That's because of *chronic* exposure.  The risk
from a single small
 dose is not very high (though obviously it depends on the dose); 
Those poor South Americans sometimes received quite a bit more than a
small dose!
I suppose it is all relative - you sitting in a room with an exposed cup
of mercury for an hour isn't too bad at all, but you sitting in a room
with an exposed cup of mercury boiling away on a hot plate is bad news.
William Donzelli
aw288(a)osfn.org