[David Cooper (OP)]

Picture something like a face mask, but it has a simple opening with a valve to let air in (with the option of putting a filter in it for people whose health is fragile), but there would be another tube leading out and down, all the way to near the ground, clipped onto clothing to stop it swinging about. When you breathe out, all the virus (if you're infected) goes down the tube and comes out low down, over one and a half metres downhill from where most people are breathing in.

The down tube could again optionally have a filter in it, and unlike a normal face mask, it wouldn't matter so much if it goes manky as you don't have to breathe in through it as well. Filtering the output may not matter though because the risk is so much lower with it coming out at ground level, added to which, a lot of the droplets will stick to the sides and fall out as liquid drops onto the floor (which can be cleaned regularly if it's in a shop or bus).

If this leads to sufficiently low transmission, people could get back onto well-ventilated public transport and move freely about shops, offices and other work places without pushing the transmission figure back above 1, thereby allowing the virus to die out gradually by not being passed on but without any further need for a severe lockdown. What matters is to keep the transmission low enough to avoid overwhelming the capacity of hospitals to cope, but also to protect the most vulnerable people who would be able to use filters for the air breathed in without having to replace them repeatedly due to them being destroyed by damp. This device would be reusable thousands of times, a simple wash with soapy water making it good as new for the next day.

(One design detail to experiment with is the bottom of the tube - it could bend back up in order not to drip on the floor. Another option would be to have a complete transparent-plastic head-surrounding container to avoid the need to have anything uncomfortable clamped round your breathing holes all day.)

[Bored chemist]

Another option would be to have a complete transparent-plastic head-surrounding container to avoid the need to have anything uncomfortable clamped round your breathing holes all day.

You need to think about "dead volume" here.

[David Cooper (OP)]

Another option would be to have a complete transparent-plastic head-surrounding container to avoid the need to have anything uncomfortable clamped round your breathing holes all day.

You need to think about "dead volume" here.

Just put the inlet high up at the back and the outlet low down at the front. The valves will do the rest. By not using filters, air will easily find the right ways in and out and breathing remains easy and comfortable.

A further thought:-

Public transport could be retrofitted with pumps to suck air out from floor level before putting it out through an industrial filter. This should be built into all public transport systems as standard, not only for this virus but for any future ones like it.

[vhfpmr]

Masks don't need to be 100% effective, or be worn by 100% of the population: