Firefighters said they were left baffled by their failure to find the source of an apparent leak that left a man and paramedic overcome by fumes.
Firefighters wearing specialist breathing equipment were called to the flat in Eversfield Road, Eastbourne, at 6.30pm last night after reports of a chemical leak.
A man had called paramedics after he was overcome by a strong smell.
A paramedic sent to treat the man was also overcome by fumes.
But despite spending three-and-a-half hours at the flat searching for the cause of the apparent leak, firefighters drew a blank.
A spokesman for East Sussex Fire and Rescue Service said: "We could not find anything - we found no root cause at all."
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