Water is used in the lungs to keep the tissue moist, but each time we exhale we expel moisture from our lungs. Under normal hydrated conditions the moisture is rapidly replaced. If we are in a dehydrated state then the tissue inside the lungs begins to coat with mucus to prevent drying. It has been demonstrated in many animal studies that an increase in water intake will reduce histamine levels and over a 2 to 3 week period restore normal hydration to lung tissue and reduce mucus build up. Once this occurs the bronchial passages begin to open and normal breathing is restored. The same histamine related effects apply to allergies, and again, significant benefits can result from an increased intake of water.

Are you having a heart disease ? If yes, than it might be the consequence of heart failure. When your left heart fails to pump sufficient blood there will be backward pressure and to the lungs. Hence, you have transudate pleural effusion.