Air carrier Flight Attendant reported a cabin fume event while parked at the gate during boarding. The APU pneumatic air was shutoff by the Captain and the passengers were deplaned.

Narrative

While boarding; when APU was turned on; it emanated toxic/carbon monoxide fumes in the cabin. Crew and pax were all smelling and complaining of the fumes. Captain and FA1 were immediately notified. The captain tried to reboot the air system; but; as soon as it was turned on again; the fumes returned with a vengeance; so the captain immediately turned off the air system and initiated deplaning of the aircraft. We were all exposed to fumes and felt lightheaded and nauseated; but; thanks to the captain turning off the air immediately; exposure was minimum.Suggestions: Not sure if event can be prevented; maybe by more frequent inspections of engines for possible engine oil leaks; and more frequent changing of air filters. However; the captain was amazing in moving quickly; by turning the air system off immediately and immediately initiating deplaning of aircraft; as; this way; exposure was minimum. This should always be procedure!Once we discovered we had a fumes event; captain and FA1 were notified; and aircraft was deplaned.