100%, a $70 CO2 sensor made a huge difference for me. I live in Germany, and there is no ventilation unless you open the window. I noticed that when I worked al

100%, a $70 CO2 sensor made a huge difference for me. I live in Germany, and there is no ventilation unless you open the window. I noticed that when I worked all day in the apartment, at the end of the day my brain would be toast, I wouldn't be capable of any really important thought, and I'd have a splitting headache and feel exhausted. Now I just open the window whenever the meter turns yellow (which actually means I just leave the window open almost all the time). Here's the one I use: https://www.amazon.de/-/en/gp/product/B00TH3OW4Q https://www.amazon.de/-/en/gp/product/B00TH3OW4Q