Heat and Fire are Yang phenomena, and differ mainly according to degree, with Fire being the more severe. Furthermore, Heat refers to an External Pernicious Influence while Fire designates an Internal Pernicious Influence.
Non-pathogenic Heat emanates from the Gate of Life (Ming Men) of the Kidneys, and when in its proper place, it is regarded as the "Clear Yang," which is the normal Yang energy that imbues all Internal Organs with vital warmth, Qi and movement. It is also a part of the External immune system or Wei Qi. Its nature is to rise and circulate. It imparts immunological, digestive function, vital warmth and movement throughout the body. The innate vitality and warmth of each Organ is designated as "Ministerial Fire".
Non-pathogenic internal Heat is not to be confused with the Pernicious Influence that occurs as a result of Exterior Heat strongly reacting with our innate Yang. That Heat is aggravated in hot or warm climates and during summer, though it appears year round. Being Yang, its qualities are hot, reactive and associated with Stagnation. In this way, Stagnation caused by Coldness can change to Heat, but when the body's resources are vanquished, it can change back to Coldness again.
The Heat Pernicious Influence causes a person to feel hot, to wear fewer clothes and covers, to crave cold foods and drinks and to be restless and re-active, even to the point of aggressiveness. There is usually thirst, possible burning sensations, strong odors, and sticky or thick excretions and secretions that are yellow, red or blood-tinged.
Heat and Fire tends to burn and scorch, causing symptoms such as high fever, flushed face and constipation. Just as a physical fire flares up and rises, so does Heat and Fire in the body. With the ascension of toxic, Unclear Yang, Heat and Fire symptoms often appear on the head and face, resulting in painful, swollen and bleeding gums, swollen, red and painful eyes and a red neck and face. Fire also tends to cause boils, sores and red skin eruptions with local redness, swelling, heat, pain and even suppuration and ulceration. In general, Heat and Fire signs are often associated with inflammation or fever.
Pathogenic Heat and Fire may accelerate the Blood flow and force it to go astray, causing various bleeding symptoms such as nosebleed, hemorrhage, ecchymosis, and Blood in the urine, stool, vomit or mucus from the Lungs. Fire can also stir up Liver Wind, causing a reckless
movement in the Blood that can affect the Mind (Shen). This results in emotional upset and insomnia in mild cases or mania and restlessness, unconsciousness, confused speech or delirium in severe cases.
External Heat invades directly, or is the product of any of the other invading Pernicious Influences transforming into Heat. Internal Fire arises from dysfunction of the Internal Organs, an Excess of Yang or Deficiency of Yin in the body, or from extreme emotional swings. It usually appears as Liver, Stomach or Heart Fire.
Fire is primarily an internal pathogenic factor. It can derive from the transformation of other exterior pathogenic factors (e.g. Wind, Cold, Summer-Heat, Dampness) but once it affects the Internal Organs, it is an interior pathogenic factor. Other causes of Fire are the excessive consumption of hot foods and alcohol, emotional stress (Stagnant Qi turns into Fire) and smoking.
When used in a narrow sense the term ‘Heat’ denotes a milder form of Fire. That being said there are some important differences in their respective symptoms, complications and treatment.
Fire is more ‘solid’ than Heat, it tends to move upwards (causing mouth ulcers, for example) and dry out more than Heat and causes dark scanty urine and dry stools. Also, Fire tends to affect the Mind more than Heat, causing anxiety, mental agitation, insomnia or mental illness.
The nature of Fire therefore is to:
To give an example, Heat in the Stomach can cause thirst, but Fire in the Stomach will cause bleeding gums, gum ulcers, and haematemesis as Fire moves upwards more than Heat and it agitates the Blood, causing bleeding.
The difference between Liver Yang rising and Liver-Fire blazing upwards is another appropriate example of the difference between Heat and Fire. Rising Liver Yang results from an imbalance between Yin and Yang within the Liver: when Liver Yang rises upwards excessively it causes dizziness, headaches, a dry throat, irritability and probably a red face. The dry throat, red face and irritability are symptoms of Heat, but not of Fire. If the Liver has excessive Fire, in addition to these symptoms and signs, there will be intense thirst, bitter taste, scanty dark urine, dry stools, pronounced mental restlessness and possibly vomiting of Blood or epistaxis.
There is an important difference in the treatment of Heat as opposed to Fire. In Heat patterns, the treatment method consists of ‘clearing Heat’ with pungent cold herbs to drive Heat outwards. In Fire patterns, the treatment method consists in ‘draining Fire’ with bitter cold herbs.
Fire often causes symptoms in the head because it is in its nature to rise strongly. For example, Liver Fire may cause redness, swelling and pain of the eyes, Heart Fire may cause tongue ulcers, Stomach Fire may cause mouth ulcers.
Fire is more drying than Heat and for this reason it causes scanty dark urine and dry stools.
Fire may cause bleeding from the nose if it affects the Liver, in one's vomit if it affects the Stomach and/or Liver, mixed with sputum if it affects the Lungs, in the stools if it affects the Intestines, in the urine if it affects the Bladder and/ or Liver and under the skin if it affects the Liver.
Fire may cause ulcers with a red and swollen rim.
A bit like a forest fire generates wind, Fire may generate Liver Wind, which would indicate that the disease has progressed to the Blood level
bypassing the Nutritive Qi level. This situation is more likely to occur in children.
The symptoms of Full Fire are a red face and eyes, a pronounced feeling of Heat, a dry mouth, a bitter taste, constipation, scanty dark urine, thirst, mental agitation, a Red tongue with dry yellow coating and a Full-Rapid pulse. When Fire enters the Blood, it may give rise to dark purple spots under the skin (macules) and bleeding.
Empty Fire arises from Deficiency of Yin and is manifested with night sweating, a feeling of Heat in the chest, palms and soles, red cheekbones, a dry mouth, afternoon feeling of Heat, a Red tongue without coating and a Floating-Empty and Rapid pulse.
Toxic Heat (热毒, Rè Dú) is similar in nature to Fire. In chronic, Interior conditions Toxic Heat develops from Fire and it has 5 main symptoms: swelling, edness, Heat, pus and pain.
Toxic Heat is manifested with swelling, redness and Heat. For example the presence of a large hot boil, a carbuncle, acne with large pustules, etc. Pus in these manifestations of swelling is also characteristic of Toxic Heat.
Finally, Toxic Heat causes pain in general. For example, a swollen prostate gland with pain in the perineum, a carbuncle, etc.
Toxic Heat can also occur as an acute pathogenic factor: in this case, it accompanies Wind Heat. For example, if a child suffers an invasion of Wind Heat that caused tonsillitis with very swollen tonsils and fluid leaking out, this indicates Toxic Heat.