ThatBritGuy:

How dangerous can a single particle be? You'll get the usual spray of products, but I'm not sure how much damage they'd do on their way through.

They don't have more energy than a baseball...

More seriously,

Sievert - Wikipedia, the free encyclopedia

In terms of SI base units:

1 Sv = 1 Jkg = 1 m²s² = 1 m²·s^-2

...

For acute full body equivalent dose, 1 Sv causes nausea, 2-5 Sv causes epilation or hair loss, hemorrhage and will cause death in many cases. More than 3 Sv will lead to LD 50/30 or death in 50% of cases within 30 days, and over 6 Sv survival is unlikely. (For more details, see radiation poisoning.)

Given that protons have a Relative biological effectiveness of maybe 10, and that the Oh My God particle has an energy of about 50 joules, a single ultra-high-energy cosmic ray could cause radiation poisoning, but only a handful of them have ever been observed and, what is more important, what reaches ground level is the shower of low-energy particles produced by the single original proton, which is scattered over a very large area.

The brainless should not be in banking. — Willem Buiter