Most of those places are already very well ventilated. You’re not catching covid in the train station, you’re catching it while sharing 1m² with 4 other people

Most of those places are already very well ventilated. You’re not catching covid in the train station, you’re catching it while sharing 1m² with 4 other people on the train. Or because you’re in a crowded restaurant and you’re between the air extract point and the person on the table next to you who has covid. I can’t see how it would be different if the air circulation is because of open windows or mechanical ventilation in these situations. Most high rise offices since the 50/60’s will have a mechanical ventilation system of some kind already but it may be quite energy inefficient. Any building with an existing air con system is already doing much more air changes than a building without ducted air. Offices need these systems for active cooling and heat rejection for fairly large parts of the year because the concentration of people and computers inside tends to overheat the building. Hotels are an interesting one, I’m not an expert on hotel design but it’s possible that some hotels use positive pressure ventilation to supply air to the rooms and passive extraction through the en-suite bathrooms. In that case when you open the room door you let a blob of Covid air out into the corridor? But the corridor should probably also be at positive pressure to stop smoke from a fire in a room from blocking the escape route. Not sure about this one. I would hypothesise that people mostly catch covid in Pubs, Clubs, Restaurants, Schools, each other’s houses and other specific hard to solve confined situations where we can’t avoid breathing each other’s exhaled air like lift cars and underground trains. If money were to be spent retrofitting heat recovery ventilation anywhere. Schools would be the best place to start.