As the U.S housing market climbs back to healthy, a third of it is less affordable now than it's been all century, according to RealtyTrac .
The firm's latest housing affordability report , released Thursday, found that 34 percent of the 1,200 U.S. counties it surveyed are at their least affordable, on average, since 2000.
At the same time, none of the counties RealtyTrac looked are considered at the "risky" end of the affordability scale.
"None of the counties we analyzed for the second quarter has regressed to the dangerously low affordability levels reached during the housing price bubble," said Daren Blomquist, VP at RealtyTrac.
Though a third of U.S. counties are less affordable these days, the upside is that 81 percent of the overall population still lives where income is enough to afford a decent home, Blomquist said.
Most affordable markets are in the middle of the country. Columbus, Oklahoma City, Omaha, Des Moines, and Minneapolis all have counties in which buyers need 20 percent or less of the median income to buy a median-priced home. Unemployment rates in these areas are also 5 percent or lower, Blomquist said.
San Francisco County, by far, leads the field of less affordable places. Eighty percent of that market is considered less affordable by RealtyTrac's data. Multnomah County, Oregon, and Travis County, Texas, home to Portland and Austin, respectively, share second place, with about 35 percent of homes there considered less affordable.
A few counties that serve mainly as vacation markets were also labeled "inherently unaffordable." These include Nantucket County, Massachusetts; Teton County, Wyoming; and Pitkin County, Colorado; as well as four of the five boroughs in New York City, and Los Angeles County.
The trend toward what the National Association of Home Builders  (NAHB) referred to as the "price out effect"  in its own recent study is of concern. NAHB found that as many as 206,269 households are effectively priced out of the market for each $1,000 increase in national median new home prices.
RealtyTrac found that if interest rates rise just a quarter percentage point, 461 counties—with a combined population of 77 million, or 30 percent of total U.S. population—will exceed their historical averages for affordability and 27 will exceed their historical peaks. If interest rates rise by a full percentage point, 630 counties, comprising 46 percent of the American people will exceed their historical affordability averages, while 59 will exceed their peaks.
The Federal Reserve  expects interest rates to rise again in 2015.