These answers are ridiculous. Common metals, such as steel/iron or aluminum, themselves do not harm the environment. Some metals, such as radioactive metals (e.g. Uranium) or toxic metals (e.g. Lead) will have adverse biological effects (harm or kill plants or animals), but I doubt that such rarely used metals were the focus of this question. It's not like filling a landfill full of scrap metal will cause global warming or induce weather change.