Study Scrutinizes Airborne Disease

The airborne transmission of infectious disease has long been misunderstood and hospitals should take stronger measures to prevent it, according to an article co-authored by a Harvard lecturer in yesterday’s New England Journal of Medicine.

Harvard School of Public Health (SPH) Lecturer on Occupational and Environmental Health Donald K. Milton wrote the piece along with Chad J. Roy of the U.S. Army Medical Research Institute of Infectious Diseases.

SPH spokesperson Kevin C. Myron said Milton is “one of our trusted authorities” on air quality and public health, and added that the article “bolsters the case for considering airborne transmission in disease.”

In a SARS study also detailed in yesterday’s New England Journal of Medicine, a team of Hong Kong scientists found evidence that airborne transmission contributed to the outbreak of SARS at a Hong Kong housing complex in 2003.

Citing that study, Milton and Roy called for a reconsideration of the way in which doctors understand the relationship between infectious disease and airborne transmission. The traditional understanding has classified diseases as either “true” airborne infections or not airborne infections at all.

“We don’t think this leads to useful thinking,” Milton said.

Instead, Milton and Roy proposed a three-tiered classification system for aerosol-transmitted infections. In this system, potentially airborne diseases are either “obligate, preferential, or opportunistic.”

An “obligate” airborne disease is one that only infects people via aerosol. Preferential airborne diseases can be transmitted in other ways, but spread primarily through aerosol. The “opportunistic” category includes diseases that generally infect people through other means, but may be transmitted through the lungs as well.

Since the classic airborne diseases—tuberculosis and measles—came under control years ago, science has largely neglected the study of airborne transmission, Milton said.

“The clinical implications of airborne transmission are particularly important for infection control in hospitals and in public indoor settings such as airplanes and schools,” Milton and Roy wrote in the article.

They specifically cited the transmission of SARS in hospital waiting rooms as evidence that hospitals need to take serious precautions.

“This may indicate a need for more general application of aggressive air sanitation measures (e.g., upper room germicidal ultraviolet irradiation) in areas where patient care is provided and in the public areas of hospitals,” Milton and Roy wrote.

Ultraviolet germicidal irradiation (UVGI) involves a system of light fixtures in the upper region of a room to kill infectious agents before air recirculates through a building, said SPH Lecturer on Industrial Hygiene Engineering Stephen N. Rudnick.

“[The lamps] look pretty much like fluorescent lights,” said Rudnick, who has done much of his work with UVGI in homeless shelters.

Milton noted that, aside from hospitals, areas where airborne transmission is of particular concern include airplanes and schools, environments where air is often recycled.

Milton said that because airborne transmission is so difficult to prove, he does not expect all scientists to adopt his ideas immediately.

“Scientists quite justifiably tend not to believe things they can’t measure,” said Milton. “I’m not saying that one cannot measure airborne infectious agents now and in the future—the problem is that it is very hard and in the past was nearly impossible. New technology makes it increasingly feasible—so now is a fruitful time to do the research.”

A spokesperson for the Centers for Disease Control and Prevention (CDC), which sets guidelines for preventing exposure to infectious agents such as SARS in hospitals, said that the CDC does not comment on articles in scholarly journals.