I WISH I could agree with Joanna Blythman that the two-metre rule is "scientifically baseless ("Scientifically baseless two-metre rule would kill our restaurants. But there is another way", Herald Magazine, May 23), but sadly I cannot. It is agreed that transmission between people occurs when virus-containing moisture droplets, shed by an infected person, reach the breathing zone of another. Live experiments, where such droplets were illuminated by laser light, showed that coughing, sneezing and even loud conversation can generate such droplets. The larger droplets tend to fall within one to two metres. The smaller droplets or virus particles continue floating in mid-air and may follow any air currents into other breathing zones. So there is an observable scientific basis for a distance factor in the transmission mechanism.

Outdoors, there is usually some air movement to disperse the floating particles and an infinite supply of fresh air to dilute any contaminated volumes. So the risk of transmission is relatively small.

Indoors, the risk depends on the particular ventilating system involved. Some air may be re-circulated, with the balance being fresh air from outside. The re-circulated air passes through filters, but these are not normally capable of removing such small particles, unless for special systems used in hospitals and clean rooms. A comment made to me by an eminently qualified environmental engineer is that "most HVAC systems are not fit-for-purpose when it comes to infection control".

Guidance notes, issued by a European HVAC technical federation, state that "the safety distance of two metres between people regarding the risk of Covid-19 is a myth". It is legislating for indoor situations and emphasising that the distance between people is largely beside the point. The more important question is – what does a particular ventilation system do, as regards virus-loaded droplets and how do you set the system to reduce the risk of virus particles staying in the breathing zone? It may be that the best thing you can do is switch the system off and open windows to produce a through-flow of fresh air (perhaps not conducive to the kind of dining-out experience we all treasure). There is a restaurant case study where one diner infected nine others, many of whom were much further than two metres away from the infected person. There was plenty of air movement from wall-mounted air conditioners, but very little fresh air ventilation.

So Ms Blythman is correct if she is implying that two-metre spacing, regardless of the context, is not the issue, but the information on HVAC systems suggests that a great deal of work needs to be done urgently on indoor environments. This applies to a large number of locations – schools, shops, offices, theatres and so on. Where is the guidance from governments?

Thomas G F Gray, Lenzie.

MAY I respond to Ian McNair's request (Letters, May 26) for an adult debate about appropriate social distancing?

It is becoming increasingly clear that there is no such thing as " The Science". Scientists, highly qualified, have differing opinions, and different governments take different advice.

I would choose neither Boris Johnson nor Angela Merkel's science, and would adopt the one-metre rule recommended by the World Health Organisation. The WHO rule would enable pubs, restaurants, clubs and coffee shops, unable to comply with the two metre rule, to open, thereby helping what little remains of our economy.

David Miller, Milngavie.