An article in last Wednesday’s NYT included a pretty remarkable statistic. Steven R. Weisman cited Diana Farrell, director of the McKinsey Global Institute, who estimates that petrodollars have held American interest rates three-quarters of a percent lower than they would have been otherwise.

The macroeconomics behind the rate supression are simple—more available dollars = more available credit = lower interest rates—but consider for a moment what it means in real terms. Wednesday’s NYT quoted the 30-year fixed mortgage rate at 5.80%, which means that a homeowner with a $300,000 mortgage would pay $1760/month. Now let’s assume that Farrell is right about the 0.75% suppression. Without the lower credit made possible by petrodollars, the same homeowner would be looking at a 6.55% interest rate, which translates into a $1906 monthly payment. The difference, then, turns out to be $146 per month, or $1,752 per year. Which means that the mortgage of our fictional American homeowner is being effectively subsidized by oil money to the tune of 8.3% overall. And that’s not saying anything about all of the other places the interest-rate suppression affects our lives (credit cards, car loans, etc.).

The troubling thing about this oil subsidy—for Americans, anyway—is not that it’s happening but that it could disappear someday very soon. For the rate suppression to benefit U.S. consumers, oil producers must want to keep their oil revenues in dollar-denominated investments. (The oil markets trade in dollars.) For decades this has made sense, as the U.S. dollar was the reserve currency of choice among foreign governments. But the value of the dollar has been slipping by the day—according to the Economist, it’s fallen 6% against a collection of other currencies since August—making dollar-denominated investments less attractive.

Indeed, Weisman’s article points to a growing reluctance among oil-producing countries to keep their skyrocketing revenues in dollar-based assets:

C. Fred Bergsten, director of the Peterson Institute of International Economics, said that while some countries in the gulf were trying to diversify their investments away from the dollar and into euros and pounds sterling, the Saudis were trying to quell that trend out of fear that the dollar will decline further and diminishing the value of their assets.

What I’ve called an “oil subsidy” could equally be a “gold subsidy,” a “heroin subsidy,” or a “banana subsidy,” should any of those other commodities generate oil-sized revenues. The important fact here is that America is an attractive market for foreign investment. To be clear: the interest-rate suppression is only contingently dependent on the high price of oil. The crucial point is the ability of dollar-denominated investments to attract investors, and that depends on their ability to hold and increase value.

In a sense, then, and looking at this from a very wide angle, the interest-rate suppression is a kind of “superpower subsidy.” It will last as long as America has the economic, political, and military strength to guarantee that the dollar is a good store of value. How long that happens is another question entirely.