  Operators' cognitive functions are impaired significantly under extreme heat
stress, potentially resulting in more severe secondary disasters. This research
investigated the impact of elevated temperature and humidity (25 60%RH, 30
70%RH, 35 80%RH, 40 90%RH) on the cognitive functions and performance of
operators. Meanwhile, we explored the psychological-physiological mechanism
underlying the change in performance by electrocardiogram (ECG), functional
near-infrared spectroscopy (fNIRS), and eye tracking physiologically.
Psychological aspects such as situation awareness, workload, and working memory
were assessed. Eventually, we verified and extended the maximal adaptability
model to the extreme condition. Unexpectedly, a temporary improvement in simple
reaction tasks but rapid impairment in advanced cognitive functions (i.e.
situation awareness, communication, working memory) was obtained above 35 WBGT.
The best performance in a suitable environment was due to more effective
activation in the prefrontal cortex (PFC). With temperature increasing, more
mistakes occurred and comprehension was impaired due to drowsiness and lower
arousal levels, according to evidence of compensatory effect in fNIRS. In the
extreme environment, the enhanced PFC cooperation with higher functional
connectivity resulted in a temporary improvement, while depressed activation in
PFC, heavy physical load, and poor regulation of the cardiovascular system
restricted it. Our results provide a detailed study of the process of
operators' performance and cognitive functions when encountering increasing
heat stress, as well as its underlying mechanisms from a neuroergonomics
perspective. This can contribute to a better understanding of the interaction
between operators' performance and workplace conditions, and help to achieve a
more reliable human-centered production system in the promising era of Industry
5.0.
