Renters in Pennsylvania with the lowest incomes have the greatest housing needs, according to a study released today by the Federal Reserve Bank of Philadelphia's Community Affairs Department.

The study, based on data collected in 2000 and again in 2005-06, found a significant decrease in the number of rental-housing units that were both affordable and available to the lowest-income renters statewide.

Areas with the greatest concentrations of lowest-income renters who faced shortages and severe cost burdens were Philadelphia's suburban counties - specifically Chester, Delaware and Montgomery - Northeast Pennsylvania, and Centre County, home to Pennsylvania State University's main campus.

Two out of three such renters had severe cost burdens, said the report, written by the department's Erin Mierzwa and consultant Kathryn P. Nelson: More than 50 percent of their incomes went to rent and utilities in 2005-2006.

Nearly half of Pennsylvania's renter households - 1.4 million, representing 2.9 million people - lived in just six counties: Philadelphia, Allegheny, Montgomery, Bucks, Delaware, and Lancaster.

The state's renters tend to be older than those in the rest of the nation, the study shows, meaning that rising rents have a big effect on those with fixed incomes.

Pennsylvania's rental-housing stock also is older, with more than two-fifths of units built before 1950, compared with 24 percent nationwide.

Liz Hersh, executive director of the Housing Alliance of Pennsylvania in Glenside, said the study demonstrated the need for some kind of government assistance to renters, as well as to landlords to rehab these older units.

"If you look at the ads, we are oversupplied at the high end as the need for affordable apartments grows," Hersh said. "There is also more competition for rental units, especially from people who have lost their homes through foreclosure."

The study sheds much-needed light on the failure to "grow the supply" of affordable rental housing, Hersh said.

"We started the decade 170,000 units short, and that shortage has increased by 50,000 units since 2000, according to the study," she said. "The market is definitely out of whack on the supply side, as well as the affordability side."

Mierzwa and Nelson said housing experts were predicting that the need for additional affordable rental housing would continue to grow and that rental housing would become an even more important aspect of national housing-policy discussions.

Contact real estate writer Alan J. Heavens at 215-854-2472 or aheavens@phillynews.com.