The fact that there's a much lower reading (regardless of sensitivity setting/units) when he puts the sensor against the front of the lens (and around the back until he presses it against the lens) would suggest that they're pretty weak particles. I'd expect the mirror, pressure plate, and back door of an SLR to block them pretty effectively.

Remember, a lot of substances aren't dangerous due to radioactivity. It's because they're a toxic metal like lead. Ingest them and you'll suffer from heavy metal poisoning rather than radiation sickness.