When we see photos of Beijing shrouded in a veil of thick smog, we’re horrified. How can the Chinese live with such terrible air pollution?

One answer is: Americans did. Back in the 1950s and ’60s, people in Los Angeles breathed some of the dirtiest air in the world.

Los Angeles still has smog, of course, but it’s not nearly as bad as it used to be. How did the city get its act together?

It took decades. Los Angeles had its first real smog attack during World War II, a smog strong enough that some people suspected a Japanese chemical attack. But it wasn’t until 1975 that the U.S. required new cars to have catalytic converters, “the key piece of technology that allowed everything to change,” according to Mary Nichols, chairman of California’s Air Resources Board. In between, there were frustrating years of scientific research, industry denial, politics, protest and an unwavering attachment to the automobile.

Los Angeles, like Denver and Mexico City, is a natural pollution trap. The surrounding mountains combine with temperature inversions to trap dirty air. Early on, smoke and fumes from steel and chemical plants, oil refineries and backyard trash incinerators - legal until the late 1950s - plagued the city.

As did pollution from automobiles. Los Angeles County had more than a million vehicles on the road as early as 1940. Just 10 years later, that number more than doubled as the post-war LA population and economy boomed. City leaders, including the Chamber of Commerce, realized that air pollution threatened tourism, real estate and agriculture.

“They’d promoted Los Angeles as this clean, healthy place,” said historian Sarah Elkind, author of “How Local Politics Shape Federal Policy: Business, Power, and the Environment in Twentieth Century Los Angeles.” So in 1947, the county established an Air Pollution Control District, the first of its kind.

No one, however, blamed the automobile at first. “People did look at tailpipes, but auto exhaust was clear and the smog was brown, so it didn’t seem like there was a direct relationship between those two things,” Elkind said.

“It took about 10 years for there to be concrete laboratory-proven evidence that the hydrocarbon emissions from tailpipes, when exposed to sunlight and nitrogen oxides, turned into photochemical smog.”

Arie Haagen-Smit, a biochemist who had been studying the flavor of pineapples at the California Institute of Technology, not only made that discovery, but fought hard to convince politicians, regulators and industry that cars were the biggest smog culprit in Los Angeles.

The oil and automobile industries pushed back on his research. Chip Jacobs, co-author of “Smogtown: The Lung-Burning History of Pollution in Los Angeles,” says a turning point came when the oil industry-funded Stanford Research Institute sent a member to Caltech to discredit Haagen-Smit’s findings.

“The best thing that happened to LA lungs was when the man from SRI came in and smeared his reputation,” Jacobs said. Haagen-Smit was furious, and vowed to prove industry wrong. He redoubled his research efforts. By the mid 1950s there was no doubt among scientists that cars were a primary factor in LA’s smog crisis.

That doesn’t mean the public believed it immediately, or that car owners were willing to cut back on driving. Or that the auto industry sprang into action.

“Los Angeles had no influence over the auto manufacturers,” Elkind said. Plus, smog wasn’t yet a national problem. “It was very easy to dismiss smog as a quirk of LA geography.”

Automakers were slow to respond, wary of any change that would add cost to their vehicles. “It’s like the stages of grief,” said Nichols. “At first you deny it. Then you fight against it. And finally you grudgingly accept it, embrace it and move on.” That process took almost two decades.

James Lents, former executive officer of California’s South Coast Air Quality Management District, says Californians started agitating for change as the science became stronger and smog’s public health dangers became clearer. On bad days, parents kept children out of school, emergency rooms overflowed, athletic events were canceled.

Local doctors were beginning to talk about possible connections between lung cancer, heart problems and smog. In 1954, as many as 6,000 people showed up to a protest meeting in Pasadena. Los Angeles’s pollution czar volunteered to sit in Haagen-Smit’s plexiglass smog chamber to prove ozone’s danger. He got bronchitis.

“It was just a toxic atmosphere,” said Jeff Slade, who grew up in Beverly Hills. “I was thinking, ‘what could you compare it to today?’ And I think you’d have to look at cities like Beijing. It hurt, literally hurt, to breathe.”

Slade’s mother, Afton Slade, was president of Stamp Out Smog, a women’s activist group based in Beverly Hills. It was one of many anti-smog groups that sprouted in Los Angeles County during the late 1950s and early 1960s. They influenced public opinion and pushed politicians to do something about the crisis.

Stamp Out Smog had Hollywood connections and a flair for the dramatic. “So they did flashy things,” Slade said. His mother presided over a media event at the Ambassador Hotel in 1964 with a birthday cake marking 21 years of smog. It had a skull and crossbones on top in frosting.

“The press just loved this kind of thing,” Slade said. They also loved it when the women brought their kids to rallies wearing gas masks, a bit of political theater that became fairly common at smog protests.

These rallies and media events were among the earliest “environmental” protests in the U.S. The word “environmentalism” wasn’t really in the vocabulary yet. Rachel Carson’s book “Silent Spring” had just recently introduced a scary new thought – that technological progress could kill us. By the early 1960s, California demanded the first anti-smog controls on cars.

“The elected officials finally believed that cars were a big part of the problem and were going to regulate them, in spite of what the automobile manufacturers said,” James Lents said.

The 60s produced a dizzying series of changes, in California and the nation. In 1963, Congress enacted the first Clean Air Act, a tacit acknowledgement that smog had become a national problem. Two years later, it called for the first national emissions standards for cars.

In 1966 the California Highway Patrol began random roadside inspections of early smog devices. A year later Congress gave California permission to set even stricter emission standards than the federal government’s.

In 1969 the Justice Department sued automakers for conspiring to delay anti-smog devices, a lawsuit ultimately settled out of court. Then, Congress enacted the law that has set the framework for U.S. air pollution regulation, the Clean Air Act of 1970.

“It wasn’t until the Clean Air Act in 1970 that you had a law that said, ‘we’re going to set an air quality standard based on a public health measurement, and then the government will go out and take whatever action is needed to reach those limits,’” Nichols said. “But that was a shift, and it was based on growing populist opposition to how bad the air was.”

California still has some of the worst air in the country. But “worst” isn’t as bad as it used to be. Ozone levels in Los Angeles are just 40 percent of what they were in the mid-1970s, and that’s with more than twice the number of cars.

In the end, the air got better not because people were willing to change their behavior, but because technology improved, according to Lents. “My belief has been that humans are very innovative,” he said. “My experience was if you pushed them a little bit, they find solutions. They just don’t like to do it because it takes time and costs money and they don’t like to push ahead.”

Lents said fighting air pollution is an ongoing battle. As the science gets better, the more is learned about air pollution’s dangers. The “goal posts” move and air quality standards get higher.

In the past few years, scientists have grown increasingly alarmed about so-called “particulate matter” pollution, which embeds deeply in the lungs and is linked to serious heart and lung problems, including an increased risk of lung cancer. Los Angeles was ranked fourth for particle pollution in the American Lung Association’s latest “State of the Air” rankings. Now California regulators are struggling to bring particle pollution under control, along with ozone.

“We don’t meet federal ambient air standards in Los Angeles,” Nichols said. “We’ve brought the levels way, way down to the point where we don’t trigger actual health alerts very often, but I’m not satisfied with that.”

Nichols says by 2030 California needs to “move people and goods” with zero emissions technology. That gives the state 15 years to get its act together. Can it do it again?