As a heat wave hits America from coast to coast, it’s hot outside — but cool inside, thanks to the triumph of air conditioning.

For most of human history, there was little people could do to avoid heat.

During the day, it drove people outside of their homes to enjoy the shade of a tree or to take a refreshing dip in a lake or river.

At night, folks in cities slept outside on their porches, roofs and even fire escapes.

When I was a kid in the suburbs of Pittsburgh, few homes had air conditioning.

Our windows were always open with several fans bringing in the cool night air, as we drifted off to sleep comforted by their wobbling sound.

Air conditioning changed everything.

Homes used to have big windows, high ceilings, cross ventilation and large hallways to dissipate heat. This is no longer necessary, and most homes are sealed shut all summer long.

Commercial buildings used to have windows that opened, but that isn’t necessary, either.

Today’s glass-plated buildings are designed to keep the light and air out, so that we are oblivious to whatever season it may be.

Without AC, heat was the great equalizer. The rich suffered just as much as the poor.

All of that changed a little over 100 years ago when Willis Haviland Carrier invented air conditioning.

Department stores and movie houses were among the first to install AC. Regrettably, the federal government soon followed.

It wasn’t until the 1920s that AC began making its way into residential homes.

Today, 90% of American homes have air conditioning, which is why tens of millions of young and old folks have been able to safely migrate to steamy places like Florida and the baking deserts of the Southwest.

AC can literally be a life saver.

Europe’s unbearable 2003 heat wave killed more than 30,000 people. That’s unconscionable at a time when a window AC unit costs a hundred bucks at any big-box store.

The good news, according to Scientific American, is that by “2050, researchers expect the number of room air conditioners on Earth to quadruple to 4.5 billion, becoming at least as ubiquitous as cell phones are today.”

Here’s even better news: Technology innovation is making AC units more efficient, less costly and less impactful on the environment.

Consider: AC requires hydrofluorocarbons (HFCs) to cool the air.

HFC refrigerants are very effective at cooling the air inside our homes, but they are potent greenhouse gases that, as they leak from aging units, are bad for the environment.

An intense competition is under way among AC manufacturers to replace HFCs with eco-friendly refrigerants and advanced filtration systems.

To further aid this effort, a global coalition of partners has joined hands to launch the Global Cooling Prize, a first-of-its-kind challenge to spur innovation in the AC industry.

Meanwhile, AC continues to be one of the greatest inventions to better the lives of millions — maybe the coolest American invention ever.