De Almeida (1995) points out that:
Chemical gas sensors can be supplied as mixed gas sensors and specific sensors. Mixed gas sensors are sensitive to a combination of volatile organic compounds (VOCs). In contact with a hot semiconductor metal oxide (e.g. tin oxide) VOCs are oxidized, decreasing the resistance of the sensor. VOC sensors are inexpensive (typically in the range $30-$50) but their response is of qualitative-type and is not specific to a particular chemical, and their sensitivity is not adequate for all applications. Additionally, the sampling chamber needs to be flushed periodically with clean air in order to restore the zero.
