I recently purchased a home and spent some time improving ventilation in it this year. This is a home in Minneapolis, so just "opening a window" is not an option, as it would be too cold in the winter and dramatically reduce energy efficiency. Its effectiveness also changes based on factors like the wind, and the temperature difference between inside and out.

The general strategy for doing this right now with existing homes is to insulate your home as well as possible for energy efficiency, then install a continuous ventilation fan. This is essentially a bathroom fan, except that it runs all the time at a constant low speed, helping to circulate the air through and out your house by "pulling" a designed amount of air through the cracks.

I didn't want to punch a new hole in the house just to do this, and already had a bathroom fan installed, so as a hack I just turned it into a whole house ventilation fan with this: https://www.aircycler.com/pages/smartexhaust

Basically you calculate how much CFM you need per hour based on the square footage of your house, and then you set it on the fan control. It still acts like a bathroom fan, except every hour it also runs for a set period of time (in my case, about 12 minutes).

The standard for this is ASHRAE 62.2. Use this to calculate the CFM for code: https://homes.lbl.gov/ventilate-right/step-3-whole-building-...

Then the formula for calculating the fan run time is on this sheet: https://cdn.shopify.com/s/files/1/0221/7316/files/AC_DOC_7_0...

And presto, you have a ventilation system without having to do a lot of work. Do _not_ try this with a crappy, rusty old bathroom fan - clean or replace the motor first, and for extra credit, use an arc fault circuit interrupter on the breaker so if the motor fails it will blow the fuse instead of potentially causing a fire.

Note: This is really just to manage general air quality and VOCs. If you want to specifically make ventilation for COVID-19, that's a different problem. They focus on Air Changes per Hour (ACH), and a cubic feet calculation is used rather than square feet. There's no "recommended amount" of ACH for managing COVID-19. You're likely improving the situation by increasing it, but I wouldn't start inviting people over after you did it. ACH is very high in ICUs but staff are still getting sick there.

RE Humidity - ideal range varies based on region and outside temperature, but this chart roughly shows it: https://lh3.googleusercontent.com/proxy/mPz-jGdLpnPGgvWR3Egf...

I have an on-furnace humidifier controlled by an ecobee for the winter, it's a huge quality of life improvement if you live in cold climates, but make sure to set it to "frost control" otherwise it won't lower the humidity based on the outside air and you can get mold in your walls. For summer, a standard house A/C combined with continuous ventilation should be sufficient to bring down humidity levels.

Finally, the "correct" air ventilation is a moving target with trade-offs and concerns of the moment. It was higher in 1925 (30cfm/person) to try to prevent tuberculosis and infectious diseases, then was lowered to 5cfm/person in the 70s during the energy crisis, and is currently at 15cfm/person. I imagine COVID-19 could make us re-consider the current recommendations. https://homes.lbl.gov/ventilate-right/ashrae-standard-622