It's not the smog or the carcinogens that are good. It's the activities that give them off that are good.
American cities are stale, boring, and just feel dead.
Asian cities in places like South Korea and China are just different. They feel vibrant and just full of energy. They're active at all hours of the day and night, 24/7 and just bustling with an energy that feels like 1920s or 1950s NYC. Sure, they have smog but they also have life. Cities like that are a dime a dozen there and nothing in America, not even NYC/ Chicago/ LA, comes close to giving an "urban lifestyle".
Have tons of emission controls, food safety standards, zoning rules, bar 2 AM closing rules, aggressive policing of drunks, etc.. yeah -- everything is clean, there's no smog, you're never going to get food poisoning, the noise is down at night.....
But you miss out on the restaurants and bars next to residential areas everywhere instead of having cities divided in half, people feeling free to do what they want without fear, cheap food carts everywhere, bars and things to do at all hours of the day and night, the energy of people everywhere on the streets all day and night.
It's a trade off. I prefer to live with the smog and other dangers and enjoy the vibrant lifestyle. If I wanted that shit, I'd move to the suburbs. American cities are suburbanized husks of their former selves now.