a word about ventilation

This is not going to be in depth, because this is not my field at all, but I've seen some online consternation from the end of last week at the report that students from Silver Lake Regional rode home on a bus with the windows open in the snow.

Yes, they had their windows open, and yes, there are districts planning on having bus windows open, and yes, we knew that.
Here's why:

Air exchange can be a key piece in cutting down on virus transmission. This excellent piece from El Pais from last week does a nice job of running through the variables--space, mask wearing, loud speaking, and others--in transmission of the virus. One of those is how much air is being exchanged: how much fresh air is coming into a room, and how much stale air is leaving the room. There are ways of measuring air turnover and so forth, but the key point is that it's both fresh air coming in and the old air from the room being pushed out.

Now as the El Pais piece makes clear, this is about combinations of things that make the chances of virus transmission higher or lower. So you're trying to add things--masks, spacing, air exchange, in Worcester's case ionization--that lower chances of transmission.

When we talk about schools--and if you watched Worcester's School Committee this summer, you caught this--we talk about three kinds of systems:

• Full ventilation systems (the AC in this particular case isn't the question), which both bring IN fresh air from outside AND ALSO push the old air out. Now, if you're someone who grew up in places that need indoor heating in particular seasons, you know why schools wouldn't necessarily have this: you're continually needing to heat that outside air! They do, though, in some cases, have settings that allow for particular mixtures of air so you're not having to continually heat entirely new air. However, they can be set for full exchange if that is what is wanted.
• Partial ventilation, which brings IN fresh air from outside BUT doesn't have a mechanical means of getting rid of the air inside. If you're an adult of about my age from the northern piece of the U.S., there's a decent chance that your memory of a school heating capacity is from the blowers along the walls. That doesn't do anything about the air that's already inside getting pushed out.
• No HVAC: many schools have just systems that just heat the air that is in the building; ventilation (the V of HVAC and thus why these buildings without that or AC don't have HVAC) isn't part of that. This is things like steam radiators. 

So if you're in the situation of either of those last two, which is many school buildings and yes, also buses, what are you going to do? You're going to open windows both to bring in outside air and to expel indoor air. Thus the fans you've seen in school windows (frequently pushing air out, though sometimes they're paired to bring air in, too), and thus also why buildings were built with things like the transom windows opening onto the hallway, which allowed for cross-ventilation in buildings when they were built, if the exterior windows also are open.

An important note if you're in Worcester (or other places doing or discussing this), incidentally, is that the ionization systems are yet another layer to all of this, which are actually cleaning the air without air exchange, and so are a whole different thing. 

This is how we end up with apartment buildings from the last pandemic that are designed to be overheated, so the windows will be open in the winter. From an energy efficiency standpoint, of course, this is...not.
On a school bus, though, there aren't a lot of options on air exchange.