There is a further element to consider. I have been looking at air-bourne transmission, in regard to ventilation systems within my own discipline. I am a Theatre Consultant and NOT an M&E Consultant, butby the nature of my job, i have to learn something of the other disciplines.
There in contradictory advice. From our own Government, the basic advice is "open windows, and keep working - service the HVAC as normal", however, a study in Europe confirms that the virus has already been transmitted to others with no direct contact, via the air-stream of a restaurant's ventilation system.
A problem occurs when air is fed at one side of the room, and is extracted at the other side, so the air is drawn through the space. Ideal normally - but not if an infected person or persons is at the up-stream end of the flow. This is also a problem for many theatre's, where we supply around 7 litres of air per person per second - vast quantities- and this is frequently fed from over-head diffusers, and also extracted at high level - thus creating all sorts of streams in various directions. I suspect that many large exhibition spaces may have similar systems.
The few Professional Risk Assessments I've seen intended to cope with the virus and working with it do not understand or address air-handling or it's interelation with the design of the building, and have limited themselves to "opening windows" which in a modern building is not necessarily adequate. Air-handling should be run at 100% Primary air, with no recycling, and professional opinion should be sought as to whether the filters can be upgraded. Do make sure Risk Assessments are comprehensive, and don't be afraid to question them!