It’s flu season. The National Institute for Occupational Safety and Health (NIOSH) conducts research on protecting health care providers and other workers from infectious diseases including influenza.  A significant portion of our research deals with understanding how the influenza virus is transmitted. Influenza is known to be transmitted through respiratory secretions containing the virus. Airborne transmission of influenza by small aerosol droplets over longer distances is debated in the literature. Coughing, sneezing, speaking and breathing all generate potentially infectious aerosols (small airborne particles). Several studies suggest small aerosol particles can carry influenza virus, but how important is this route of transmission? If patients can readily infect others via aerosols produced during coughing, speaking, sneezing, and breathing, then interventions such as patient isolation and cohorting (grouping those exposed to a similar disease), increased air ventilation and filtration, air disinfection, and the use of respirators or other personal protective equipment may help to protect healthcare workers and other patients from the illness. Such precautions will not likely be implemented without data to support transmission as such interventions can be costly and time-consuming. A summary of one NIOSH study on influenza A virus detected in coughs and exhalations follows. Additional transmission research is available on the NIOSH website.

NIOSH researchers compared aerosol particles containing viable influenza virus generated during coughs and exhalations[i]. In this study, 61 adult volunteer outpatients with influenza-like symptoms were asked to cough and exhale three times into a spirometer. Fifty-three test subjects tested positive for influenza A virus. Of these, 28 (53%) produced aerosol particles containing viable influenza A virus during coughing, and 22 (42%) produced aerosols with viable virus during exhalation. Thirteen subjects had both cough aerosol and exhalation aerosol samples that contained viable virus, 15 had positive cough aerosol samples but negative exhalation samples, and 9 had positive exhalation samples but negative cough samples.

Viable influenza A virus was detected more often in cough aerosol particles than in exhalation aerosol particles, but the difference was not large. Because individuals breathe much more often than they cough, these results suggest that breathing may generate more airborne infectious material than coughing over time. On the other hand, as coughing involves much higher air velocities than breathing, coughing may spread the virus further in a given location. Thus, both mechanisms for producing infectious aerosols may be important depending upon such factors as the distance from a patient, the timescale, the infectious dose, and the air flow within a room.

The fact that the number of aerosol samples with viable influenza was not significantly greater for coughing than for exhalation is consistent with the theory that much of the influenza-laden aerosol produced by infected people originates in the deepest parts of the lungs rather than in the upper airways.

More research is needed but this study is consistent with previous studies that show aerosol particles containing viable influenza virus are produced by infected individuals both during coughing and during exhalation. These findings support the idea that airborne infectious particles could play an important role in the spread of influenza.

Visit the NIOSH Flu at Work page for guidance on preventing seasonal flu in the workplace.


[i] Lindsley et al. (2016) Viable influenza A virus in airborne particles expelled during coughs versus exhalations. Influenza and Other Respiratory Viruses, Doi: 10.1111/irv.12390.