SIR – In reading MacDonald’s letter in your paper (dated November 21st), I was led to wonder if he had actually understood either of my previous letters, and I resent his insinuation that I am a liar.
First, a little calculation. The current positive test rate for Covid-19 in Ireland is currently averaging about 500 a day (an overestimate, I know). If you accept my earlier assumption of about 50% detection, this means there are about 1,000 new cases a day in the country.
Over the 14-day period widely quoted, this suggests that there are about 14,000 people in the country currently infectious with Covid-19 – about 0.3% of the population – three in a thousand.
The thrust of MacDonald’s argument is that the wearing of a face mask reduces the risk of onward transmission from this 0.3%. Indeed, a number of studies have been carried out on droplet transmission outward through a face mask when correctly worn, showing a range of effectiveness depending mainly on the mask construction, and all show some effect on outward transmission, even the humble scarf.
However, throughout my letters I have been trying to show the potential for a face mask worn past its saturation time to increase the risk of infection to the wearer, and the paper to which I referred dealt with this matter.
I originally estimated about 15 minutes based on my own experience – this may be too low, but the principle still applies.
In other words I have been trying to bring home to people – the 99.7% currently not infected – some of the potential dangers of using face masks incorrectly and overlong (thereby potentially increasing their own risk of infection) and regarding them as a ‘magic carpet’ solution. I admit to myself using a face shield rather than a face mask – apart from its other advantages – it is a 100% barrier against spray transmission of the virus both ways, although diffusion transmission can still occur.
I do think that anyone who wishes to criticise a position – whether they are qualified in critical reasoning or not – should first take the trouble to read and understand what is being said.
John Lucas, MA (Oxon),
Gurteen, Ardgroom, Beara.