Paraquat is a weed killer once promoted by the United States for use in Mexico to destroy marijuana plants. Research found that this herbicide was dangerous both to workers who applied it to the plants and to people who smoked the marijuana harvested from them.

Paraquat is now banned in the United States, although it is still legal for some uses in Mexico. As most marijuana smoked in the U.S. is now domestically grown, the risk of paraquat lung to American marijuana smokers is minimal.

Paraquat may cause lung damage by inhalation, it may cause esophageal erosions or ulcers from ingestion and absorption through the gut, or it can cause skin breakdown through contact with intact skin.

Severe ingestion of Paraquat can rapidly cause death from respiratory failure due to a form of proliferative alveolitis, or inflammation of the air spaces in the lungs. This impairs oxygen transport to the blood and may lead to acute respiratory distress syndrome (ARDS).

Administering oxygen to improve the blood oxygen level may actually increase the damage to the lung in such cases of ARDS. Chronic exposure to Paraquat, may cause pulmonary fibrosis, or a stiffening of the lung tissue.

Paraquat may also damage the kidneys, liver, mouth, and esophagus. Death may occur from perforation of the esophagus, or from acute inflammation of the mediastinum, the area that encases major blood vessels and airways in the middle of the chest.