Ultramicroscopicvolcanicsiliconiosis?

My nine-year-old son came home today from a summer day camp to inform me that the longest word in the English language is ultramicroscopicvolcanicsiliconiosis. Is that so? It is not in my dictionaries and a Google search returns zilch. Does anyone know for sure? Can you quote an authoritative source? I would accept the Oxford Unabridged, but don't have a copy at home.

The word apparently refers to a lung disease caused by the inhalation of fine volcanic ash.