The Science of COVID-19: Coronavirus: Avoiding these 4Ds will help you avoid COVID-19

Dr. Zulfikar Bux
Assistant Professor of Emergency Medicine, Vanderbilt Medical Center

Data is now emerging to help us zone in on hotspots for COVID-19 transmission, so that we may be able to manoeuvre around them and sustain our livelihoods. The most convincing data thus far are from two studies which showed high transmission rates of the virus indoors and very limited transmission in the outdoor setting, where free-flowing air dispersed virus particles quickly. The first study was carried out in Denmark, where 3030 participants were studied; while the other study was done among 1848 U.S Marine recruits in South Carolina. Experts concluded from these studies that COVID-19 transmission was predominantly indoors, and even occurred in those that were wearing standard surgical/cloth masks. They found that the “4Ds” were the main risk factors that predicted COVID-19 transmission in the indoor setting.
Today, I will expand on the 4Ds, and why you should avoid such settings if you want to avoid COVID-19.

Density
Density, meaning the number of people in a room, was a major factor in predicting COVID-19 spread. The basic idea was that the more persons you have in a closed space, the likelier COVID-19 would spread if one or more person had the infection. Viral particles would be expelled from the infected person’s mouth and nostrils, and will “hang around” in the air longer, because of poor air circulation in a closed space. With everyone breathing, the odds become higher of breathing in air with viral particles, given the limited air-to-person ratio.

Duration
The longer one spends in a closed space with someone that is infected, the likelier would one become infected even if one were wearing a standard surgical/cloth mask. An infected person in a closed space would expel more viral particles into the air every time they breathe, sneeze, cough or speak. It is obvious that the longer someone spends in such an environment, the higher the chance of them being exposed to expelled viral particles becomes, because they would be exposed to a higher concentration with time. Also, think about being in a room where the virus is moving around; the longer you spend in that room, the likelier become the chances of you coming into contact with it as it moves around.

Dimensions
The smaller the dimensions of a closed space, the higher the chance of getting infected. As an infected person expels virus particles into the air, the particles will disperse faster once there is sufficient space for them to move into. However, in smaller spaces, the virus particles become stagnated, and would circulate in that space for some amount of time. It is therefore obvious that one’s chances of coming into contact with viral particles would be higher when one is in a smaller, closed space.

Draft
In closed spaces, where there is limited entry of fresh air and poor airflow speed, chances of COVID-19 transmission become higher. When viral particles get expelled into the air, they would remain floating for a while if the air is stagnant. However, if there is free flow of fresh air, or if the air is mechanically propelled by fans, the viral particles would be quickly dispersed, and not float around and infect persons that come into contact with them.
Offices, restaurants, homes, bars, gyms, cinemas etc. are examples of closed spaces where the risk of transmission would be higher. Therefore, it is obvious that we should minimise our presence with others in these and other indoor spaces as much as possible. If we have no choice, then ensure that these spaces are large, with minimal occupation, and have free-flowing air; and we should minimise the time we spend in them, when possible.
While infections still occur with standard mask use in these spaces, the occurrences are far less, and masks would help minimise severe infection risk by decreasing the amount of virus that gets into you. So, we should continue to practise mask wearing, but change our lifestyle to ensure we avoid the 4Ds as much as we can, and carry out our exposures with others in the outdoor setting as much as possible.

Article submitted as part of the Ministry of Health’s COVID-19 public information and education programme. For questions, email [email protected]