I thought it was antidisestablishmentarianism

Pneumonoultramicroscopicsilicovolcanoconiosis

Thants the dust which comes out of volcanoes and can get into your lungs and kill you.
Normally you would see a sign saying that with a gas mask.

I thought the REAL longest word was gammaaminonucleic acid

and I thought the rest were just jokes.....

Antidisestablishmentarianism.

What a word is is somewhat debated, but the longest non-agglutinative "word" is given by wikku9 above me. The post length limit prevents it from being displayed here in full.

http://en.wikipedia.org/wiki/Longest_word_in_English
Depends, LOLWIKIPEDIA LOLOLOLOL

the word above isn't technically word it is a chemical

the longest technical word in the dictionary is Pneumonoultramicroscopicsilicovolcanoconiosis

As Felix said, it depends on what you classify as a word. That huge long word is essentially the entire protein capsule of a virus, (woot, didn't need to look it up!). As for the lodging of silicon particles in your lungs, that to is a bit iffy, as it is a medical condition and you would not find it in a normal dictionary