Over the years, researchers of human comfort have established the variables that affect a human's thermal sensations and they have established the ranges of these variables within which the average person is comfortable.

Six primary comfort variables
These "comfort" variables include air temperature, relative humidity, air motion, and mean radiant temperature. The mean radiant temperature is the average temperature of all of the surfaces that surround the person in question. These four variables are called the "environmental variables" because they represent the environment surrounding the body.

A second set of variables, called the "personal" variables, are controlled by the individual. The two are the clothing insulation value, termed the "clo" value, and the metabolism rate, with units of "met" as described above.

Introduction

Human Physiology

Comfort Control

• variables
• parameters
• indices
• designing

Animations

SIX PRIMARY COMFORT VARIABLES

Comfort conditions have been difficult to visualize. Most people, mistakenly, equate comfort with an air temperature range. One hundred years ago, Willis Carrier, developed a method that allows us to visualize two of the variables -- the combination of air temperature and relative humidity that exist in a space. His tool is called the Psychrometric Chart. Psychrometrics, which Willis Carrier imagined and developed, is the study of air and water vapor mixtures, and is the scientific basis of the air conditioning industry. Below we see a psychrometric chart that outlines the conditions at which most sedentary humans are comfortable; one envelope is for winter comfort and one for summer comfort. There is a difference because of the amount of clothing worn in the two different seasons. Any point located on the chart establishes the temperature (dry bulb) and the amount of water vapor in a unit quantity of air. If we were to select one temperature at which most humans would be comfortable year round, it would be around 74F (23C).