The selective measurement of formaldehyde and benzene is not readily achieved without recourse to sample gas pre-conditioning. That usually means pneumatics which enable the target gas to be separated, either chromatographically (‘micro-GC’) or chemically (‘benzene tubes’, ‘chem tubes’). In such devices the sensor is only part of the cost, and supporting pneumatics, and in the case of tubes, manual handling and disposal, raise the cost and limit the deployment of selective devices. There is now a large market for sensors of fairly low power demand (10’s mA), at a price point of ~$100’s or less, which measure many, if not most of the VOCs that impact on air quality to provide a measure of the total VOC concentration (TVOC). This lack of selectivity would at first glance seem to fatally compromise such a sensor. After all, many VOCs such as as ethanol cause little concern at concentrations many thousands of times the concentration at which benzene or formaldehyde, say, are harmful. However, generic VOC measurements are attractive and effective because:

• unpolluted air contains very low TVOC levels, enabling measurement of any polluting VOC at low concentrations.
• in polluted air, the source and identity of VOCs can often be anticipated e.g., hydrocarbons in a service station.
• VOCs often ‘co-present’: a TVOC signal may act as a marker for the presence of a particular hazardous VOC, eg benzene co-presented with other VOCs, or even the detection of a generic condition, eg a kitchen in need of ventilation.
• the ability to sense many VOCs with one sensor provides a commercial economy of scale