For those who never handled the air system of a ship, I tried to simplify the drawing and text into a few words:
Close the air re-circulation valve in your air (conditioning or heating) system to get a healthy environment in the cabins & in the wheelhouse.
The air system on a ship is not the same as at your house. The best way is to compare the air system with a system in a car. Also in cars there is a recirculation button. If it is hot outside, you can either open the windows or use this button when you have an airco in your car.
Another use of this re-circulation button in your car is to keep the smell out when you are behind a truck in a traffic jam.
Using a re-circulation button or valve is just the same as saying: I don’t want the air from outside in my breathing area. I am happy with the air which I have.
Of course you can not do this for a long time as you will suffer from low oxygen. So in a ship you will put this valve partly open. In newer or sophisticated systems this valve will be controlled by a temperature sensor. The sensor controls the valve then automatically by a servo motor.
The drawing is a simplified version of how an air system works (from an airconditioning system). I made a drawing straight from our school books. I just simplified it by showing fresh air and contaminated air by different colors.
This is how all ventilation systems work. The recirculation valve is there to cool the ship also in tropical conditions. Same in your car, in offices, in shopping malls and airplanes.
A recirculation valve is just a valve and it reduces the cooling capacity which the system needs to cool down the cabins and the wheelhouse of your ship. Or if you are sailing in colder conditions, the valve helps to keep your ship warm.
As you can see, there is a contamination problem. If there is a flue virus on your ship. Lot’s of you will get the same flue.
I assume that the spread of all virus have more or less the same characteristics. They travel through air and if there is no flow in the air, it will fall to the ground. Harmless.
With an air system in place, like all ships, offices and transport now a days, the airflow is not as in a laboratorium. It flows rapidly. If you ever stand next to the ventilators, you will have problems with your ears. The noise of those fans more or less indicates the power of that airflow.
It makes perfectly sense, at least to me, closing this recirculation valve on ships in offices, in shopping malls, in theaters will help to keep you safe.