Scant Risk for SARS-CoV-2 From Hospital Air

Laird Harrison

December 28, 2020

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Everywhere they look within hospitals, researchers find RNA from SARS-CoV-2 in the air. But viable viruses typically are found only close to patients, according to a review of published studies.

The finding supports recommendations to use surgical masks in most parts of the hospital, reserving respirators (such as N95 or FFP2) for aerosol-generating procedures on patients' respiratory tracts, said Gabriel Birgand, PhD, an infectious disease researcher at Imperial College London, United Kingdom.

"When the virus is spreading a lot in the community, it's probably more likely for you to be contaminated in your friends' areas or in your building than in your work area, where you are well equipped and compliant with all the measures," he told Medscape Medical News. "So it's pretty good news."

The systematic review by Birgand and colleagues was published in JAMA Network Open.

Recommended precautions to protect healthcare workers from SARS-CoV-2 infections remain controversial. Most authorities believe droplets are the primary route of transmission, which would mean surgical masks may be sufficient protection. But some research has suggested transmission by aerosols as well, making N95 respirators seem necessary. There is even disagreement about the definitions of the words "aerosol" and "droplet."

To better understand where traces of the virus can be found in the air in hospitals, Birgand and his colleagues analyzed all the studies they could find on the subject in English.

They identified 24 articles with original data. All of the studies used reverse transcription–polymerase chain reaction (PCR) tests to identify SARS-CoV-2 RNA. In five studies, attempts were also made to culture viable viruses. Three studies assessed the particle size relative to RNA concentration or viral titer.

Of 893 air samples across the 24 studies, 52.7% were taken from areas close to patients; 26.5% were taken in clinical areas; 13.7%, in staff areas; 4.7%, in public areas; and 2.4%, in toilets or bathrooms.

Table. Air Sampled for SARS-CoV-2 RNA

Location Air samples, n Positive (%)
ICU rooms 107 25.2
Non-ICU patient rooms 364 10.7
Toilets/bathrooms 21 23.8
Clinical areas 237 8.4
Staff areas 122 12.3
Hallways 16 56.3
Outdoor public areas 8 37.5


Among those studies that quantified RNA, the median interquartile range of concentrations varied from 1.0 x 103 copies/m3 in clinical areas to 9.7 x 103 copies/m3 in toilets or bathrooms.

One study found an RNA concentration of 2.0 × 103 copies for particle sizes >4 μm and 1.3 × 103 copies/m3 for particle sizes ≤4 μm, both in patients' rooms.

Three studies included viral cultures; of those, two resulted in positive cultures, both in a non-ICU setting. In one study, 3 of 39 samples were positive, and in the other, 4 of 4 were positive. Viral cultures in toilets, clinical areas, staff areas, and public areas were negative.

One of these studies assessed viral concentration and found that the median interquartile range was 4.8 tissue culture infectious dose (TCID50)/m3 for particles <1 μm, 4.27 TCID50/m3 for particles 1 – 4 μm, and 1.82 TCID50/m3 for particles >4 μm.

Although viable viruses weren't found in staff areas, the presence of viral RNA in places such as dining rooms and meeting rooms raises a concern, Birgand said.

"All of these staff areas are probably playing an important role in contamination," he said. "It's pretty easy to see when you are dining, you are not wearing a face mask, and it's associated with a strong risk when there is a strong dissemination of the virus in the community."

Studies on contact tracing among healthcare workers have also identified meeting rooms and dining rooms as the second most common source of infection after community contact, he said.

In general, the findings of the review correspond to epidemiologic studies, said Angela Rasmussen, PhD, a virologist with the Georgetown University Center for Global Health Science and Security, Washington, DC, who was not involved in the review. "Absent aerosol-generating procedures, healthcare workers are largely not getting infected when they take droplet precautions," she said.

One reason may be that patients shed the most infectious viruses a couple of days before and after symptoms begin. By the time they're hospitalized, they're less likely to be contagious but may continue to shed viral RNA.

"We don't really know the basis for the persistence of RNA being produced long after people have been infected and have recovered from the acute infection," she said, "but it has been observed quite frequently."

Although the virus cannot remain viable for very long in the air, remnants may still be detected in the form of RNA, Rasmussen said. In addition, hospitals often do a good job of ventilation.

She pointed out that it can be difficult to cultivate viruses in air samples because of contaminants such as bacteria and fungi. "That's one of the limitations of a study like this. You're not really sure if it's because there's no viable virus there or because you just aren't able to collect samples that would allow you to determine that."

Birgand and colleagues acknowledged other limitations to their. The studies they reviewed used different approaches to sampling. Different procedures may have been underway in the rooms being sampled, and factors such as temperature and humidity could have affected the results. In addition, the studies used different cycle thresholds for PCR positivity, they write.

JAMA Netw Open. Published online December 23, 2020. Full text

Laird Harrison writes about science, health and culture. His work has appeared in magazines, newspapers , and online publications. He is at work on a novel about alternate realities in physics. Harrison has taught writing at San Francisco State University, UC Berkeley Extension and the Writers Grotto. Visit him at lairdharrison.comor follow him on Twitter: @LairdH.

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