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Reviewing the pandemic: Air was always the biggest risk factor

The surface risk from the coronavirus was very low in comparison with the risk of infection from air transmission, new research reveals.

Passengers are returning to public transport in London but its operator says it is struggling due to pandemic losses
Passengers are returning to public transport in London but its operator says it is struggling due to pandemic losses - Copyright AFP/File Behrouz MEHRI
Passengers are returning to public transport in London but its operator says it is struggling due to pandemic losses - Copyright AFP/File Behrouz MEHRI

Air transmission of SARS-CoV-2, the virus behind the coronavirus pandemic, was much higher than surface transmission. This finding comes from the University of Michigan, based on a two-year study.

The research looked at public spaces on the University of Michigan’s Ann Arbor campus. This included classrooms, rehearsal rooms, cafeterias, buses, gyms, student activity buildings, and ventilation and air ducts.

In terms of the difference between air and surface transmission, the risk of surface-to-human contact was 1,000 times lower than airborne transmission.

According to the lead researcher, Professor Chuanwu Xi: “We also found that the total case number within the campus was significantly higher in weeks with positive environmental samples than in nonpositive weeks.”

The data therefore shows the infection risk from fomites was much lower than initial research suggested.  

Xi adds: “Considering the recurring epidemics caused by respiratory infectious diseases in recent years, our study reinforces the relevance of integrating multiple environmental surveillance methods for modeling and risk assessment.”

To gather the data, Xi and colleagues used wetted wall cyclone bioaerosol samplers, which suck in large volumes of air using a pump and capture any virus particles in the air. For surfaces, researchers used swab kits.

Across the period August 2020 and April 2021, the scientists collected 256 air samples and 517 surface samples. The analysis showed that positivity rates were 1.6 percent and 1.4 percent, respectively.

In addition, the probability for infection was about 1 per 100 exposures to SARS-CoV-2 aerosols through inhalation but it stood only at 1 in 100,000 from contaminated surfaces.

These outcomes are based on simulated scenarios and conducted during lockdowns in a college campus. No samples were collected in spaces with large gatherings of people and some samples were only collected when few people were present.

Therefore, extrapolation of the findings the general population and health care settings should be done with caution. However, it is likely that the difference between air and surface transmission rates would remain unaltered.

The researchers hope the data advances understanding of infectious diseases and mitigation efforts during a future pandemic, especially with preparing for future outbreaks of respiratory diseases with similar transmission mechanisms.

In terms of understanding risks, the focus of effort should be with physical spaces and distancing measures.

The findings appear in the Journal of Exposure Science and Environmental Epidemiology, titled “Monitoring SARS-CoV-2 in air and on surfaces and estimation of infection risk in public buildings on a university campus.”

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Written By

Dr. Tim Sandle is Digital Journal's Editor-at-Large for science news. Tim specializes in science, technology, environmental, business, and health journalism. He is additionally a practising microbiologist; and an author. He is also interested in history, politics and current affairs.

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