The presence of four oxo-fatty acids in human lung lavage has been correlated with cotton mill exposure. Specific syntheses have been developed which definitively prove the structure assignment and make available larger amounts of these compounds for further biological testing. The same compounds have not been detected in any of the possible environmental sources thus far investigated, suggesting that they may be produced by the lung. An autoxidation pathway has been proposed to account for their chronic presence in the lungs of cotton mill workers. While the mechanisms for the accumulation of these compounds in the lung is not clear, their presence may, at least in part, account for the pathological lesion observed in chronic byssinosis.