Coal smoke from homes and factories made the air dirty. Wet weather
made the air damp. When dry air mixed with damp air, the air became
cloudy and heavy. It became yellow smog. People breathed in a lot
of dirt, which made them cough. Sometimes the smog weas so thick,
people could not see through it. The fog was always worse on the river.
Lightermen said that when it was smoggy, they could find still their
way on the river because they knew all the smells which came from
warehouses along their route.