[nudephil (OP)]

Tim asks:

Are air conditioners in shops and offices a danger for spreading the coronavirus?

[alancalverd]

No. Though it depends on the system.

The worst an aircon can do is to recirculate what is already in the air, but as it sucks air away from the occupied level then  recycles it through a filter, even if the filter is missing it has increased the mean path length of any infected droplets and thus reduced the concentration to which you might be exposed by direct exhalation.

A full aircon system usually involves cooling and drying the air, so any "active" moisture is likely to be condensed and removed through the drain.   

[evan_au]

Are air conditioners in shops and offices a danger for spreading the coronavirus?

I would say "yes".
- There was a case in a restaurant, where the airflow past an infected person resulted in other infections "downwind".
- There have been cases in hospitals, where tracing the airflow showed that air from the room occupied by infected patients flowed into the corridor outside, near the nurse's station
- If there is no systematic airflow, the cloud of virus particles diffuses radially in all directions, at a slow rate; social distancing means that you will be exposed to a lower concentration of virus. However, if there is steady airflow, the virus particles will spread out more quickly in that direction, at a steady rate; this will quickly exceed social distancing limits.

An important parameter is how many times per hour the air is changed for "outside" air (which presumably will have a lower concentration of SARS-COV2 particles). You could increase this rate by opening windows - only many modern offices have windows that cannot be opened. You must rely on the airconditioning system to exchange the air for you.

The best situation is if there have been no infected people in the building - you cannot spread something that is not present.
- A high-quality HEPA filter will greatly reduce the number of virus particles released from the airconditioning system - but your average office worker is not aware of the design of their airconditioning system, and has little visibility of the airflow through the office
- The next best situation is if everyone is wearing a mask, so an infected person will spread fewer virus particles into the air around them, and an uninfected person will breathe in fewer virus particles.
- So while there are significant amounts of virus circulating (and the vaccine rollout has not reached you yet), it is wise to work from home, if you can...

[alancalverd]

OK, I'll qualify my reply by saying it all depends on intentional design.

Having worked in positive-pressure operating theaters and negative-pressure radionuclide and biohazard facilities, I'm always interested in what goes where when designing new clinics, but it seems that not everyone takes the same gloomy attitude to HVAC!   

[Bored chemist]

If there is no systematic airflow, the cloud of virus particles diffuses radially in all directions

But that's essentially impossible.
Things like heaters (and lights), not to mention people and air vents will move the air around.
If you are at all lucky, the net effect of the air con will be to remove and dilute the virus.
If you are very unlucky, the aircon blows Mr Sneezy's exhaled virus at you. But, in doing so, it saves the diners at the other end of the room.

I'm intrigued by the implication that people in restaurants should eat with their masks on.