The six climatic evils include wind, cold, summer-heat, dampness, dryness and fire. These are normally called "the six climatic factors," i.e. the six natural climatic variations. The six climatic factors represent the natural conditions within which all living things exist, and they do people no harm. Chapter 25 in Plain Questions says: "People live on what is produced by the sky and earth, and grow and develop according to the law of the seasons." In other words, human beings depend on the air and nutrient essence existing between the sky and the earth, and live, grow, and develop in accordance with the seasonal law of germinating, growing, and maturing.

Moreover, human beings have learned how to deal with the seasonal changes through their living experiences and obtained certain adaptive abilities, so the six natural climatic factors generally can not cause people to suffer from disease. But the climate may become a pathogenic factor under limited circumstances; for example, when climates change abnormally, i.e. the energetic levels of the six climatic factors become too high or too low and do not correspond to their respective season (for example, an abnormally cool spring, or an abnormally warm autumn ), or when climates change wildly. Then they will invade the human body and cause diseases if the body's energetic level of genuine-qi is low, and if its ability to resist disease is weak. Only under these circumstances are the six climatic factors termed "the six climatic evils." (They are also termed the "six yin." The Chinese word "yin" here means overacting, penetrating and dispersing. ) The six climatic evils belong to the category of exogenous pathogenic factors.