Someone smuggled a minute amount of Polonium 210 out of the former Soviet Union into England in a capsule in 2006.

The smuggler needed to find an intermediary with connections to test and verify the sample.

The intermediary crossed paths with Litvinenko or one his associates in London prior to October 16th, 2006.

Polonium 210 self-heats. The resulting pressure forced am opening in the seal.  A few micrograms seeped out the capsule. They got on the clothing fibers or person of Litvinenko or one of his associates.

There was then cross-contamination among Litvinenko and his associates Lugovoy, Kovtun, Scaramella.

A tiny amount-- a few microgram s-- eventually fell into Litvinenko's food, poisoning him.