Lowtech Graph lists low-tech options for every need of a home. Here you can compare them, find tutorials to build them, and link them together into a circular system. Why low-tech?
Getting rid of the carbon dioxide people breathe out, which builds up in a closed room and makes it stuffy and hard to concentrate in. In a house this is done by ventilation rather than by anything dedicated, which is why nothing is catalogued here: the answers sit under passive and mechanical ventilation.