Beyond droplets: Is flu airborne in near-range?

Study finds viable small particles in coughs

A recent study of flu aerosols may lead to a new way of thinking about disease transmission. A patient's cough spews tiny particles that remain viable and can penetrate to the lower regions of the lungs, according to a report from West Virginia University in Morgantown.

That finding may require a new paradigm of a near-range spread — something between the droplet model, which assumes the transmission risk is from the splash of a cough or sneeze, and the airborne model, in which disease particles remain viable over time and distance.

"It's time to change this paradigm," says Lisa Brosseau, ScD, CIH, associate professor in the School of Public Health at the University of Minnesota in Minneapolis, an industrial hygienist who specializes in respiratory protection and aerosols but was not involved in this study.

"We [need to] make people aware that there are likely to be very small particles when you're standing near a patient and you can breathe them in," she says. "What you should be protecting yourself from is not something that sprays onto your face but something that makes its way around the edges of a surgical mask and into your mouth and nose."

This concept is controversial because it points to the need for respirators rather than surgical masks to prevent transmission. The Centers for Disease Control and Prevention already has recommended the use of respirators when health care workers are performing aerosol-generating procedures, such as intubation or bronchoscopy, on patients with influenza because of the increased volume of aerosolized particles.

Brosseau says her support for respirators ultimately stems from a focus on patient safety. Health care workers who inhale the particles and become infected with influenza can then unknowingly transmit it, she says. "I'm concerned about their passing infection on to patients," she says.

A continuum from droplet to airborne

In the West Virginia study, researchers recruited 58 students with influenza-like symptoms. Of the 47 who had confirmed H1N1 influenza, viral RNA was collected in the coughs of 38 students, or 81%. About two-thirds (65%) of the RNA was found in particles less than 4 microns, or in the respirable range.1

"We always talk about aerosol versus droplet transmission as if there's a distinction, but in fact it's a continuum," says William G. Lindsley, PhD, research biomedical engineer with the Health Effects Laboratory Division at the National Institute for Occupational Safety and Health in Morgantown and lead author of the study. "As particles get bigger, they have more inertia and they're heavier."

More importantly, Lindsley and colleagues found viable virus in cough particles of two of 21 samples tested — despite the fact that method used to collect aerosols can lead to inactivation of the virus.

"What we're finding is that there's a certain amount of airborne flu, [and] a certain amount is viable, so potentially there's a risk of inhaling particles."

Lindsley concedes that it's not known whether those small particles could actually cause illness. "There are going to have to be other studies to determine what the infectious dose is," he says. Researchers in the flu division of the Centers for Disease Control and Prevention are using ferrets to study contact and airborne transmission of flu.

Lindsley's study also suggests that some influenza patients may be "super-spreaders." Almost half (45%) of the viral RNA collected from cough aerosols came from just four of 38 students with influenza.

In this flu season, Lindsley and colleagues are recruiting more students to further test the viability of small flu particles.

Reference

1. Lindsley WG, Blachere FM, Thewlis RE, et al. Measurements of airborne influenza virus in aerosol particles from human coughs. PLoS ONE 2010; 5: e15100. doi:10.1371/journal.pone.0015100. Available at www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0015100.