15 Hz does not cause cancer but there are frequencies below that which might be harmful.
15 Hz is the frequency used by the Terminator zapper and some others. It was chosen because the human body ( nerves ) can feel it easier. The zapper's strength is the same at 15 Hz, 2.5 kHz, and 30 kHz but the nerves have a limited range of sensitivity which means that we do not feel the higher frequencies as well.
As a matter of point, while the body can not feel the higher frequencies as well, in some situations, they may be more effective.