The human cognitive performance under hot and humid exposure is a critical issue for people working under hot and humid environments, for the considerations of both their health and safety. In order to find an indicator of the cognitive performance, the electrocardiogram (ECG) data were recorded continuously for 2.5 hours with portable Holter, when participants were exposed to 4 hot-humid environmental conditions and conducting different cognitive tests. The ECG data then were analysed to obtain the HRV indices that were used to establish a rel...