Dr. John Lee wrote:To think about this in relation to face masks, you first need to have a handle on just how small viruses are. A human hair is about a tenth of a millimetre thick. A typical bacterium (such as the human pathogens
E. coli or
S. aureus) comes in at about one thousandth of a millimetre, so you could line a hundred up across the width of a hair. A coronavirus particle is about 10 times smaller still, so a thousand would fit across a hair. This extreme smallness was instrumental in the discovery of viruses in the nineteenth century: they were the infective agents, left in solutions that had been passed through ‘ultrafilters’ which had removed all other known pathogens.
So to filter out viruses effectively you need a filter with a very fine mesh indeed, even assuming that all the air goes through the filter. And you need to think about which way the air is going: breathing in (the idea being to protect you) or breathing out (ostensibly to protect others if you have the virus).
A
recent study looked microscopically at pore sizes in low-cost facemasks of the types common in developing countries such as Nepal, made from various cloth materials. Here’s a sample they showed in the study.
They found pore sizes of roughly one to five human hair widths – the pore sizes got slightly bigger after washing. So in relation to viruses, these masks are doing little, except possibly reassuring their wearers.
The surgical-type facemasks, more likely to be used in developed countries, are a bit better. They have pores typically three times larger than the virus particles, rather than the one to five thousand times larger for the cloth masks. Better, but still not good enough to filter out viruses. A laboratory study by the
Health and Safety Executive looking at influenza virus, which is a similar size to coronavirus, found live virus in the air behind all surgical masks tested. They tested masks on a human volunteer using an ‘inert aerosol challenge’ (a simulated sneeze), and on a breathing dummy head using a live virus aerosol challenge. The numbers of particles were reduced by a factor of two for the human volunteer, and six for the dummy head; probably not very effective in reducing infection when infective aerosols – the droplets emitted from someone’s cough or sneeze, or even during talking – may contain hundreds of thousands, or millions, of tiny particles.
But the thing to understand about this science is that those breathing-in factors represent the very best that could possibly be achieved. The masks were adjusted ‘in order to obtain the best fit possible’ and ‘the test subject was asked to remain still during the test’. Obviously, the dummy head was completely still – in real life this just doesn’t apply. Masks don’t fit snugly, people move all the time, the mask material gets damp and air gets around the side. What if you sneeze, cough, burp, sigh, yawn, or readjust the mask? What if you touch your face to ease an itch caused by the mask? More air gets round the side. The reduction factors in actual use, over a day rather than half a dozen measurements, are likely to be much nearer 1 – no protection at all – than those measured in the lab.
What does this mean in terms of protecting yourself when you breathe in? It seems very much along the lines of wear one if you want to, but don’t expect it to stop you getting the virus from aerosols – which are likely to have an important role in viral transmission, especially in crowded or busy environments. In other tests, masks are better at filtering out large droplets. But of course the real-life caveats of moving and breathing around the side of the mask also apply, and large droplets may settle on your skin, hair, and clothes, as well as the outside of the mask, and may get into the air when you take off your mask and coat at home, rub your face and put your hand through your hair after a hard day’s work. The mask may protect you a bit, but it may well not.
What about protecting others? Surgeons wear masks mainly to protect their patients from particulate and potentially infective matter falling out of their noses, mouths, moustaches and beards into a patient’s open wound. We’re talking about big particles here (human hair width) and bacterial infections, not viruses. Even for this situation, which is universally observed, it is surprisingly difficult to generate watertight scientific data about the
effectiveness of surgical masks in preventing bacterial infections. For viral infections there is little data, but again, the pores allow viruses through and much of the air you breathe out goes around the side. When a person is infectious with a virus it is estimated that they may shed one hundred billion virus particles a day – that works out at about ten million per breath. A mask won’t stop you putting these particles into the air around you. In fact, with a damp mask you’ll be blowing aerosols and larger particles sideways, directly at your socially distanced colleagues two metres away. And if wearing a mask tempts you to feel that you’re not going to infect anyone else, you may also be less likely to observe the two-metre rule. So does wearing a mask protect others if you’re infectious? There’s little direct evidence to say that it does, and quite a lot of straightforward reasoning to suggest it doesn’t.