The age of industry is the age of steel. Steel provides the support for modern buildings, the bodies of modern cars and even the stylings of modern appliances. Iron is the primary constituent of steel, and records of its extraction and employment reach back more than 100 years. By combining this data with information on the metal's return as scrap or waste, Daniel Mller of Yale University and his colleagues derived the first estimates of the total stock of iron in use and elsewhere in the U.S. "Over the last century, there has been a lot of mining activity; metals have been put into society," he explains. "Where are they now?"

The researchers used information from the American Iron and Steel Institute, United Nations trade statistics, and U.S. Geologic Survey data among other sources to estimate that between 1900 and 2004 the U.S. has brought into use 3.2 billion metric tons of iron. "A very large fraction of the metals that have been mined are still in some sort of product," Mller adds. "Such in-use stocks are becoming important mines for the future." In fact, because the amount of metal in use is roughly equivalent to the amount of metal still in the ground, recycling alone could supply future demand for iron and steel. "There is demolition of buildings and infrastructure, cars are going to be scrapped. There is every year already now a large amount of scrap becoming available," he says. "You can put the scrap directly into the steel furnace and eliminate the most energy-intensive step."

That step--the conversion of iron ore to iron in a blast furnace--uses 75 percent of the total energy required to turn iron into steel. And with U.S. iron ore declining in quality--a drop from as much as 60 percent iron during World War II to as little as 25 percent per metric ton of ore--the energy demand is only increasing. Fortunately, however, the overall picture also reveals that U.S. per capita demand for steel has stabilized at roughly 12 metric tons since 1980. "Since then it hasn't grown or maybe even slightly decreased," Mller notes.

This study does not reveal why iron use has leveled off, though Mller suspects things like the substitution of lightweight aluminum for steel in cars, but it does reveal a glaring hole in how the U.S. is dealing with its scrap iron: 77 million metric tons of iron leave use every year, but only 57 million metric tons reach the recycling stage (and only 42 million metric tons make it back into use). Some of this iron is lost to the black market or left in obsolete products, but questions remain. "This study is aiming at quantifying the entire iron system and trying to identify the key processes where materials may be lost or taking paths that we don't want them to take," Mller says.

And by deriving such exact estimates for U.S. iron consumption, the researchers hope to understand how much steel developing countries such as China or India might ultimately employ. The team has turned its attention to finding steel saturation points for other mature economies. "This would give us a very strong tool to look at emerging economies to see where they could reach their plateaus," Mller says. And perhaps do a better job of minimizing energy use and maximizing reuse, recognizing their new steel-built cities as the iron mines of the future. The research was published online this week in Proceedings of the National Academy of Sciences USA.