The causes of asthma include internal and external factors. The internal factor lies in the retention of phlegm and fluid inside the body. The retention of phlegm results from the deficiency of the lung, spleen and kidney. The fluid metabolism of the human body is closely related to the lung, spleen and kidney.

The external factors of the disease mainly include the six exogenous pathogenic factors with wind-cold and wind-heat being the major ones. If exogenous pathogenic factors invade the lung, the lung will fail to disperse and descend, causing reversely rising qi intermingled with latent phlegm. In this case, phlegm and qi jointly obstruct the lung, developing into asthma. Besides, if people contact special objects like pollen, lampblack, hairy dust, etc.

In this case, the latent phlegm is disturbed and rises with the rising qi, developing into asthma. In short, the onset of asthma is the result in which the external causes exert influence on the internal ones.
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Source:http://www.tcmadvisory.com/