When friends cautioned me about Beijing’s notorious air pollution recently, ahead of my first visit there, I brushed it off. It was an old story, and having grown up in northern New Jersey in the era of unregulated industrial air pollution and open garbage burning on the Meadowlands, I figured I could handle it. But I began to have second thoughts on the flight in from the north, when we crossed a mountain ridge and the clear air turned instantly to dense smog. It was still 20 minutes to touchdown.

After a day or two in the city, I felt as if I had taken up cigarettes. Same burned-out feeling at the back of the throat, with bits of airborne grit catching on the epiglottis. Same clearing of the throat by soft coughing. It got worse over the weekend, when regulations limiting cars on the road don’t apply. Coming back into the city on a Sunday afternoon was like a slow apocalypse. The air was a filthy brownish gray, and pedestrians, many of them wearing white face masks, walked hunched over as if through a rainstorm. Buildings emerged ghostlike from the haze a half-mile ahead and vanished again behind.

But I was a novice. It turned out that this was a relatively normal winter day for Beijing, with the air quality index at just 269. That’s rated “very unhealthy” by the World Health Organization, and many times worse than the maximum safe exposure level, but nowhere near those headline-making, sky-darkening days when the Beijing index has topped 700.

Back in New Jersey, the air quality index was generally under 50, and it reminded me how lucky we are to have relatively strong laws and regulations to protect our air. These are the same protections that President Donald J. Trump loudly promised during his campaign to undo on his first day in office. Indeed, the new Republican-dominated House of Representatives this month passed a Regulatory Accountability Act, which will give the new president power to roll back an array of governmental regulations, including 50 years of environmental protections — with as little public notice as possible. It could undermine even the Clean Air Act of 1972 and for the first time oblige regulators to put corporate profits ahead of public health.