Indoor spread of respiratory infection by recirculation of air.

Stimulated by an outbreak of measles that was spread throughout a school by the ventilating system, we looked into the possibility of preventing this hazard in air conditioned buildings. The amount of air recirculated by air conditioning systems increases as the temperature difference between indoor and outdoor air increases and often exceeds 70 per cent. Germicidal u.v. radiation in central supply ducts seems almost ideally suited for disinfecting recirculated air, being effective, safe, and cheap. The effectiveness of disinfecting recirculated air in blocking person to person transmission of airborne infection can be predicted to be great at the beginning of a potential outbreak and negligible during an established epidemic. Infection introduced by the air conditioning process, recently implicated in causing Legionnaires' Disease, might also be prevented. Air disinfection would supplement immunization in the control of respiratory infection and might be cost effective.