Cross-sensitivities: what confuses each sensor

No physical sensor reads only what its label says. Every instrument has cross-sensitivities, responses to inputs other than the target, and a useful dashboard knows about them and corrects for them. Here are the practical ones for Terrestream.

The SEN66's VOC index is moderately humidity-sensitive: at very high RH (above 80%), the index rises mildly even with no actual VOC change. The algorithm partially compensates, but on a sudden humidity step (say, a hot shower) the VOC reading will tick up. The dashboard recognizes the humidity-step + VOC-step coincidence and labels it accordingly. Just as important is what the VOC index is not: it is non-specific and cannot isolate individual compounds. Terrestream does not measure, report, or detect formaldehyde or benzene, and must not be used to detect either (see formaldehyde and benzene).

The NO_x index responds to any oxidizing gas, primarily nitrogen oxides as intended, but also ozone (rarely high indoors but possible) and some halogenated compounds. Outdoor ozone infiltrating during a high-O₃ afternoon can produce a small NO_x-index rise that has nothing to do with combustion sources.

The PM optical sensor measures scattering. Anything that scatters infrared light at the wavelengths used will register: water droplets (fog, steam), some cooking aerosols (oil mist) above true PM. A steamy bathroom or boiling-pasta kitchen produces inflated PM readings that decay quickly as the droplets evaporate. Fonollosa et al. review broader MOX drift and cross-sensitivity issues.

How the dashboard handles this: a small set of correlation tests run continuously and any apparent spike is checked against likely cross-sensitive companion signals before being labeled with a confidence score. A "VOC notification" with simultaneous humidity step is labeled lower-confidence than a VOC notification with flat humidity. The chyron surfaces the uncertainty when relevant.