A single cosmic ray can cause cancer if it hits just the right cell in just the right way. Doesn't mean it will and nearly all of the time it doesn't, we get hit with cosmic rays without realizing it all the time, but sometimes it will cause cancer.

For what it's worth,
"As a note, for inhalation (the exposure of highest risk), breathing in 5,000 respirable plutonium particles of
about 3 microns each is estimated to increase an individual’s risk of incurring a fatal cancer about 1% above the U.S. average."
[link to www.evs.anl.gov]

1% increased risk from 5,000 particles of 3 microns each doesn't sound like the end of all of humanity even in the worst case scenario. Not saying this isn't extremely bad, and if the worst case scenario occurs some people will get lung cancer and die, but unless I'm missing something here or the data on plutonium health risks is completely faked, it won't wipe out humanity either.