As far as I know, it's no more dangerous or larger than it normally would be. I'm in the UK and I felt a cold draft......though I doubt it's anything to do with the volcano and more to do with the fact that the UK is, on the whole, freezing:P (although we have had one sunny warm day and it looks like today will be the same....yay!).
The only problem so far is the whole airspace thing....in fact they could probably get away with flying some planes, it's just a health and safety precaution. In 1982, British Airways and Singapore Airways jumbo jets lost all their engines when they flew into an ash cloud over Indonesia.
The ash sandblasted the windscreen and clogged the engines, which only restarted when enough of the molten ash solidified and broke off.
So for safety, most countries in Northern Europe, and any country affected (depends on the wind) have grounded flights.
There is one more danger, and that's if the ash settles. Some of it I believe has already settled in Scotland. To that end, folks here are being advised to stay ondoors if they have respiratory problems. And otherwise healthy people are told to go indoors, if they smell a strongacidic smell, sulphur, or rotten eggs.
As far as I know it wil not reach or cause a problem for the US.