High-flowrate air samplers allow thorough monitoring for airborne SARS-CoV-2

High-flowrate air samplers are able to detect airborne SARS-CoV-2 in hospitals, outperforming even surface sampling, according to a recent Singapore study. Factors such as the site of deployment and distance of the air samplers from patients affect the viral load detected.

“Our findings continue to support the suitability of using air sampling as a tool for environmental surveillance of airborne SARS-CoV-2 in hospital environment. This study further demonstrated that opting for higher air sampling flowrate improves the chance of successful airborne SARS-CoV-2 surveillance especially in sites that are highly ventilated or in situations where air samplers could only be placed at a certain distance from the patient,” the researchers said.

One isolation ward and two open-cohort wards at the National University Hospital, all catering to patients with laboratory-confirmed coronavirus disease 2019 (COVID-19), were included in the study. From February to May 2020, during the peak of the first wave of cases in the country, all three wards were fitted with air sampling devices placed at various locations, including in the restroom, beside patient tables, and in donning and doffing areas for personal protective equipment (PPE).

Subsequent RNA extraction and reverse-transcriptase quantitative polymerase chain reaction (PCR) protocols were performed to confirm the presence of the virus. Surface samples were also collected and assessed for comparison.

Initially, air sampling in the isolation ward was conducted at a normal flowrate of 50 L/min, which was unsuccessful in detecting airborne SARS-CoV-2, despite being performed in triplicate. When the researchers increased the sampling flowrate to 150 L/min, the success rate increased markedly and they detected the virus in 60 percent to 87.5 percent of samples. [Indoor Air 2021;doi:10.1111/ina.12930]

The location of the air samplers and their distance from the patients also affected the detection rate. Samples collected within 0.9 metres of the patient in the negative-pressure isolation rooms tested positive for SARS-CoV-2 RNA on PCR. In contrast, a sample collected from a device placed beside a windowsill, 3 metres from the patient’s bed, tested negative.

The same was true in the open wards, where air samplers placed in or adjacent to patient care areas provided samples that were likely to test positive for SARS-CoV-2 RNA. In contrast, samplers placed >9.8 metres away from patients produced negative air samples. One notable exception is the PPE donning zone, which while designated as a clean zone, yielded one sample that tested positive for the virus.

Comparing between the two types of wards, positive detection rate was 27.5-percent higher in the isolation ward but was statistically comparable. When considering only samples from contaminated areas, such difference in positivity rate dropped to 12.5 percent. On the other hand, average viral load tended to be higher in the open ward.

Notably, air samplers outperformed surface sampling for the detection of SARS-CoV-2. Of the 73 surface samples collected, only seven tested positive (9.6 percent), almost all of which had been taken from toilets.

“In hospitals with a high daily census of COVID-19 patients, employing a routine air surveillance program with a high success rate could prove beneficial in detecting the presence of the virus early in certain unsuspecting spaces/rooms for better protection of the involved healthcare workers,” the researchers said.

“Future air surveillance studies will need to be tested in locations outside of hospital environments where mass gatherings occur for rapid and sensitive high throughput communal testing at the population level,” they added.