That seems counter-intuitive to me that the lower humidity causes greater problems I suppose that with high humidity, sweating becomes useless, so the high temperature combined with low humidity highlights the limits of our ability to sweat.

The subtitle is more informative: Scientists testing deadly heat limits on humans show thresholds may be much lower than first thought

The use of the wet bulb temperature as a measure for human survivability doesn't work at low humidities.

It doesn't seem like the wet bulb model accounts for ratio of surface area and volume.

The instrument they use is the sling psychrometer, a long cylindrical tube [1]. If I had to hazard a guess, the ratio of surface area to volume is much higher for the bulb than it is a human.

Evaporative cooling is a function of surface area--more surface area => more cooling capacity. I'd be curious to see how accurate the model if the surface area to volume ratio were more human-scale.

1. https://www.sciencedirect.com/topics/engineering/sling-psych...