Question

what is carcinogenic substance?

Answer 1

A substance that can directly induce cancer, this also includes highly radioacive isotopes.
Chemical compunds that can induce cancer usually does so by interfering with some part of the cell machinery responsible for producing, maintaining or repairing the DNA, or it can react with the DNA directly and thus damage it. It can also interfere with proper signalling within the cell so that it cant read the environment properly and thus not stop dividing when it should. Radioactive isotopes can be carcinogenic since the radioactivity that they emit can cause damage to the DNA, for example double strand breaks.