I wish I had a nickel for every time I've seen or heard that written or said.
Radiation increases the risk of cancer, it doesn't "cause" it in a deterministic sense. If I hit a window with a hammer the glass will shatter. That is a deterministic cause and effect relationship.
Instead, cancer becomes more probable with greater radiation doses, it is a stochastic cause and effect relationship. Every additional bit of radiation increases the risk that someone will get cancer in the future.
Okay, maybe you think I'm parsing words....one particle of plutonium "contributes" to cancer, is still very onerous, right?
From Argonne National Labs comes:
"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."
So, the increased cancer risk is very small from a single particle of plutonium.
Another often heard myth is that one pound of Pu distributed over the face of the Earth will kill everyone. Here is an old link that explains why that's not true (some of the numbers are old, but the logic is sound).