You’re correct about the skin penetration, but the risk with tritium is in inhalation. Experiments with mice demonstrate carcinogenesis. The increase in cancer risk from inhaled tritium in humans is unknown, but the substance is considered dangerous enough that it’s a pain in the ass to get the certifications to use it in your laboratory. It definitely should not be considered harmless.

I worked with Stephen Bodner on the fusion program mentioned in the fine article. We did our experiments with deuterium. The reason we did not use DT, which would have been better for the experiments, is because nobody wanted to (1) go through the hassle of getting the lab certified to handle tritium' (2) get anywhere near the stuff. It is considered very hazardous.