When was smog a problem?

Can you tell me when smog first emerged as a major environmental concern? I'm curious to know if it was a recent issue or something that has plagued societies for a longer period of time. Additionally, were there any particular historical events or technological advancements that contributed to the emergence of smog as a problem? I'm also interested in knowing if there have been any significant changes in the severity of smog over the years and what measures have been taken to mitigate its impact on public health and the environment.