The air smelled like bleach. Noses ran and eyes stung. Visibility was cut down to three city blocks. Photos of Los Angeles taken on this day in 1943 show a city shrouded in thick, biting smog.

In the midst of World War II, people thought the city was under attack. But as they’d discover, they were just having the first experience of a new phenomenon that would become one of L.A.’s defining characteristics. When the same kind of smog rolled into the city next summer, wrote Marla Cone for the Los Angeles Times, residents knew what to do–stay inside.

“As residents would later find out, the fog was not from an outside attacker, but from their own vehicles and factories,” writes Jess McNally for Wired. “Massive wartime immigration to a city built for cars had made L.A. the largest car market the industry had ever seen. But the influx of cars and industry, combined with a geography that traps fumes like a big bowl, had caught up with Angelenos.”

Industrial smoke had elicited complaints in Los Angeles since at least 1903, wriote Cone, but what one government report referred to as a “hellish cloud” was something much more significant. The problem until the 1940s, she wrote, was that nobody knew exactly what smog was or how it was caused.

Shutting down a plant that was blamed for the unprecedented smog didn’t help, writes McNally, and the problem just got worse. It wasn’t until the early 1950s that it became clear that the exhaust from the cars everyone was using to get around was causing most of the smog.

“Back then, Angelenos knew, air pollution was a matter of life and death,” Cone wrote. They watched as it wreaked havoc elsewhere: A 1948 smog incident caused the deaths of 20 people in Donora, Pennsylvania and made half the town sick, and in 1952 London’s killer fog took the lives of thousands.

The region has been regulating smog since that first attack, but the struggle continues. Smog continues to pose a deadly threat in Los Angeles, Mike McPhate wrote earlier this year for The New York Times. Researchers from New York University found that L.A. haze is a factor in shortening the lives of more than 1300 people each year, McPhate reported. “Los Angeles invariably ranks first in the American Lung Association’s annual ‘State of the Air’ survey of most-polluted cities,” writes Amanda Fortini for Slate.

The South Coast Air Quality Management District, which oversees air quality in Los Angeles and Orange County, is working to change that, McPhate wrote: “All sides recognize that industries under the agency’s regulatory power—including refineries, ports and warehouses—must adopt cleaner technologies.”