The conventional wisdom is that China doesn’t hold much direct risk for U.S. banks. Sure, if China were to have a financial crisis, and if it were to fall into recession — a long way to go, considering its recent annual growth of 7% — that would do serious damage to the global economy. American banks and the U.S. economy would feel that. But in terms of direct exposure to China, U.S. banks don’t stand to lose a lot.

According to a report last week from debt rating agency Fitch, U.S. banks have a collective $83 billion in direct loan exposure to China. That isn’t very much in the scope of the U.S. banking sector. JPMorgan Chase JPM , alone, for instance, has over $700 billion in total loans. The entire U.S. banking sector has nearly $15 trillion in assets. For U.S. banks, China is still not that big of deal.

That appears to be changing, though. Fearing a credit bubble, a year ago, Chinese authorities sharply raised domestic interest rates to slow lending made by local banks. That opened up a huge gap between U.S. interest rates, which are around 0.50% for short-term interbank loans, and Chinese interest rates, which are 5% for short-term banks loans.

And so it appears there is easy money to be made borrowing money from the U.S. and Europe, and lending it in China. The trade’s been growing for a while, but it has attracted attention lately. And it’s a bit dicey, at least for the U.S. banks.

In the last week or so, a number of large banks have been trying to figure out if they were taken in a loan fraud scheme in which a Chinese commodities trading firm in the port of Qingdao pledged the same collateral to a number of lenders. The scheme has shed light on the fact that a large portion of the overseas lending that Chinese companies are doing to capture the interest rate spread has been with loans collateralized with commodities.

Goldman Sachs estimates that commodities-based lending has resulted in an inflow of $110 billion of foreign currency into the Chinese economy in the past four years, or about a third of all new short-term debt in China in that time.

Citigroup C appears to be the only large U.S. bank that has fallen victim to the Qingdao fraud. But most large U.S. banks make commodities-based loans, and the practice has been growing recently. Last year, banks around the world lent $687 billion to commodities-based businesses, according to Bloomberg, though much of those loans were made directly to the companies and secured by raw materials. JPMorgan was the top U.S. lender worldwide, at $57 billion, followed by Wells Fargo WFC , which gave out $47 billion.

Many of these loans might not be factored in the estimates that Fitch and others have put together to calculate U.S. banks’ exposure to China. For instance, sometimes commodities loans are structured as derivatives or swaps. Fitch says its China loan estimates do not factor in derivatives or guarantees.

What’s more, when it comes to commodities, some banks might not even realize they are taking on China risk. The Citi loan that ended up sending money to China by way of the Qingdao port was made to Swiss commodities trading firm Mercuria. The commodities that Mercuria used to finance the loan were stored in Qingdao, on loan from a Chinese commodities company named Decheng Mining.

And unlike other types of lending, banks typically don’t break out just how much money they have lent out based on accounting. “I have never seen any bank balance sheet that has that information,” says Haoziang Zhu, an economics professor at MIT who has studied commodity-based lending. “It’s hard to track.”

And while the China-to-U.S. carry trade may seem like easy money, there are ways it can go bad. A drop in the yuan, which is down 3% in the past year, would cut into profits because a debtor company would need more of those to pay back the dollars they owe in the U.S. loan at the end of the transaction. Also, if you are financing a transaction with commodities, and commodities prices rise, that could erase some of your gains.

That said, as long as interest rates between the U.S. and China remain this wide, the China carry trade will carry on.