As with most Total VOC sensors on the market, including the one Airthings has selected, they detect a large number of different gases which all contribute to the total amount of VOCs.
The Airthings TVOC sensor measures Total VOCs, not individual VOCs.
This provides a combination, a summary of all of the VOCs that are in your air in one easy to read statistic. It is the total amount of all VOCs in the air. Therefore, a high reading does not mean there is a dangerously high level of one particular VOC. Instead, the device is calibrated to ensure certain gas mixtures that are common in a home (cooking, human presence, cleaning etc) are accounted for.
The sensor will show a ppb (parts per billion) for the Total VOC mix which the device is factory calibrated for.