Metric scales: coronavirus and covermask

Since March 2020, the world has faced a situation unbelieveable for these times of technological advances.

The situation is a pandemic caused by the SARS-CoV-2 virus or type 2 severe acute respiratory syndrome coronavirus. In the news, it is referred as Covid-19 that means coronavirus of 2019. Yes, the virus appeared in the city of Wuhan, China at the end of 2019 and spreaded around the world since the spring of 2020.

The use of covermasks has been popularized as a shield to minimize the risk of infection by this virus.

But, why not all of the covermasks are effective and what is the reason of the effectiveness for the god ones? Well, we can use the metric scales of the Decimal System of Units for explaining this situation.

The coronavirus is around 100-160 nm (nanometers) in size. The prefix ‘nano’ (the n) means 10^-9 or you have to multiply the given quantities by 10^-9 to find the quantity in meters. Thus, the coronavirus has a size of 0.000000100 m to 0.000000160 m, that is, about 100 to 160 billionths of a meter.

But keep in mind the minimum value and convert it to a smaller unit, say millimeters, which is one thousandth of a meter. To write 1 m in millimeters, you have to multiply it by 1000 (or 10³):

1 m = 1000 mm

As the prefix ‘milli’ means 10^-3, to convert a quantity from milimeters to meters, you have to divide by 1000 (or multiply by 10^-3).

Coming back to the coronavirus, its minimum size is 0.000100 mm, that is, it is 100 millionths of a millimeter. It is still difficult to imagine yet.

Let us use something smaller and familiar to compare the size: a human hair.

The average human hair is 60 microns thick, if it is very thin, it could be about 10 microns. The micron or micrometer (mm) is one millionth of a meter or one thousandth of a millimeter. To convert a quantity from millimeters to micrometers (microns), you have to multiply by 1000.

1mm = 1000mm

The minimum size of the coronavirus in micrometers (or microns) is 0.100 mm, that is, 100 thousandths of a micron, or if we simplify it, 1 tenth of a micron. Since a super-thin human hair is of the order of 10 microns, the size of the coronavirus would be 1-hundredth of that super-thin hair. If we scale the width of the thin hair as a meter, the size of the coronavirus would be 1 centimeter.

Now, let us see what happen with the covermasks.

Cotton fabrics have fibers with of 15 to 25-mm width, so if we have a fabric, no matter how tight it is, will allow particles smaller than 15 mm to pass through, among them, the coronavirus that is practically a hundred times smaller of that size. Actually, covermasks made of cloth allow particles of 80 mm or less (the coronavirus, among them) to pass through.

On the other hand, many people wear surgical or medical padded masks. This fabric does not allow the passage of 94% of particles smaller than 10 mm. We might think that the pores of the fabric are close to that size. However, the coronavirus is much smaller. Studies around the world says that this virus is transmitted by respiratory droplets of 5 mm size, which allows most masks of this type to be more effective.

The most recommended covermasks against the coronavirus is the one labeled as N95, which is made of synthetic polymers that form a ‘non-woven’ fabric that filters out 95% of particles of more than 0.3-mm size. This measurement is less than the respiratory drop size but still larger than that of the coronavirus itself. The N95 covermasks, however, comply with a United States health regulation and are the ones used by medical doctors around the world.

Of course, there are people who wear nylon masks with pores as small as 0.2mm, twice the size of the coronavirus. Surely, the filtration of this virus is much more effective but the problem with these masks is the difficulty of breathing.

Actually, the problem of transmission of the coronavirus in the air depends not only on its size but also on its speed (caused by breathing or a sneeze) while leaving the mouth of a patient. For ordinary people, following social distancing measures in living with other people, as well as avoiding meetings with people with whom they do not live, is the best way to reduce the risk of contagion.

Regarding the use of the mask, medical doctors advise that it is best to use any mask instead of nothing. However, with the use of metric scales you can imagine the size of the virus and the latticework or pores of the mask, to realize what you are really facing in this epidemics time.