Why did the Toms River Chemical/Ciba-Geigy plant change their schedule for use of the smokestack as noted in the nonfiction book, Toms River?

The smokestack of the Toms River Chemical/Ciba-Geigy plant was one of the only times that people could actually see the pollution the factory creates. Almost symbolically, the factory management changed the schedule so that it was mostly used at night, when no one could see the colored smoke.