Study warns of COVID-19 aerosol transmission risk between closely spaced buildings

GUANGZHOU -- A study by Chinese researchers has warned of the risk of COVID-19 aerosol transmission between two closely spaced buildings.

Researchers from China Center for Disease Control and Prevention (CDC) and local CDC workers in Guangdong jointly conducted the investigation based on a locally transmitted case reported in May in South China's Guangdong province.

They found that the index case (the first identified local case) and a close contact (later diagnosed as an imported case) were located at two different buildings in a Guangzhou hospital at the same time before the diagnosis.

Scientists already know that COVID-19 can be transmitted by aerosol, but transmission usually occurs in confined spaces.

In Guangdong, buildings with high proximity are called "handshake buildings" or "kissing buildings." In the case of the Guangzhou hospital, the two buildings have a distance of only 50 cm and share an exterior ceiling, forming a relatively enclosed space.

Virus sequencing showed that the viruses infecting the two cases shared the same origin. The epidemiological investigators speculated that aerosol transmission might be the transmission path in this case.

In the following field simulation to prove the theory, the researchers used fluorescent microspheres with similar aerodynamic characteristics to the COVID-19 virus at six sites related to the two cases in the two buildings.

Fluorescent microspheres are round spherical particles that emit bright colors when illuminated by UV light to show the diffusion of aerosol particles in the air.

According to a report published in China CDC Weekly, there were clear aerosol transmission paths from the location of the close contact and the index case. And the transmission was mainly affected by the airflow from switching the air conditioner on and off, as well as opening and closing doors and windows.

It noted that as long as air conditioners were switched on, the particles can still circulate slowly between the two buildings even if doors and windows were closed.

The study suggested that more attention should be paid to the risk of aerosol transmission in close-proximity buildings. It also noted that isolation wards and routine outpatient areas in hospitals should maintain adequate distance, and the hospital should check airflow layout in isolation wards.