If you place the sensor right next to the table or right next to (or above) the bed, etc., breathing can adversely affect the sensor’s readings and cause it to report higher readings than it actually is. This is because a person is breathing out (among other pollutants) roughly 100 times the concentration of carbon dioxide that is in the outdoor air! The outdoor concentration is usually below 450 ppm, and then a person exhales air that contains 40,000-50,000 ppm!
The best location is 1 – 1.5 meters above the ground, about the height of the switches. If we take carbon dioxide itself, although it is heavier than air, it usually mixes well in a normal space due to human movement and natural infiltration or ventilation, so the location of the sensor in terms of height above the ground is not that important. In a really tight bedroom it is a good idea to place it at head height.
The air circulates poorly in the corner of the room and the sensor might not detect elevated levels of air pollution. Similarly, behind furniture, where the sensor may also get a lot of dust – increasing the risk of clogging the filter.
In the case of windows (whether open or closed) and doors – especially front doors – the sensor can be affected by outside air due to possible leaks. This can falsely reduce the readings and the room could then be under-ventilated based on the biased information from the sensor.
Do not place the sensor in areas where the humidity or temperature changes (very) rapidly. The sensor would not benefit from direct contact with liquids – so place the sensor where water or chemicals cannot splash on it.