HONG KONG, Nov. 29 (UPI) — Influenza is usually spread through direct contact and droplets, but under some conditions transmission via aerosol is possible, Honk Kong researchers say.

Dr. Nelson Lee of the Chinese University of Hong Kong and colleagues say examined an influenza outbreak via aerosol — or gas — transmission in their own hospital in Hong Kong.

Lee, the study author, says on April 4, 2008, seven inpatients in the hospital’s general medical ward developed fever and respiratory symptoms and ultimately, nine inpatients tested positive for influenza A.

The cause of the outbreak was believed to be an influenza patient, admitted March 27, who received a form of non-invasive ventilation on March 31, and was then moved to the intensive care unit where he was located right beside the outflow jet of an air purifier — which created an unopposed air current across the ward.

“We showed that infectious aerosols generated by a respiratory device applied to an influenza patient might have been blown across the hospital ward by an imbalanced indoor airflow, causing a major nosocomial outbreak,” Lee says in a statement. “The spatial distribution of affected patients was highly consistent with an aerosol mode of transmission, as opposed to that expected from droplet transmission.”

N95 respirators may need to be considered when aerosol-generating procedures are performed on influenza patients, Lee suggests.

The findings are published in the journal Clinical Infectious Diseases.