THE instruction to wear masks has brought into focus the massive and complicated topic of filtration. Your average member of the public/journalist has no clue.
We filter liquids and gases to remove many undesirable solids, also other liquids and gases.
The simplest filters are sieves. Sieves have holes, particles smaller than the hole pass through, bigger ones can’t. The problem is that the sieve soon blocks, meaning it has to be thrown away or washed. Example, sand bed filters for water. The water flow has to be periodically reversed to wash the dirt out of the filter. Also garden riddle and tea strainer.
Then we have filtration in depth – the filter has a mesh of ever-decreasing spaces. Example, the oil filter in your car.
Some filters work using momentum. The fluid is forced around tight corners and the solids lodge in these corners. Example, the capture of fly ash from the combustion gases from coal fired boilers.
There are filters that use an electrostatic field to cause solids to adhere to the filter medium, for example power station combustion gas scrubbers.
There are also filters to remove unwanted gases. These have a chemical that reacts with the unwanted gas, e.g. poison gas masks and carbon dioxide filters used in submarines.
Finally, face masks. Viruses are too small to be filtered. In days gone by scientists spent a lot of time peering into microscopes looking for them. They are so small they can be ‘seen’ only by use of an electron microscope. They were referred to as ‘unfilterable’ bacilli.
So how does the face mask work? The holes in the fabric are far too big to trap a virus mechanically. The face mask is damp from your breath, and the minute particles of mucus and saliva containing the virus stick to the fabric. Think of a forest of fly papers. Its other feature is that it works both ways, i.e. the air passing through can reverse. However, when you take the mask off, it dries out and the viruses can be released which is why ideally it should be destroyed/disposed of immediately.
Face masks fail because they can’t be sealed to your face, i.e. there’s an air leak all round the edge. Beards make it even worse. A subsidiary benefit is that they stop you touching your nose/mouth, the main method of transmission of the disease.
Masks with an ‘exhale valve’ are useless as a biological protection: they are intended to protect against airborne dust.
The life of viruses outside their hosts is very variable. Some like it hot, some like it cold, Covid-19 apparently likes it hot and cold. (Bad news). Some can’t survive desiccation. More bad news: our coronavirus apparently can for 72 hours.