A compound in the blood, bilirubin, has been identified to reduce risk of lung cancer, chronic obstructive pulmonary disease and all-cause death, based on a study consisting of half a million adults.

Bilirubin (formerly referred to as hematoidin) is the yellow breakdown product of normal heme catabolism. Heme is found in hemoglobin, a principal component of red blood cells. Bilirubin is excreted in bile and urine, and elevated levels may indicate certain diseases. It is responsible for the yellow color of bruises, urine (via its reduced breakdown product, urobilin), and the yellow discoloration in jaundice.