Aerosols, which contain small respiratory particles, are increasingly viewed as a potentially important way of transmitting the coronavirus. The WHO also recently pointed out the possible role played by these aerosols, based among other things on earlier research by Bonn and collaborators about the influence that proper ventilation has on the spread of aerosols. The researchers have now investigated how long aerosols persist within the confined space of a hospital elevator.
Simulating a single cough
The researchers mimicked a series of coughs using a specially designed spray nozzle. In this way, they dispersed a controlled quantity of droplets, with the same size distribution as evaporated respiratory droplets from a single cough. To be able to detect the aerosols in the elevator, they used a laser to illuminate the aerosol particles, so that the particles could be counted. The experiments were performed in elevator cabins during normal operation, which means that the door is open about 10-20 percent of the time. ‘We found out that during such normal operation, it takes 12 to 18 minutes before the number of aerosol particles decreases by a factor of one hundred,’ says Bonn. ‘When the elevator doors are permanently open, this time reduces to 2 to 4 minutes.’
Corona-infected sputum (the mixture of saliva and mucus that is coughed up) from hospitalized but mildly ill patients can carry between ten thousand and one billion RNA copies of the virus per milliliter. The value of one billion corresponds to roughly one virus particle per aerosol droplet with a radius of 5 micrometres. Loud speaking may produce up to a few hundred thousand droplets per minute, whereas a single cough can already produce a few million droplets. Breathing the air after speaking or coughing by an infected patient inside an elevator implies a potential intake of tens up to many thousands RNA copies of COVID-19 per minute, depending on the infected patient’s infectivity. The minimum infectious dose is not known; the severity of the corona symptoms is however believed to be proportional to the dose that causes the infection, implying that transmission by aerosols may lead to relatively milder symptoms than transmission by direct contact.
The researchers recommend to leave elevator doors open for a longer period whenever possible, and to either avoid talking and coughing in elevators or wear a proper face mask. They also point out the importance of optimizing the ventilation and increasing the mechanical ventilation capacity. Van Rijn says: ‘It turns out that the ventilation inside all studied elevators in idle position automatically shuts off after 1-2 minutes. This can of course easily be prolonged by reprogramming the action control software. In most hospital elevators the ventilator is present in the ceiling and exhausts air from the cabin towards the elevator shaft. A possible measure is reversing the flow direction of the ventilator, creating a downflow of fresh air from the ceiling towards the floor of the elevator cabin.’
The research was carried out by physicists of the UvA Institute of Physics together with medical researchers Aernout Somsen (Cardiologie Centra Nederland), Leonard Hofstra and Reinout Bem (both at Amsterdam UMC).
Cees van Rijn, Aernout Somsen, Leonard Hofstra, Ghassan Dahhan, Reinout Bem, Stefan Kooij and Daniel Bonn: ‘Reducing aerosol transmission of SARS-CoV-2 in hospital elevators’, in: Indoor Air, 23 September 2020. https://doi.org/10.1111/ina.12744