3.2 Health impacts of the thermal environment

3.2.1 Heat strokes

Heat stroke is the most serious heat-related condition. It occurs when the body becomes unable to control its temperature: the body's temperature rises rapidly above 40°C, the sweating mechanism fails, and the body is unable to cool down. Body temperature may rise even further within 10 to 15 minutes, and many of the important functions of the body will begin to shut down. Heat stroke can cause death or permanent disability if emergency treatment is not provided. Infants, children and the elderly are more vulnerable to heat illness than other age groups. 

3.2.2 Effect of uniform temperature indoors

Human physiology is developed before our ancestors started creating indoor environments. Our bodies are designed to function well in environments where there is temperature variation. Human thermo-regulatory processes are stimulated when the temperature indoors varies within the comfort range (see section 3.1.4). In most buildings, the temperature increases during the day as the outdoor temperature increases. Sunlight through windows increases the temperature, while overcast weather can reduce the temperature. The seasons also have an impact, as the temperature is often cooler in winter than in summer.


Recent research shows that slightly cool temperatures (e.g. at or below 20°C) may stimulate unnoticeable shivering, which increases metabolism. As obesity is becoming an increasing problem, increased metabolism can have a positive health impact (van Marken Lichtenbelt et al., 2009; Bluyssen, 2013).

A “sunroom” can be the space in a home where temperatures vary the most and where it is most enjoyable and healthy to spend time.

3.2.3 Sleep quality

The temperature in the bedroom has an impact on sleep quality. There are large personal and cultural variations in preference. Some prefer always to have their bedroom window open and the heating switched off, even in mid-winter, while others prefer the bedroom temperature to be the same as in other rooms of the house. Some prefer thin sheets or blankets, while others prefer a thick duvet.

Surprisingly few studies give a scientific response to this discussion. There is no clear guideline as to what temperature and bedding will give the best sleep quality. What is known is that overheating reduces sleep quality. The bedroom should not be too warm, and it is particularly important that the time at which you fall asleep is not too warm. CIBSE Guide A states that sleep may be impaired above an operative temperature of 24°C (Laverge et al., 2011; CIBSE, 2006).

Bluyssen, P. M. (2009) The Indoor Environment Handbook, RIBA Publishing.
CIBSE Guide A. (2006) Environmental design. Chartered Institution of Building Services Engineers, London.
Laverge, J., Janssens, A. (2011) Physiological and Sensory Human Response to IEQ Indicators While Asleep. Proceedings of Indoor Air 2011, Austin, Texas
Van Marken Lichtenbelt, W. D., Vanhommerig, J. W., Smulders, N. M., Drossaerts, J.M. a F. L., Kemerink, G. J., Bouvy, N. D., Teule, G. J. J. (2009) Cold-activated brown adipose tissue in healthy men. The New England Journal of Medicine, 360(15), 1500–8.