You don't have (the option for) ventilation shafts in your window frames[0]? Here in the Netherlands (and north-west EU) many homes have those, they're designed to allow fresh air to circulate without the need for opening a window. Most houses have those except for really old ones (which have 'natural' ventilation, i.e. creeks) or really new ones (which have a central ventilation system).
[0] for example https://www.duco.eu/Wes/CDN/1/Products/ProductImages/2016-08...