Luzius MeisserMy hunch is that we do not only pay too little attention to the effect on plants, but also to the effect of elevated CO2 levels on humans. High CO2 levelsSee more above 2000 ppm are known to cause headaches and concentration problems. Yet, many ecological "Minergie" heating systems prioritize saving energy and keep the air just fresh enough to keep people from complaining. I for one would feel more comfortable if fresh air was the first priority. At schools in particular, I think that would enable students to think faster and more clearly as the can breathe better air.