NEW YORK (CNN/Money) - The number of U.S. metropolitan areas experiencing booms in real estate prices spiked 72 percent in 2004, according to a report from the Federal Deposit Insurance Corporation (FDIC).

Still, that represented just 15 percent -- a total of 55 -- of the country's 362 metro areas, leading some to question whether the reports of a real estate bubble have been greatly exaggerated.

Does that 15 percent exposure threaten to infect regions beyond the already feverish markets of the Northeast, California, and Florida? Or does the 15 percent represent a modest number of homeowners who could be hurt by a market turndown?

Before last year, Richard Brown, the FDIC's chief economist, would have told you that real estate prices had stayed relatively in check. "Prices had been growing faster, but not that much faster," he said. By the end of 2004, though, prices were rising at an accelerated pace. "

The biggest change the FDIC found was that only 33 of the 55 booms can be explained by local factors, such as supply constraints.

Instead, national factors -- notably more relaxed financing options -- are growing in importance.

Sub-prime lending, zero-interest loans, zero-down payment, and low-interest ARMs all make home buying more affordable. That means more buyers pursuing the limited supply of houses for sale, boosting those home prices. These lending vehicles also leave homebuyers more leveraged than in the past, which can eventually add to market volatility.

In addition, swiftly appreciating markets attract real estate investors and speculators, who are much more likely to bail in a downturn than buyers who are living in their homes.

How vulnerable are the boom markets?

The FDIC defines boom rather conservatively, as a 30 percent or greater rise in median home prices, adjusted for inflation, during a three-year period.

But many U.S. housing markets have had far greater annual appreciation than 10 percent.

Las Vegas was up more than 50 percent in 2004 and several California and Florida regions gained 30 percent or more. Still, those sizzling markets represent a small percentage of the country. Overall, there was an 11 percent increase last year in median home prices.

That did, however, far surpass increases in personal income (5.8 percent) and rental prices (2.7 percent). In the past, periods when housing grew much faster than those metrics were usually followed by a period of stagnation.

The FDIC report points out that busts that have followed booms have tended to be rather mild, more stagnation that catastrophe. No housing bust that followed a boom during the past 25 years exceeded a 20 percent average price drop.

Furthermore, the correlation between boom and bust seems weak. The FDIC reports, "In just 9 of 54 unique boom episodes prior to 1998...did a bust subsequently occur within a five-year window."

As a matter of fact, FDIC data revealed that more housing price busts (12) occurred in markets that hadn't gone through booms than in ones that did. They are more likely to result from collapses in local economic conditions than from mere run-ups in prices.

The most severe housing price drops in recent years occurred during the late 1980s in places that hadn't boomed -- Casper Wyoming, Lafayette Louisiana, Midland Texas, and Anchorage Alaska, so-called oil-patch cities, during down years in the petroleum industry.

So even if housing prices are not booming in your region, it doesn't mean they won't drop. And if your region is booming, you don't necessarily have to brace for a collapse.

But that's based on history. The wild card this time around is the possibility that those new mortgage lending practices that have led to more highly leveraged home buying have introduced a fundamental change to the housing market.

If interest rates climb precipitously, it could leave many homeowners with far larger monthly mortgage payments than they have now. The danger, according to Brown, is not simply that they would lose some of their investment, it's that they could lose their homes.

If that happens, it would also add inventory to the market, forcing prices down. Instead of housing busts resulting in small price downturns we may see large percentage losses.

In any case, what residents of the nation's hottest housing markets should remember, Brown says, is that "Booms don't last forever."

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