A few math steps later, the researchers concluded that $10 billion in change, or about $90 per household, was "idle."
Yikes. If that's really true, never mind the environment. What about world poverty?
The numbers are debatable. U.S. Mint spokesman Michael White says it makes no estimates of coins not in circulation.
And the for-profit company has plenty of reasons to drum up a cause - including the 8.9 percent it deducts from much of the change sorted at its 15,000 kiosks nationwide. Not exactly chump change.
But at the core of this notion - circulating coinage instead of squirreling it away - is a basic tenet of green: Reuse stuff. Don't make something new if it's already lying around.
One thing all U.S. coins have in common is copper. Oddly enough, our iconic copper coin, the penny, has the least. Each one is 2.5 percent copper, 97.5 percent zinc.
Some of the copper comes from recycling old coins. Most coins last 30 years, and every one that's worn out and returned to the mint goes into new coins, White says.
Still, that's just a fraction of what's needed.
Coinstar, in Bellevue, Wash., hired the Sustainable Research Group in Grand Rapids, Mich., to look at the impacts of primary copper production and copper mining - or, in this case, not mining the copper needed to replace the coinage it says is idle.
Coinstar officials somehow got from that data - they won't share it - to a rather gimmicky "change for the Earth" campaign that claims recirculating just 10 percent of all those coins would save water equal to 82 million showers. Or the carbon emissions from 12,619 cars. You get the gist.
Once again, we can quibble with the statistics. But clearly, mining has impacts.
And metal prices continue to rise. For two years in a row, producing both the penny and the nickel has cost more than their face value. (It costs 1.2 cents for the penny, 7.3 cents for the nickel, according to the U.S. Mint.)
Far be it from me to suggest that collectors should cash in their treasures, or that little kids with piggy banks are harming the Earth.
But it's disconcerting to think that we treat our coins so cavalierly. We throw them in fountains. We stash them in closets. Many people don't even stoop to pick up fallen change on the sidewalk.
So as I pondered the possibility of a $90 mini-motherlode in my own house, I was elated. Maybe I could even feel sanctimonious while spending it on a massage. Or shoes.
Alas, poking around under the couch cushions produced only crumbs and cat hair.
A small dresser dish held paper clips, perfume samples, and 73 cents. My off-season purses: one lousy dime.
In the end, the total was $27.03.
The time rummaging might have been better spent contemplating larger currency issues. Such as whether we even need pennies anymore - a notion the treasury secretary has actually broached.
The nation made 7.4 billion pennies in 2007, more than half of them at the Philadelphia Mint.
Meanwhile, I've been holding up checkout lines far and wide as I fumbled in my wallet so I could fork over the exact change.
Contact staff writer Sandy Bauers at 215-854-5147 or firstname.lastname@example.org.
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visit her blog at http://go.philly.com/greenspace.