Car exhaust fumes contain carbon monoxide and lead which are highly poisonous. In big cities, the exhaust fumes build (5)_________ and pose a health hazard to human beings. Pollution of the sea by oil threatens marine life. Much of the oil comes from ships that clean their fuel tanks while at sea. Offshore oil wells also discharge vast amounts of oil into the sea. Pollution of the sea can also be caused by oil spills. An oil spill may occur as a result of accidents involving oil tankers.