Over time, a light switch will wear out. When this happens, a small spark on the electrical contacts will be emitted whenever you turn the light on and off. Soon the contacts won't meet properly, and electricity will flow more freely, creating heat. This is a fire risk.
The best answer would be: That by using a new toothbrush you get the best results from group 3. It would appear that a used toothbrush sterilized or not, is less effective that a new one.