Readers will know of my pet peeve on this issue. It turns out this has come up as a viewer complaint at the BBC several times and they actually have a policy on it, though like many media organizations they don't consistently follow their own guide.
You can see many examples simply by searching google images for "air pollution". The people riding bikes with masks are in actual pollution. The rest of the photos on the first page are mainly steam plumes. Note how the photographers like to catch the steam at dusk or backlit so they look dark and sortof smokey.