HCHO: Here's an article that may help to explain the issue:http://www.ukcpi.org/features/formaldehydeTVOC: According to WHO, VOC contains Formaldehyde, d-Limonene, toluene, acetone, ethanol (ethyl alcohol) 2-propanol (isopropyl alcohol), hexanal. Orange peel contains limonene (limonene), citric aldehyde, as well as tangerine ketone, hesperidin, inositol, vitamin B1 and so on. Among them, alcohols, aldehydes, and ketones all react electrochemically with the sensor diaphragm, leading to increased readings.