Answers
1. Lead was once widely used in the United States as a gasoline additive but it was banned due to health risk.
2. Radon is a radioactive gaseous decay product of uranium that is found in rocks, soil and water.
3. Radon is a leading cause of lung cancer in the United States because the by-product of this can get trapped in the lungs.
4. Mercury poses a health risk to humans who eat large quantities of marine fish such a tuna and swordfish.