Workers slow down and make more errors, but the signal behind it is rarely recorded
Field studies of workers consistently find that heat stress degrades cognitive performance, but they mostly capture the output of the brain rather than its internal signals. Petrochemical workers tested at the start, middle, and end of shifts showed more commission errors and longer response times as heat stress accumulated, along with fewer correct responses on a working-memory test [1]. Taxi drivers performed worse in precision, speed, attention, and error counts during hot summer evening shifts than in cooler conditions [4]. These are behavioral changes, not recordings of brain activity or autonomic nervous system activity.
That gap matters for the question. Neither the petrochemical study nor the taxi-driver study measured EEG, heart-rate variability, or other physiological signals, so they can show that heat impairs performance but cannot say which brain or autonomic pathway is responsible [1][4]. A review of indoor heat studies likewise notes that the field has largely relied on cognitive task outcomes and calls for future work to integrate sensitive brain-activity measures [3].
The one direct brain-signal study found a nonlinear link, not a simple on-off switch
The most direct evidence comes from a controlled indoor study of middle-aged manual workers that measured EEG, the brain's electrical activity, while varying heat stress levels. It reported nonlinear relationships between heat stress and cognitive outcomes [2]. In plain terms, performance did not simply decline in a straight line as heat rose; the relationship changed direction or steepness at different heat levels. The study decomposed the EEG signal into standard frequency bands (delta, theta, alpha, beta), which are broad categories of brain-wave rhythm [2].
Because this evidence is available only as an abstract-level summary, the specific EEG bands that tracked impairment, and the size of the effect, are not established here. What it does support is a caution: treating heat stress as a simple dose that linearly reduces cognition may misrepresent how the brain responds [2].
In animals, heat plus ozone points to inflammation and a leaky blood-brain barrier
A rat study offers a possible biological route rather than a measured human signal. Rats exposed to heat stress showed cognitive decline and neuronal damage, along with activation of glial cells (the brain's support and immune cells), neuroinflammation, and signs of a disrupted blood-brain barrier, the selective filter that normally protects the brain [5]. When heat exposure was combined with ozone, these effects worsened [5].
This is evidence of a mechanism in rats under combined heat and ozone exposure, not proof that the same process explains impaired cognition in heat-exposed workers. It is consistent with the idea that inflammatory and barrier changes could contribute, but it does not identify a signal that could be monitored in a workplace [5].
Task difficulty and timing change how heat hits performance
The size of the heat effect depends on what the worker is doing and when. A review of indoor hyperthermal environments, defined as air temperatures of 30 °C or higher during low-intensity office-like activity, found that complex tasks are more susceptible than simple tasks under identical heat conditions, and that heat intensity and exposure duration interact [3]. The same review describes a speed-accuracy trade-off under heat stress, likely driven by psychological effects, and notes that factors such as age, gender, expectation, motivation, and mental health may modulate the effect [3].
These qualifications sit beside the performance findings from workers: the petrochemical results varied by test and by point in the shift [1], and the taxi drivers' performance dropped sharply specifically in summer evening shifts [4]. A single average estimate of heat's cognitive cost would therefore miss when and for whom the impairment is largest.
Sources used in this answer
Evaluating the effect of heat stress on cognitive performance of petrochemical workers: A field study
Petrochemical workers showed increased commission errors and response times and reduced correct responses as heat stress accumulated across a shift, indicating declining cognitive performance.
Thermal stress and cognitive performance in heat-exposed manual workers: An EEG investigation in a controlled indoor environment
In a controlled indoor study of middle-aged manual workers, EEG measurements revealed nonlinear relationships between heat stress and cognitive outcomes.
How heat stress impairs cognition: A review of cognitive performance in indoor passive hyperthermal environments
A review of indoor hyperthermal environments found a speed-accuracy trade-off under heat stress, greater susceptibility of complex tasks, and modulation by human factors such as age and motivation.
The Impacts of Heat Stress on the Cognitive Performance Parameters of Taxi Drivers
Taxi drivers showed reduced precision, speed, and attention and more errors and mental fatigue under summer heat stress, especially during evening shifts.
Combined exposure of heat stress and ozone enhanced cognitive impairment via neuroinflammation and blood brain barrier disruption in male rats
In rats, heat stress induced cognitive decline with glial activation, neuroinflammation, and blood-brain barrier disruption, and co-exposure to ozone worsened these effects.