UK government guidelines as follows from the Hansard's
7th December 2004 Official Report (Commons Hansard), Vol.428, Col. 420W,
Air Passengers (Chemical Exposure)
To ask the Secretary of State for Transport whether UK airline captains experiencing cockpit or cabin air contamination event serious enough for pilots to use emergency oxygen have a responsibility to inform passengers to which chemicals they might have been exposed. [202745]
Charlotte Atkins: Pilots are required to use oxygen as a precautionary measure in all cases of suspected cockpit air abnormalities irrespective of severity of event. Flight crew are not professionally qualified to verify the cause of air contamination or identify what chemicals if any, passengers may have been exposed to. The captain has discretion to inform passengers of an event.
Merv.