Ingesting alpha emitters is not a good idea. Won't do much harm at a distance, as even a few inches of air will stop the alphas, but inside the body they will do damage. Don't think Dr. Cohen was really serious.
The specific toxicity of plutonium is about the same as caffeine. I do not suggest mixing plutonium with your morning cup of joe.
Professor Cohen was absolutely serious. He believes that the health risks of foregoing nuclear power are far greater than nuclear power ever could be, by orders of magnitude. Alternative energy sources all present higher health risks, and the alternative of foregoing economic activity has health implications. Longevity correlates with prosperity more strongly than any other identified factor. Being poor makes you unhealthy. Basically, the trade-off of foregoing nuclear power is that we trade prosperity and health for poverty and illness.
BTW, Cohen estimates that if nuclear waste were simply discarded into the environment, into the Mississippi for instance, about one in 10,000 atoms would ever be ingested by a human being (a surprisingly high amount) but the health effects would still be less serious than the poverty resulting from foregoing nuclear power. Again, no one is recommending careless disposal of nuclear waste when far more sensible alternatives are available, but it just shows how irrational and hysterical the arguments about nuclear power have become.