It's an impossible situation to manage unfortunately reducing risk in all situations to zero is not feasible so we have to mitigate risk to an acceptable and workable level and avoiding making the exceptions reasons to invalidate the rule - maybe using driving as an example some lunatics will drive at 60 in a 30 limit and cause mayhem it dosent mean we should ban driving for everyone- if masks are indeed a positive action then better to use them whenever possible than not at all - seems though the UK needs to be told what to do rather than be guided on best practice and allowed to make their own judgement