Drops or particles? It’s not either/or

Recent research into COVID-19 suggests that healthcare systems need to move beyond the idea that pathogens are spread via droplets or aerosolized particles. Patients can generate the full range of respiratory particles.

The myriad problems with health care systems in the United States and around the world uncovered by COVID-19 continue to preoccupy medical researchers as they peel back layers to find even more dimensions to the pandemic that has raged since March 2020. For example, in the early months of the pandemic, experts struggled to understand how the virus spreads. Paula J. Olsiewski, PhD, a contributing scholar at the Johns Hopkins Center for Health Security, said: Infection control today® (IT®) 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 like a virus in the air.”

But COVID-19 can also spread through droplets. It’s not either/or.

That is a point repeated in a judgement published earlier this month in the Annals of Internal Medicine (TARGET). Michael Klompas, MD, MPH, a professor of population medicine at Harvard Medical School and an infectious disease expert, and his colleagues argue that the distribution of droplet and aerosol transmission is misguided and should be stopped.

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

Another group of pathogens are aerosolized and spread through tiny respiratory particles that humans produce when they talk and breathe. Aerosols tend to stay in the air much longer than droplets and travel much further.

if IT® be aware in October 2020, perhaps the best analogy for COVID-19 would be how cigarette smoke might linger and spread in an enclosed environment, such as a bar (when smoking was allowed in those establishments). In such a situation, social distancing of 1.8 meters offers little protection.

Measles and tuberculosis have also been held up as two examples of viruses that spread in this way. Aerosol precautions include N95 masks, negative pressure chambers, 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, shows that humans generate the full range of respiratory particles, not droplets or aerosols. Droplets can remain in the air for a long time, as aerosols and respiratory viruses are not picky about the size of the particles they piggyback on, although aerosols can cause most transmission, in part because humans produce aerosols just by talking and breathing.

The determining factor for transmission, Klompas and his colleagues wrote, is “infectious dose” — the amount of virus a person is exposed to. Infectious dose is the product of time and exposure concentration, or how much virus is in the air, the review says.

Poor ventilation can allow aerosols loaded with viruses to accumulate and increase the exposure concentration and thus the infectious dose. Good ventilation, HEPA filters and ultraviolet disinfection can reduce the amount of virus floating in the air. Time is a factor because the longer a person breathes with air containing contaminated aerosols, the greater the infectious dose.

“Source strength” — or how much virus an infected person spits 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 with Klompas at Harvard; 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 possible policy responses included in the: TARGET judgement:

  • Consider creating a unified set of respiratory precautions for all respiratory pathogens rather than distinguishing between airborne pathogens and droplets
  • Consider the use of higher level respiratory protection (e.g. N95 respirators) in the care of all patients with active respiratory viral infections
  • Allocate airborne infection isolation chambers for pathogens historically associated with long-distance transmission and for patients with high viral loads
  • Reinforce minimum ventilation standards for non-clinical areas
  • Consider the use of higher-level respiratory protection such as N95 respirators for all long-term, face-to-face encounters when the incidence of SARS-CoV-2 is high in the community
  • Consider using overhead chamber or 222nm ultraviolet air disinfection or HEPA filters to reduce transmission risk in inadequately ventilated areas.
  • Consider dropping the concept of aerosol generating procedures
  • Instead, risk stratify patients based on viral load, disease severity, or expected duration of encounter and proximity to airways.

This article originally appeared in Director of care®.

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