Step on a jetliner. Sit in a doctor’s waiting room. Take your kid to day care. If it’s flu season, we’re wagering you’re wondering: Am I inhaling enough influenza virus to get sick?
“Yes, if you stick around for an hour.” That’s our bare-bones summary of a new measurement of airborne influenza A virus in these three locations. The study calculated how long virus particles would remain aloft and therefore be subject to inhalation.
Influenza A is the most common and dangerous genus of influenza virus.
Bigger particles can carry more virus, but they also settle far faster than tiny ones.
Among the half of the samples that contained influenza A, the average cubic meter of air contained 16,000 influenza A particles. Since we breathe roughly one liter per inspiration, this means that when influenza is present, one would inhale 16 pieces of flu virus with each breath.
Breathing that air for an hour would be enough to make half of us sick, says Linsey Marr, an associate professor of civil and environmental engineering at Virginia Tech, the study’s corresponding author.
The study sorted the flu-carrying aerosol particles by size, Marr says, which “allows us to say how long the virus particles will stay suspended in the air. We found virus in aerosols small enough to remain suspended for hours.”
About 64 percent of the virus particles were held in aerosols no more than 2.5 millionths of a meter across.
Because few previous studies of influenza A measured the size of the aerosols that carried them, Marr says, “We did not know if they were held in very large droplets that would fall out in a few seconds or minutes,” and thus be less likely to transmit disease.
Virus: Just another pollutant?
Marr, an air-pollution engineer who says she became interested in viral transmission after her three-year-old came home sick from day care “about every two weeks,” says hunks of influenza A virus are about one-tenth of a micron across, and thus difficult to identify.
Using a version of polymerase chain reaction that detects viral RNA, Marr and her collaborators, including Elankumaran Subbiah, an assistant professor of virology at the Virginia-Maryland Regional College of Veterinary Medicine, examined air samples during flu season.
Half of the samples were influenza free, a strong indication that nobody in the area had the flu. But when influenza A was present, the researchers calculated that the air would contain enough to infect 50 percent of the people after one hour, Marr says.
Such an estimate is necessarily rough, as many factors can affect the health consequences of any exposure to the virus:
Individual differences in viral exposure and immune effectiveness
Previous inoculation with or exposure to influenza A
The infectivity of the virus, which degrades over time
“We detected total virus RNA, but that does not tell us whether the particles are infectious,” says Marr, since they could have been inactivated due to ultraviolet light or another cause. Previous studies, many of them decades old, suggest that influenza A can remain infective for two to three hours, but those estimates vary widely, Marr says.
Furthermore, although in general smaller particles can go deeper, and cause more disease, the body is also “pretty good at removing virus in the nasal tract,” she points out.
“The research is an important step in better understanding the nature of influenza transmission,” says Jonathan Temte, a professor of family medicine at University of Wisconsin-Madison, and a specialist on immunization. “Influenza continues to be a very significant respiratory pathogen which can spread significantly and almost without abatement, especially when there is little immunity such as we saw during the pandemic of influenza A(H1N1) last year.”
However, Temte says, “This study is limited” because it relied on only 16 air samples, and was performed “when there was almost no transmission of influenza in North America. Hence, the researchers were able to detect influenza at a time when very few individuals were getting sick. From a clinician’s viewpoint, it is necessary to show that viable influenza viruses can be carried in an airborne form and that transmission can occur in this mode and this occurs at a high enough rate to be of significance.” Although the study is an interesting application of technology, “Making the translation from technology to real patient care in the next and necessary step,” Temte says.
Now that the study has confirmed with measurement what we’ve long known — that flu virus can float in the air — we wondered whether we are more likely to catch flu by breathing or by touching our schnozz after shaking hands with an infected person. The answer is not yet clear, says Marr, “but we have showed that the airborne aerosol route is possible.”
Will this intensify fear of flying? Perhaps. “This fear has always been there — if someone is sick, it’s possible that virus is floating around,” says Marr. Face masks could help contain influenza, “But without a proper fit, you short-circuit the mask, and it doesn’t do much good. What would be even better is if infected people wore masks….”
– David Tenenbaum