those statistics.

There have also been an unusually high number of drownings. On Tuesday alone, four people died while swimming in lakes, rivers, and quarries across the U.K. In total, 15 people have drowned while trying to beat the heat. Wildfires have also been a problem, with U.K. firefighters having to battle twice the number of blazes as last year.

The heat wave, the U.K.'s first prolonged one since 2006, is expected to continue for several more days. Stay cool, U.K.!