In air environments, cold stress generally produces severe discomfort before any effect on health occurs. There is therefore a strong behavioural reaction to cold and many methods used for its avoidance: clothing, activity, shelter, etc. Human thermal environments that can be described as cold would lead to a tendency for heat loss (or negative heat storage) from the body. There is sometimes some semantic confusion. An environment with an air temperature of 5°C may be described as ‘cold’. However, an active, heavily clothed person in that environment may be hot and sweating into clothing in an attempt to lose heat. When the person rests then the previously warm to hot human thermal environment of 5°C air temperature, becomes cold and heat loss and discomfort are exacerbated by damp clothing. In human thermal environment terms, the person has gone from a hot to a cold environment, whereas the air temperature has not changed.