I had never heard of ear-based HR monitors, so I googled them, pretty cool actually! Searching pub med, I found the following:
This one https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6413847/ found: “The Jabra earbud had a mean absolute percent error (MAPE) of 3.14 ± 6.13%, and a high correlation with the benchmark, r(c) = 0.939, performing well in all three conditions.”
This one https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0238569 found: “Garmin Fenix 5 (MAPE = 13%, LOA = -32 to 162, rC = 0.32), Jabra Elite Sport (MAPE = 23%, LOA = -464 to 503, rC = 0.38”
And here is a systematic review https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7596679/ that concluded: “Although non-optical methods may have the advantage of no light source and reduced power consumption, the performed studies suggested the feasibility of HR/PR measurements only under basic resting conditions in small groups of subjects. Sensitivity to user’s motion may impair performance under exercise conditions, requiring further optimization of the electronics and sensor placements within the earpiece, and noise reduction in the readout circuitry. On the other hand, methods allowing to acquire an ECG, instead of a pulsatile signal, suggested the possibility to trace cardiac activity beyond HR estimation.”
My take away is: theres a lot of variability but it seems like they’re fairly comparable to wrist-based HR monitors. Which is to say, better than nothing but not super reliable.