COVID-19 Spread: Droplets or Particles? It’s Not an Either/Or


Recent research into COVID-19 suggests that health care systems need to move beyond the idea that pathogen spread happens either via droplets or aerosolized particles. Patients can generate the full range of respiratory particles.

The innumerable problems with the health care systems in the United States and the rest of the world exposed by COVID-19 continue to keep medical investigators busy, as they peel away layers to find yet more dimensions to the pandemic that stomped about since March 2020. For instance, in the beginning months of the pandemic experts struggled to understand just how the virus spreads. Paula J. Olsiewski, PhD, a contributing scholar at the Johns Hopkins Center for Health Security, told Infection Control Today® (ICT®) in a Q&A in December 2020, that it “took the [Centers for Disease Control and Prevention and the World Health Organization] a long time to recognize that this virus lingers in the air as an airborne virus.”

But COVID-19 can also spread by droplets. It isn’t either/or.

That’s a point reiterated in a review published earlier this month in the Annals of Internal Medicine (AIM). Michael Klompas, MD, MPH, 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.

There are viruses like influenza and mumps that spread by relatively large droplets produced by coughing and sneezing and that fall to the ground relatively quickly. Doctors, nurses, and other clinicians are advised to wear face masks to block the droplets.

Another group of pathogens are aerosolized, spreading via minute respiratory particles that people produce when they talk and breathe. Aerosols tend to stay suspended in the air for much longer periods of times than droplets and travel much farther.

As ICT® pointed out in October 2020, perhaps the best analogy for COVID-19 would be how cigarette smoke might linger and spread in an enclosed setting, like a bar (back when smoking was allowed in those establishments). In such a situation, 6-foot social distancing offers very little protection.

Measles and tuberculosis have also 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.

Klompas and colleagues argue that research into COVID-19 and the virus that causes it, SARS-CoV-2, demonstrates that 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 on, 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 “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 a factor because the longer a person spends breathing in air that has contaminated aerosols, the greater the infectious dose.

“Source strength”—or how much 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, MD, MPH, and Meghan Baker, MD, ScD, who are at Harvard with Klompas; Donald Milton, MD, DrPh, of the University of Maryland School of Public Health; and Surbhi Leekha, MBBS, MPH., of the University of Maryland School of Medicine—discuss 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 AIM review:

  • Consider creating a uniform set of respiratory precautions for all respiratory pathogens rather than differentiating between airborne vs. droplet pathogens
  • Consider using higher-level respiratory protection (e.g., N95 respirators) in the care of all patients with active respiratory viral infections
  • Allocate airborne-infection isolation rooms for pathogens historically associated with long-range transmission and for patients with high viral loads
  • Reinforce minimum ventilation standards for nonclinical spaces
  • Consider using higher-level respiratory protection such as N95 respirators for all prolonged, face-to-face encounters when the community incidence of SARS-CoV-2 is high
  • Consider using upper-room or 222-nm ultraviolet air disinfection or HEPA filters to decrease transmission risk in inadequately ventilated spaces.
  • Consider retiring the concept of aerosol-generating procedures
  • Risk-stratify patients instead on the basis of viral load, severity of illness, or anticipated duration of an encounter and proximity to the respiratory tract.

This article originally appeared in Managed Healthcare Executive®.