After this next layer of reading, I feel I have moved from asking which sensors might be interesting to having a much clearer idea of what the sensing system actually needs to do.
The strongest pattern across the papers is that one physiological signal on its own is not enough. Heart rate, EDA, HRV and temperature can all change for many reasons, particularly when somebody is moving through a real environment. The more defensible approach is to combine physiological change with movement, context and the person's own baseline, and then use that information cautiously.
So my current thinking is moving towards:
personal baseline + physiological change + movement + airport context + previous responses → cautious interpretation → check whether this is a good moment to prompt → transparent check-in → traveller decides
This is a much clearer system than where I was a few weeks ago.
Based on the research so far, the strongest current sensing combination is:
EDA + PPG / pulse + accelerometry with skin temperature as a secondary contextual measure.
I am currently treating each signal as having a different role rather than expecting them all to tell me the same thing.
EDA
Useful for detecting changes in electrodermal activity and physiological arousal.
Recent comparison work suggests it can remain informative during movement, but it still does not tell me what the change means.
PPG / pulse
Useful for continuous pulse-rate trends.
The EmbracePlus validation study showed that wrist PPG can remain reasonably accurate during motion when accelerometry is used to help deal with artefacts.
Accelerometry
Possibly the most important contextual channel.
It can help identify whether a physiological change happened while the person was walking, rushing or relatively still, and can also help improve the quality of PPG interpretation.
I am now much less convinced that HRV / PRV should be a primary live trigger during movement. It can still be useful, but the driving-context research reinforces how cautious I need to be when interpreting HRV-derived measures outside controlled conditions.
Skin temperature remains interesting, particularly because both wrist and ring devices can measure it, but at the moment I see it as a secondary contextual signal rather than the main trigger.
The Oura study has made personal baseline feel essential rather than optional.
A universal threshold such as: If heart rate rises above X, offer support… does not fit the evidence.
The more defensible question is: Has something changed meaningfully compared with what is normal for this person? Even then, the system still does not know why the change happened. The baseline simply gives the sensing a more personalised reference point.
At this stage, I think the hardware decision has narrowed to two genuinely plausible routes: