How to distinguish between fever caused by infection, and environment and or exercise-induced hyperthermia and heat illness?


Updated: 22 May 2020


Be careful to not mistake hyperthermia for fever.


A heat-stressed individual may also be ill (with mild to severe symptoms of overheating) and can potentially be mistaken as being febrile. Therefore, it is important that they are monitored until hyperthermia subsides. If symptoms persist, seek medical advice immediately.


If someone has been exercising and/or was exposed to heat, let him/her rest in a cool environment for at least 30 minutes. If body core temperature remains elevated during this time, it may be fever – consult a health expert immediately and explain the person’s condition. If you observe a substantial drop in body core temperature (of 0.5°C or more, towards the normal 37°C) and the individual feels better after resting in a cool environment, it is more likely to be heat-stress related. In this case, ensure that the individual is hydrated and has no other indications of COVID-19 infection.


Differential diagnosis between heat illness and COVID-19 is critical to facilitate accurate testing, diagnosis and treatment, and prevent contraindications of treatment.

What can be done?

Recommendations for the public as well as health, occupational and sports/exercise specialists should stress the difference between hyperthermia induced by the environment or exercise, and fever.


A distinct symptom of viral infection and heat stress is a marked elevation of the internal body (core) temperature. In cases of viral infection, the increase in body core temperature is due to fever and is used as one of the initial screening criteria for COVID-19 (Guan et al., 2020). Fever (82%) was the most common first symptom (Tian et al., 2020). In cases of heat stress caused by physical work and/or heat exposure, the elevated body core temperature can, in extreme cases, affect respiration and well-being (Sawka et al., 2011). A febrile person will likely try to defend the elevated core temperature (by shivering) as opposed to someone with exertional hyperthermia. This is however NOT fever and should not be confused as a sign of viral infection.


Be careful with the interpretation of axilla, infrared tympanic or forehead thermometers since they may be as much as 1-2°C higher or lower than actual body temperature. These types of thermometers are poor screening tools for detecting temperature abnormalities. In contrast, nonvascular central thermometers, including rectal thermometers, show excellent agreement with the gold standard  pulmonary artery catheters. This confirms their clinical accuracy and utility (Niven et al, 2015).


Guan, W. Ni, Z. Hu, Y. Liang, W. Ou, C. He, J. Liu, L. Shan, H. Lei, C. Hui, D. S. C. Du, B. Li, L. Zeng, G. Yuen, K-Y. Chen R, Tang C, Wang T, Chen P, Xiang J, Li S, Wang J, Liang Z, Peng Y, Wei L, Liu, Y. Hu, Y. Peng, P. Wang, J. Liu, J. Chen, Z. Li, G. Zheng, Z. Qiu, S. Luo, J. Ye, C. Zhu, S. and Zhong N. Clinical Characteristics of Coronavirus Disease 2019 in China. New England Journal of Medicine. Online:, 2020

Niven, D. J. Gaudet, J. E. Laupland, K. B. Mrklas, K. J. Roberts, D. J. and Stelfox, H. T. Accuracy of peripheral thermometers for estimating temperature: a systematic review and meta-analysis. Annuals of Internal Medicine Vol. 163, pp. 768-77, 2015

Sawka, M. N., Leon, L. R., Montain, S. J. and Sonna, L. A. Integrated physiological mechanisms of exercise performance, adaptation, and maladaptation to heat stress. Comprehensive Physiology, Vol 1, pp. 1883-928, 2011.

Tian, S. Hu, N. Lou, J. Chen, K. Kang, X. Xiang, Z. Chen, H. Wang, D. Liu, N. Liu, D. Chen, G. Zhang, Y. Li, D. Li, J. Lian, H. Niu, S. Zhang, L. and Zhang, J. Characteristics of COVID-19 infection in Beijing. Journal of Infection, vol. 80, pp. 401-406, 2020.