Pu 239 can be put in highly toxic radioactive isotopes.
As you know radio isotopes are mostly generated from the radioactive decay reactions and the heaviest material naturally occurring is U238.
Pu239 is mostly created in artificial nuclear reactions. The danger of the radio isotopes is measured by their half lives. Radio isotopes having longer half lives are treated as highly toxic and in this case half life is ~24000 years.
Short lived radio isotopes can be stored in a container and after sometime they are gone and that container pose no radioactive hazard. However long lived radio isotopes pose radiation hazard in case of leak from container. They can contaminate soil, water and air and results in radio toxicity.
Hence you may put radio-isotopes in low toxic (half life few days), mid toxic (half life few years) high toxic (half life much longer than human life) ranges.
Just imagine you put Pu239 in a Stainless Steel container, do you think that the container can hold that isotope for ~50000-100000 years and that's why it is highly toxic.
I hope it will help
EDIT:
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Regards
EDIT
@dmckee you are technically right. The toxicity also depends on the type of the radiation they emit (gamma, neutron, alpha or beta), among them neutron is most hazardous.
Any inert molecule is like radioactive particle with infinite decay time (which simply pose no health hazard). But I am talking about the really radioactive compounds, for example mildly radioactive $U^{238}$has ~$10^6$ disintegration per seconds per mole.
The toxicity I am talking about is not per person but for society. If one person dies with short lived isotope only he dies but if long lived isotope is released you may imagine what havoc may happen. It goes inside you and keep on emitting the high energy particles which will result in increased cancer or other deceases in whole area and this will last for thousands of years.
Yes it also depends on actual decay rates, for example thorium, which is even milder radioactive material, is a naturally occurring radio isotope in several places in the world, resulting in natural radioactivity far greater than the acceptable levels worldwide and people are still living there. Over thousands of years they apparently gain higher resistance to the radioactivity than others (I am not sure about it).
I hope I have clarified your objections (at least partially)