Airborne transmission worse than thought

Current thinking on how most communicable respiratory infections are spread – by large droplets over short distances or by coming in contact with contaminated surfaces (face-to-face) – needs to be reconsidered, according to Donald Milton, lecturer on occupational and environmental health at the Harvard School of Public Health and co-author of a perspective in the April 22 issue of the New England Journal of Medicine. The perspective coincides with the report by Yu et al. in this issue on airborne transmission of SARS.

The authors argue that in certain cases, illustrated by the SARS Amoy Gardens apartment complex outbreak, where aerosolized, fine-particle spread of SARS was implicated, aerosol transmission of respiratory infection has been overlooked. This is due in part because the most widely recognized aerosol-transmittable diseases, measles and tuberculosis, are spread by airborne transmission and have been controlled, and because infectious aerosols are extremely dilute with fine particles that are difficult to collect and culture, obscuring their role in disease spread.

Calling for a critical re-evaluation of the phenomenon, the authors offer a novel way to classify aerosol-transmitted airborne diseases, dividing them into three categories: obligate, an infection that is only initiated under natural conditions via aerosols that are deposited in the lung; preferential, caused by agents that can naturally initiate infection by multiple routes but are predominately transmitted by aerosols deposited deep in the lung and opportunistic infections that require trafficking by pulmonary cells to produce full-blown disease and may include viral rashes such as measles or smallpox. The authors note all three have one common element, the capability of creating infection from a minute dose delivered to the lung in a single airborne particle. By their definition, the Amoy Gardens SARS outbreak would most closely resemble an opportunistic aerosol-transmitted airborne disease.

Milton and his colleague, Chad Roy, principal investigator in the Center for Aerobiological Sciences at the U.S. Army Medical Research Institute of Infectious Diseases, conclude that a better appreciation of aerosol-acquired infection is needed, especially in environments where ventilation systems in airplanes, apartment buildings, schools, and hospitals circulate and recirculate air that may at times contain infectious airborne aerosol-acquired diseases and viruses. They call for better measures to be taken toward sanitizing the air, through tools such as upper-room ultraviolet irradiation and through research that will help control the spread of airborne infection.