Introduction
This topic covers the objective, instrument-based and calculated ways of describing the whole thermal environment of a workplace or dwelling at once. A single air temperature reading is not enough, because the human body responds to air temperature, humidity, air velocity and thermal radiation together. Objective methods are divided into physical methods, which measure the factors themselves, physiological methods, which record the body's response, and integral methods, which combine the factors into one index.
Why complex evaluation is needed
The effect of each factor depends on the others. High humidity slows sweat evaporation, so the same air temperature feels hotter on a humid day. Air movement cools a person mainly by convection and evaporation, and it warms rather than cools when the air is hotter than the skin. Radiant heat from hot surfaces can dominate the heat balance even when the air temperature is moderate. For this reason all four factors are measured together, at the place where the worker or resident actually stands, at the height of the body (usually around 1.1 m for a standing adult, with additional levels for seated work).