As noted in an earlier thread, there is good evidence that higher humidity reduces the spread of airborne viruses, and the spread of COVID-19 appear to be reduced in tropical countries.
The air in commercial aircraft is notoriously dry even in humid weather. The problem is air outside the plane is about -40 F at flight altitude, so the cabin air is desert-dry by the it gets heated to room temperature.
Would it make sense to add portable humidifiers to commercial flights, at least during the COVID-19 outbreak?
Low humidity also exists in airports, train stations, trains, buses, subways, etc. during winter weather. Would it make sense to add portable humidifiers to those locations as well?
From what I see the costs and risks of adding humidifiers is trivial compared to the likely benefit.
Here is a link to the earlier thread that discusses the benefits of higher humidity: