Whether pulmonary arterial hypertension (PAH) is serious depends on the cause and the severity of the PAH. It is generally categorized into mild, moderate, and severe. If the PAH is caused by diseases such as pulmonary embolism and is severe, this condition is very serious and may lead to sudden death. If it is chronic pulmonary arterial hypertension with also a severe degree and lasts for a long time, it can lead to right heart failure, and subsequently right heart failure may cause left heart failure, which is also a very serious issue. If the pulmonary hypertension is a reversible, mild condition, such as due to embolism in the pulmonary artery branches which resolves after the embolism is cleared, then the PAH can easily recover, and in this case, it is not serious. However, if it is caused by chronic lung diseases such as chronic obstructive pulmonary disease or pulmonary interstitial fibrosis leading to chronic pulmonary hypertension, it is quite serious because it is a progressively worsening disease. The pulmonary artery pressure will not decrease, and over time it will gradually worsen, eventually leading to heart failure.