Authorities say the 51-year-old victim was overcome inside a Manorhaven Boulevard building just before 3:45 a.m.
He was taken to St. Francis Hospital in Roslyn in unknown condition.
The building is home to a jewelry manufacturer, and cyanide is a byproduct of some jewelry production.
Manorhaven Boulevard was closed between Cottonwood and Dunwood roads as hazardous materials crews investigated. As a precaution, a "very limited" evacuation was ordered around the Manorhaven business.