Getting accurate death tolls for smog events is difficult because they don't just kill immediately — smog leaves some with slow-burning, life-long health complications. That said, let's look at Donora, Pennsylvania. According to research published in the American Journal of Public Health, 20 people died during a smog event there, and 43% of the population (or, about 6,000 people) suffered lifelong consequences from it.
So, what happened? Donora is the home of two massive industrial plants: the Donora Zinc Works, and American Steel and Wire. Smog was pretty normal, so when it settled in during the final days of October 1948, it was business as usual. Until suddenly, it wasn't.
Charles Stacey survived it, and recalled (via National Geographic) turning on the radio to hear about a Pennsylvania town "where people are dropping dead." He wondered where it was, and found out later that it was his own neighbors who had watched a high school football game, participated in a Halloween parade, and then started to die. The first died at 2 a.m. on October 30, and over the course of the next 12 hours, 17 more would die. Over the following decades, residents would suffer from higher-than-normal instances of things like heart and respiratory disease, and cancer.
The cause — along with, of course, the pollution — was a temperature inversion that trapped cold air beneath warm and sandwiched the pollution between the town and the upper, warmer air. The disaster ultimately kick-started the passing of the Clean Air Act.