Check out why we spend used bills faster than new ones.

We’ve all heard the phrase dirty money, but when you think about it all used money is probably filthy considering the number of hands it’s likely passed through.

A new study shows that people spend their old, manhandled money more quickly than the crispy, new stuff.

The psychology behind the decision is pretty simple – people dislike contamination.

That is, unless we are being observed, in which case cash spenders prefer to flash their smooth, clean bills.

You may be relieved to know that one-dollar bills only circulate for about 21 months. Once deemed too damaged to be viable all paper currency is returned to either the Federal Reserve, or the Bureau of Engraving and Printing, or the US Mint. These agencies destroy around 5 million unfit bills daily.

Here are a few fun facts about the birth of fresh US currency.

In 2012 about 8.4 billion new notes were produced at a rate of 35 million a day.

The average cost of each bill was just under 9 cents.

Two Bureau of Engraving and Printing facilities in Fort Worth and Washington DC used a combined 11.1 tons of ink per day in 2012 to produce them.