Officials believe people reported who feeling sick on a US subway may have been reacting to pepper spray used by police to restrain a man.
Officials said a substance released by the man who was arrested was now thought to be perfume.
The pepper spray is likely to have caused 35 people to complain of nausea, headaches and dry throats after the incident in Temple Hills, Maryland, outside Washington DC.
The incident prompted a full-scale bio-terrorism alert but US health secretary Mr Tommy Thompson said the mystery substance appears to be perfume.
"We haven't done the analysis yet. It appears right now that it looks like perfume," he said.
Police say the symptoms suffered by bystanders were likely to have been caused by pepper spray used by officers to restrain the man, who had fired a shot from a handgun after spraying the substance from a pump-action bottle.
The incident came as fears grew over bio-terrorism being behind the outbreak of anthrax in southern Florida, which has claimed the life of British-born journalist Mr Bob Stevens.
People who complained of being affected at the subway station were decontaminated by firefighters wearing special protective rubber suits.
Prince George County chief of police Mr John Farrell said: "It appears at this point to be an isolated incident."
PA