Donora, Pennsylvania.

Meuse Valley, Belgium.

London, England.

Environmentalists will recognize those names as sites of deadly smog events in the mid-1900s. In Donora, in October 1948, 27 people died and hundreds became sick. In the Meuse Valley incident, back in 1930, 60 people died and thousands developed severe breathing problems. In London, just four years after Donora, a staggering 4,074 people died and tens of thousands of people became seriously ill.

Common to all three tragedies were two key elements. First, large factories in each area had been spewing enormous amounts of pollutants into the air, the most deadly being sulfur dioxide. And second, Mother Nature came calling in the form of something called a temperature inversion.

On most days the air is coolest higher in the atmosphere and warmest nearest the ground. Air isn’t a terribly good conductor of heat energy, so most of the sun’s energy warms Earth’s surface. Sometimes, though, air in higher elevations becomes warmer than air at the surface. That layer of warm air then traps the cooler air below, putting a kind of lid on the area. Temperature inversions happen with some regularity throughout the world and are particularly common in valleys. When moisture clings to the air during an inversion, you’ll see fog. Typically fog “burns off” during the morning, dissipating when surface air warms.

Sometimes, though, fog lingers. Combine that with pollutants also being held down by the layer of warmer air above and you’ve got the makings of a tragedy. People begin breathing air with increasingly higher concentrations of sulfur dioxide, fluorine, and other toxins from factory fumes. When sulfur dioxide combines with water vapor, the result is sulfur trioxide, a dangerous toxin and the primary component of what we know today as acid rain. That chemical change also begins robbing the air of oxygen and increasing the potential that humans, farm animals, and pets will suffocate and die. The longer the inversion lasts, the more deadly the effects.

Donora’s inversion lasted six days before rain finally broke it up. In London and the Meuse Valley, the inversion lasted five days. In all cases the death toll mounted quickly. One undertaker in Donora, Rudolph Schwerha, talked not long after the event with journalist Berton Roueché of The New Yorker about his return home after an arduous 2-mile journey in dense, black fog to pick up a body.

My wife was standing at the door. Before she spoke, I knew what she would say. I thought, Oh, my God — another! I knew it by her face. And after that came another. Then another. There seemed to be no end. By 10 o’clock in the morning I had nine bodies waiting here. Then I heard that DeRienzo and Lawson, the other morticians, each had one. Eleven people dead! My driver and I kept looking at each other. What was happening? We didn’t know. I thought probably the fog was the reason. It had the smell of poison. But we didn’t know.

We know now, of course, and today our air is significantly cleaner than it was then. It’s not perfect, not by a long shot, but it is much improved from the last century. Let’s hope the air becomes even cleaner this century.