Polonium-210 and Lead-210 are the two main culprits of it, still present after the efforts of the industry in the task of removing them from the final product before selling it. Nowadays, this is not a secret anymore and everyone should take notes to avoid exposing themselves to this source of ionizing radiation.
There's who affirm that even comparing the radiation intake with the fact of living in areas storing ionizing radiation like Fukushima or Chernobyl, an average smoker takes more. Today such claims usually come with technical gear capable of measuring to a certain degree of accuracy, normally high.
Any risk to the most important consequence of all, should be avoided as a top priority from any corner, specially in an evolved society. It's incredible how tobacco continues being sold in the public domain., to young people or any other.