Why might CO2 and heat together be worse than either alone?
One plausible reason is that both factors stress the body in ways that can add up or multiply. High indoor CO2 levels, which often signal poor ventilation, have been linked to reduced performance on working memory, mental workload, and visual concentration tasks in office settings [3]. Separately, high temperatures can cause heat strain and affect cognitive performance [4]. When combined, the physiological burden may exceed what the body can compensate for, especially during tasks that require sustained attention or complex processing.
In the mask study, wearing a surgical mask at the higher temperature (28°C) led to decreased oxygen saturation and increased eardrum temperature, and accuracy on arithmetic tasks dropped significantly compared with 26°C [2]. This suggests that the combination of mask-induced rebreathing (which raises CO2) and heat may impair the body's ability to maintain normal blood oxygenation and temperature regulation, which in turn could affect cognitive performance. The study authors concluded that when outdoor air supply rates are inadequate, a lower ambient temperature would reduce the negative impact of wearing a surgical mask on cognitive performance [2].
When do CO2 and temperature act more independently?
The interaction is not always present. In the same mask study, when participants did not wear a mask at 28°C, accuracy on a high-load memory task decreased as bioeffluent concentration (CO2) increased, but at 26°C the main effect of higher CO2 was an increase in reported non-clinical symptoms like throat dryness rather than a clear drop in cognitive accuracy [2]. This suggests that temperature can change which cognitive domains are most affected by CO2, or whether CO2 effects appear at all.
Other research has focused on CO2 alone. A systematic review and meta-analysis of short-term exposure to indoor CO2 found negative effects on cognitive task performance, particularly for complex tasks, but did not examine temperature as a moderator [5]. Similarly, an office study found that increasing CO2 from 500 to 2500 ppm impaired working memory, mental workload, and visual concentration, but temperature was not varied [3]. These findings show that CO2 can harm cognition on its own, but they do not tell us whether temperature would change that relationship.
What does this mean for real indoor spaces?
For practical purposes, the evidence suggests that in hot conditions, especially when ventilation is poor (indicated by high CO2) and people are wearing masks, keeping temperatures lower may help protect cognitive performance [2]. The mask study specifically recommends lower ambient temperatures when outdoor air supply rates are inadequate [2]. This is a concrete, actionable insight for classrooms, offices, and other indoor environments where masks are used or where ventilation cannot be easily improved.
However, the interaction may depend on the specific task and the population. The mask study included 16 male and 16 female sedentary subjects in a climate chamber, so the results may not generalize to all settings or activity levels [2]. The office study that found CO2 effects used 30 participants and did not vary temperature [3]. A study on female students in a hot climate examined combined effects of temperature and CO2 but its abstract does not provide detailed results [1]. Another study on frontal thermal radiation in a 32°C environment mentioned CO2 but did not explicitly test interaction [4]. So while there is evidence of interaction, the exact conditions and magnitude remain uncertain.
Sources used in this answer
Combined effects of ventilation rates and indoor temperatures on cognitive performance of female higher education students in a hot climate
The study investigates combined effects of indoor temperature and CO2 levels on cognitive performance in female students in a hot climate, but the abstract does not report specific interaction results.
The effects on cognitive performance of wearing a surgical mask at two different temperatures and outdoor air supply rates
In climate chamber experiments, bioeffluent concentration (CO2) and wearing a surgical mask had interactive effects on cognitive performance; at 28°C versus 26°C, mask wearers had significantly lower accuracy on multiplication and addition tasks, and without a mask, higher CO2 at 28°C reduced high-load memory accuracy.
Integrated Approach to Evaluating the Effect of Indoor CO2 Concentration on Human Cognitive Performance and Neural Responses in Office Environment
Increasing indoor CO2 from 500 to 2500 ppm had a detrimental effect on working memory, mental workload, and visual concentration in an office setting, but temperature was not varied.
Thermal and cognitive effects of frontal thermal radiation on body regions under low-activity conditions in a 32° C hot environment
The study examines thermal and cognitive effects in a 32°C hot environment with mention of CO2, but does not explicitly address interaction or independence of CO2 and temperature effects.
Short-term exposure to indoor carbon dioxide and cognitive task performance: A systematic review and meta-analysis
A systematic review and meta-analysis found that short-term exposure to high indoor CO2 concentrations negatively affects cognitive task performance, particularly for complex tasks, but did not examine temperature interaction.