Paraquat lung: Lung disease caused by the contact herbicide paraquat, which selectively accumulates in the lungs and is highly toxic. The prognosis depends upon the severity of exposure. Paraquat lung is rare because the herbicide must be directly inhaled to cause the disease. Paraquat lung emerged as a health concern in the 1970s, when the US government sprayed paraquat aerially over some illegal marijuana fields. Some of the sprayed plants survived and were sold, causing paraquat lung in purchasers who smoked the product.