The traditional dividing line between transmission by droplets and smaller aerosols is incorrect, say the authors of a Annals of Internal Medicine review paper, and infection control measures need to reflect the new understanding that respiratory viruses spread in respiratory particles of all sizes and perhaps mainly in aerosols.
For year, experts viewed contagious respiratory pathogens in a binary way.
There were viruses like influenza and mumps that were spread by relatively large droplets produced by coughing and sneezing that fell to the ground relatively quickly. Doctors, nurses, and other clinicians were advised to wear facemasks to block the droplets.
The other group of pathogens traveled aerosols, minute respiratory particles that people produce when they talk and breath. Aerosols tend to stay suspended in the air for much longer periods of times than droplets and travel much farther. Measles and tuberculosis have been held up as two exemplars of viruses that spread this way. Precautions against aerosols include N95 masks, negative-pressure rooms, ventilation and high-efficiency particulate air (HEPA) filters.
But in a review published earlier this month in the Annals of Internal Medicine, Michael Klompas, M.D., M.P.H., a professor of population medicine at Harvard Medical School and an infectious disease expert, and his colleagues argue that division of droplet and aerosol transmission is misguided and needs to be retired.
Research into COVID-19 and the virus that causes it, SARS-CoV-2, has helped reshape the understanding of respiratory virus transmission, the review argues. People generate the full range of respiratory particles, not either droplets or aerosols. Droplets can stay aloft for long periods like aerosols and respiratory viruses are not picky about the size of particle that they hitch a ride in, although aerosols may account for most transmission, partly because people produce aerosols just by talking and breathing.
The governing factor of transmission, wrote Klompas and his colleagues, is governed by “infectious dose” — the amount of virus a person is exposed to. Infectious dose is product of time and exposure concentration, or how much virus is in the air, the review says. Poor ventilation can allow virus-laden aerosols to accumulate and increase the exposure concentration and, as a result, the infectious dose. Good ventilation, HEPA filters and ultraviolet disinfection can decrease the amount of virus floating in the air. Time is factor because the longer a person spends breathing in air that has contaminated aerosols, the greater the infectious dose.
“Source strength”— or much a virus an infected person is spewing into the air in respiratory particles — is another factor in the complicated question, the reviewers explained.
Klompas and his colleagues — Chanu Rhee, M.D., M.P.H., and Meghan Baker, M.D., Sc.D., who are at Harvard with Klompas; Donald Milton, M.D., Dr.Ph., of the University of Maryland School of Public Health; and Surbhi Leekha, MBBS, M.P.H., of the University of Maryland School of Medicine — discussed some of the implications of the current understanding of respiratory virus transmission for infection control policies and programs.
Here is a list of potential policy responses included in the Annals review: