Energy recovery ventilator. It will exchange indoor air with outdoor air while maintaining indoor heat and humidity. Unfortunately small window mounted models are hard to come by. Most are whole-home systems.
This is the way. As others mention opening window is your cheapest option, but you lose a lot of your heating and cooling efficiency. We installed a whole house ERV for about $10K and saw a significant drop in CO2 readings.
Using a proxy through the `HTTP_PROXY` or `HTTPS_PROXY` environment variables has its own issues. It relies on the application respecting those variables—if it doesn’t, the connection will simply fail. Sure, in this case since all other network connection requests are dropped you are somewhat protected but then an application that doesn't respect them will just not work
You can also have some fun with `DYLD_INSERT_LIBRARIES`, but that often requires creating shims to make it work with codesigned binaries