Contamination of the environment will cause exodus of western specialists. A non-nuclear explosion at Chernobyl contaminated the whole Europe. A nuclear explosion will be felt all over the world - but the fallout in the region will be significant.
However we should consider (such as be sure) that Iran, being under attack, will destroy oil wells of Saudi Arabia, Kuwait, and whoever else is in the area. They can use missiles, but it's more likely that they will send special operations forces.
Politically, an unprovoked nuclear strike on Iran will turn the USA from the world's banker to the world's madman. Want to count how many countries will be happy to punish the USA in the UN? How about all of them, maybe except the UK? Trade embargoes will destroy the US economy; not that it is in such a great shape today.
Veto.