COVID in the air that we breathe
COVID-19’s airborne transmission poses a challenge to hospital infection control, says an Australian biosecurity expert.
Studies that show the SARS-CoV-2 virus can travel as far as 4.8 metres are challenging the 1.5-metre rule for physical distancing, according to Professor Raina MacIntyre, the Head of the Kirby Institute’s biosecurity program.
Professor MacIntyre spoke to Association members at a safe work webinar last month.
She says one study showed a viable virus 16 hours after aerosolisation. Another found the virus could be detected “everywhere – on the floor, the bedrails, on locker handles, the cardiac table, electric switch, the chair, the toilet seat and flush, and the air exhaust vent” in a hospital setting.
Large droplets emitted by infected patients travel much further than conventionally thought, Professor MacIntyre said. While the WHO has defined “large droplets”, which are assumed to fall close to the patient, as anything greater than five microns, Professor MacIntyre argues that droplets up to a hundred microns can potentially be airborne.
“Droplets that are less than a hundred microns will stay in the air and can be inhaled. But there’s this belief in hospital infection control that only … smaller airborne particles [travel a] great distance. The notion that one or two metres is a safe distance is therefore an ‘arbitrary rule’,” she said.
The initial response to the virus from the WHO was to focus on its spread by droplets and direct contact, Professor MacIntyre said.
“But the WHO response wasn’t based on any evidence; it was only based on an assumption.”
One recent study has found that some of the earliest studies that did not detect viable virus in the air were using air sampling methods that appeared to kill the virus. A more recent study “used a different kind of air sampler and found a viable virus in the air samples up to 4.6 metres in the absence of aerosol generating procedures”.
Speaking transmits more than coughing
Known outbreaks of coronavirus have also clearly demonstrated airborne transmission. In one case somebody was infected after passing by the open door of a patient room multiple times. Other infections have occurred on buses and restaurants, despite people having no close contact with the infected.
“Other outbreaks include a choir where people rehearsed indoors for two and a half hours with physical distancing and [yet] 86 per cent of them got infected.”
In an outbreak documented in China, the virus travelled well beyond the five people first infected on the 15th floor of a building.
“There was a vacant apartment on the 16th floor and the bathroom of that apartment was covered in virus … which means it became aerosolised through the sewage pipes and then deposited. People on the 25th and 27th floors also got infected,” Professor MacIntyre said.
“The transmission of SARS-CoV-2 really is about the air we breathe,” Professor MacIntyre said.
While we may think of coughing and sneezing as two key sources of virus transmission, in absolute terms far greater amounts of virus are emitted when people are speaking, singing or shouting, she says.
“The total amount of aerosol that’s generated is probably far greater from speaking and breathing because those things happen constantly. Whereas sneezing and coughing happen very occasionally.”
Aerosols accumulate in closed settings
The virus can build up in the air of an infected patient’s room.
“Essentially, one minute of loud speaking can produce thousands of droplets per second. And at least a thousand virus-containing droplet nuclei could remain airborne for more than eight minutes. Opening a window will disperse the aerosols, but unfortunately, many health facil-ities don’t have windows that can be opened.”
Professor MacIntyre cited one study that investigated the effect of HEPA air purifier filters in classrooms. Without air purifiers, the concentration of the airborne virus will increase in a closed room, whereas if you add an air purifier it decreases substantially.
Professor MacIntyre said: “I think we need to shift our mindset from transmission being a one hit event where somebody sneezes and this ballistic droplet comes and lands on your eye or in your nose, to [understanding] a more cumulative exposure.
“In a closed setting, where there’s no good ventilation, the aerosols are just going to keep accumulating like cigarette smoke, but you can’t see it. The longer you’re in that closed space, the greater your risk, and that risk is there just from patient’s breathing or speaking.”
Ordinary air conditioning recirculates the air rather than bringing in fresh air. An effective air conditioning system in the presence of SARS-CoV-2 needs to replace and purify the air, she said.
Because the virus is shed in faeces, toilets are another huge risk. When toilets are flushed’ you “get massive upward aerosolisation of virus particles”.
Since many hospital toilets do not have lids, there is the “potential for 40 to 60 per cent of the particles to rise above the toilet seat lid, leading to widespread contamination in bathrooms. Once aerosolised, this material can be deposited on surfaces and then re-aerosolised by human activity, like shaking out the bedsheets.”
Professor MacIntyre urged hospital staff to lobby for lids on all toilets.