Sorry if I am asking a stupid question. I recently had an argument with my friend which among this two is more dangerous: 3H or 235U(not considering its decay chain products). I was under the impression that 3H has an effective dose about 1000 times smaller than 235U due to its weak beta-radiation. Various sources seem to confirm my guess that at least per *activity* tritium is one of the least dangerous radiactive iostope. However my friend said because tritium has a shorter-half life and lighter nuclei, it has very high activity/gram rate, about 10^11 higher than 235U which means it is in fact "millions of times of more dangerous" than 235U. His argument sounds convincing but a little contradictory to what I was originally impressed. I know 235U itself isn't that dangerous as media reported but I never thought it is "millions of times of safer" than tritium... So is he correct on this argument? Thanks.