Industry experts have questioned the reliability of a Chinese report that claims air conditioning was responsible for spreading the coronavirus at a restaurant in Guangzhou in January, writes Neil Everitt.
The Cooling Post was made aware of the report, compiled by teams of researchers for two Chinese disease control and prevention centres, some two weeks ago. It is due to be published in Emerging Infectious Diseases, the open-access and peer-reviewed journal published by the US Centres for Disease Control and Prevention.
The story has recently been picked up by the mainstream media across the world, with the UK’s Daily Express going as far as to refer to it as a “terrifying new study”.
Sadly, and particularly for the fact that it is to be published in a “peer-reviewed journal”, the report lacks vital facts.
ASHRAE has been prompted to comment that the conclusions of the report “are not well-supported by evidence”. I would go further to say that the report is based on ill-informed suppositions, but I’ll let you decide for yourselves.
The report says that family A in the diagram above travelled from Wuhan and arrived in Guangzhou on January 23. On January 24, the index case-patient (A1) ate lunch with three other family members (A2–A4) at the air conditioned restaurant in Guangzhou.
Two other families, B and C, sat at neighbouring tables at the same restaurant. Later that day, patient A1 experienced onset of fever and cough and went to the hospital. By February 5, a total of nine others (four members of family A, three members of family B, and two members of family C) had become ill with Covid-19.
According to the report, the only known source of exposure for the affected persons in families B and C was patient A1 at the restaurant. It therefore supposes that the virus had been transmitted to at least one member of family B and at least one member of family C at the restaurant and that further infections in families B and C could have resulted from within-family transmission.
The distances between patient A1 and persons at other tables, especially those at table C, were all greater than 1 m. However, the report maintains that strong airflow from the air conditioner could have propagated droplets from table C to table A, then to table B, and then back to table C.
I think it is reasonable to assume that, as the report states, while the virus remains in the air for only a short time and travels only short distances, generally less than 1m, a strong airflow would increase what have become our accepted safe distances. That would be the case whether indoors or out: whether outside in a strong wind, or inside in a room with an open window, a ventilation fan, a draught from an open door or, as in this case, an air conditioning unit.
Incredibly, the report makes no mention of the airflow rate from the air conditioning unit at the time. It appears that that vital piece of information has not been recorded. It doesn’t even tell us what type of air conditioning unit was involved. It does, however, mention that the ac unit was above the occupants of table C, suggesting it was most likely a high-wall DX split system.
Also, the diagram clearly shows an extractor fan on the wall opposite the ac unit. This could have affected airflows, but there is no mention as to whether this fan was actually operating on the day in question or not. Neither, it seems, were any swabs taken from the fan for traces of the virus.
The report suggests that the airflow may have been strong enough to pick the virus up from the infected person on table A, infect at least one person on table B and then return across the room to infect at least one person on table C. Yet, none of the staff or other diners in the restaurant were infected. It concludes that the lower concentrations of aerosols at greater distances might have been insufficient to cause infection in other parts of the restaurant.
Frustratingly, while table C was right beneath the air conditioning unit, six smear samples from the air conditioner from both the inlet and outlet of the air conditioning unit all proved negative.
Finally, the report concludes, to prevent spread of COVID-19 in restaurants, it recommends strengthening temperature-monitoring surveillance, increasing the distance between tables, and “improving ventilation”.
Perhaps someone could explain what “improving ventilation” means? More ventilation? Less ventilation? No ventilation? More filtered ventilation?
Commenting on the report, Bill Bahnfleth, ASHRAE’s Epidemic Task Force chair, ASHRAE Environmental Health Committee voting member and 2013-14 ASHRAE presidential member said: “The conclusions of the EID article are not well-supported by evidence. It’s not clear whether airflow patterns caused by the air-conditioning system contributed to the infections that occurred for several reasons. First, contaminants in air diffuse from the source to other locations via air circulation, regardless of whether air-conditioning systems are present or not. Second, there is no way to differentiate air distributed by the air conditioner with air in adjacent areas; mixing of air occurs naturally and sourcing potentially infected air requires a lot of assumptions. In the restaurant report, patrons and servers were constantly moving around in these spaces, which caused a significant amount of redistribution of contaminated air within the dining area.”
The Cooling Post has posed a number of questions to both the report’s authors and the Centres for Disease Control and Prevention. Neither have so far responded.
There is a lot we don’t know about this disease. This report, if conducted correctly, could have filled in some of those gaps, providing some very useful evidence as to the effects of the indoor environment on the transmission of this disease. As it is, we have what appears to be a report from a bumbling bunch of Inspector Clouseau’s who’ve trampled all over the crime scene.
Worst of all, perhaps, the report appears to have been taken seriously by the USA’s leading national public health institute.
The report can be found here