A flash vaporizer (34) provides a constant flow of vaporized hydrogen peroxide or other antimicrobial compounds for rapidly sterilizing large enclosures (10), such as rooms or buildings. The vaporizer includes a heated block which defines an interior bore or bores. The flowpath created by the bore or bores may increase in cross sectional area as the hydrogen peroxide passes through the block to accomodate the increase in volume during the conversion from liquid to gas. The vapor is injected into dry air in a duct (23) that circulates it to the large enclosure.
