COVID-19: How to avoid super-spreading events on the metro

COVID-19 hit public transit systems in New York, US and London, UK badly, causing several deaths

By K Nagaiah, G Srimannarayana, Phaniraj G
Published: Wednesday 09 September 2020
Sanitisation process underway in Delhi metro trains. Metro services were allowed to be resumed across the country recently. Photo: Twitter / Delhi Metro Rail Corporation / @OfficialDMRC

Metro train services in major cities across India were cleared to run from September 7, 2020, for the first time since March, even though novel coronavirus disease (COVID-19) cases significantly increase every day.

COVID-19 hit public transit systems in New York, United States and London, United Kingdom badly, causing several deaths, including those of the train operators, other employees and passengers.

There is, thus, a need to exercise abundant caution to ensure safety measures in the running of the metro. It becomes imperative for metro services to run with safety measures against the SARS-CoV-2 virus that causes COVID-19, especially given the potential of the metro to become a hub of viral aerosol transmission and super-spreading events (SSE).

Virus-laden aerosol droplets — tiny particles of less than five micrometres and droplets that range from 5-50 microns — are emitted by COVID-19 patients from their mucosal membrane when they talk, breath, shout, sing, etc.

These droplets fall to the ground — because of their weight and gravity — while viral aerosols linger in the air as they are lighter and float for a few hours when their moisture shield dries up.

How aerosols are transmitted

Six-foot social-distancing fixes droplet spread to a large extent, but studies have found viruses 15 feet away from patients in hospitals due to aerosol transmission. Aerosols are, however, a little more complex to deal with, unless additional measures are taken.

Aerosol virions (virus particles) expelled by the patients can circulate in poorly ventilated spaces. Anyone breathing in them can get the viral load. The time spent in such spaces is also directly proportional to air quality and crowding.

Aerosols can sneak into heating, ventilation and air-conditioning (HVAC) systems and are subsequently spread from recirculated air. Metro trains and stations rely on air-conditioning. Special attention must, thus, be paid to minimise aerosol spread by ensuring ventilation and air circulation.

SSEs are incidents where one person with a viral load can spread the virus to tens of others in a specific area and in a short duration of time. Thousands of such SSEs were documented across the world.

The distinct pattern in these cases is they occur mostly in indoor settings, with aerosol transmission being the suspected culprit. We need to prevent SSEs and make metro systems safe.

To aid people to remember and watch out for aerosol transmission and to avoid SSE, we propose a Triple A (AAA) system.

  • Area: Avoid crowded spaces with too many people hanging around
  • Activity: Minimise talking, singing, shouting, exercise and dancing in crowded areas
  • Air quality: Avoid places with poor ventilation, high carbon dioxide (CO2) concentration and little air circulation, especially when air reeks of body odors or strong smells, including those of garlic, tobacco or perfumes

Metro systems can avoid SSEs

Metro trains have their doors and windows closed when in transit. They always rely on air-conditioning. Metro stations are mostly indoors or enclosed outdoors, with their entrances and exits being indoors having long corridors.

There is established evidence from studies published in reputed scientific journals of the virus being reportedly airborne, with aerosol spread being one of the reasons along with fomite and droplet transmissions.

The World Health Organization and the Union Ministry of Health and Family Welfare called for hand-washing and social-distancing, along with wearing masks.

Metro trains are, thus, potential aerosol transmission hubs and need special attention. The Union Ministry of Housing and Urban Affairs, in its standard operating procedures, made a cursory mention of improving ventilation.

But given the scope for any malfunctions in ventilation, additional measures must be taken.  A well-documented study attributed the super-spread of influenza cases in a commercial airplane in Alaska to a ventilation failure. The metro systems should not take such chances.

The CO2 level indicates surrogate aerosol activity and must be monitored on trains and stations. They should, if possible, indicate CO2 levels on light-emitting diode boards to alert people of possible aerosol spread. Metros must use at least 12 air changes per hour to dilute aerosol. High-efficiency particulate air filters usage must be explored in HVAC systems.

Further precautions

Passengers must be asked to wear masks at all time, not to talk or use phones or eat during their transit, as these can unleash viral aerosols in the air. Public announcement systems must carry the message repeatedly to remind passengers to remain silent on the trains. Train carriage windows can be opened to further improve the ventilation. France and Japan are already doing so on some of their trains.

Every fifth window must be opened on both sides of the train with a mesh. Train doors must open at every station at least for a minute or two to ensure the flushing of air periodically. The station entrances, exits, concourses, food courts, toilets and corridors should use industrial fans to provide for additional air circulation.

It is not advisable to keep food courts open till cases decline, as people remove their masks while eating. However, if they do open food courts, there needs to be a reconfiguration of seating arrangements to ensure social-distancing. Such food courts must serve food only in disposable packages.

Tables must have plexiglass shields and interiors of food-vending areas, including food stored in glass cases, must be shielded to avoid contamination. Food court staff should be advised to wear masks and shield themselves with fans.

Toilets need to have additional ventilation and air circulation and crowding must be avoided. Exhaust fans to flush out air must be used and opening windows must be encouraged. The toilets need to be sanitised every couple of hours, with the provision of soap and hand sanitiser must.

The use of elevators should be restricted and strict capacity limits must be imposed. Crowding on escalators and stairs should be dissuaded as well. Passengers should be discouraged from spending too much time inside stations to avoid exposure.

Trains and stations must have one-way entrances and exits to avoid passengers running into each other. Special coaches must be reserved for senior citizens, the differently-abled, pregnant women and parents with infants to help them avoid crowds.

Enforcement marshals must be deployed in trains and stations to oversee and maintain compliance.  Employees must be periodically shuffled and be advised to not be stationary at one place for a long time. They must be tested randomly to ensure their safety.

Given the potential of the metro system to transmit aerosols, it is imperative to follow the triple A system, improve ventilation, air circulation, follow safety measures like hand hygiene, wearing masks and social-distancing to keep SSEs from happening.

Views expressed are the authors’ own and don’t necessarily reflect those of  Down To Earth.

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