Although much is understood about lung cancers, there are still many things and situations that are unclear. For example, it is unclear why some families have a history of individuals with lung cancer and no clear risk factors. The same is true for some patients who develop lung cancers with no clear risk factors. Another occurrence such as drinking water that has high arsenic concentrations may increase lung cancer risk but how the process happens is not clear. In addition, why adenocarcinoma in the lungs is more common in nonsmokers than smokers is also not known.