Whether a pleural effusion should be tight or not is mainly determined by the nature of the pleural effusion. If a small amount of pleural effusion is caused by inflammation or tuberculosis, it does not matter if it can improve quickly within 2 weeks after treatment. However, if the pleural effusion is caused by primary tumor in the lung or pleural metastasis of tumor elsewhere, a small amount of pleural effusion is serious because it is pleural fluid caused by tumor, and the amount of pleural fluid will increase quickly. So a small amount of pleural effusion, if it is caused by malignant tumor metastasis, can also be life-threatening.