Poverty, long known to be a major factor behind the HIV epidemic in urban areas, is such a powerful force that income and related measures are better predictors of who will get infected than whether a person exchanges sex for money, according to a new federal study of heterosexuals in 24 cities.

The study, published Thursday by the Centers for Disease Control and Prevention, was too small to break out findings on Philadelphia or the other cities. But it helps explain why Philadelphia has some of the highest HIV rates in the country, as Philadelphia is among the most impoverished of big cities.

The report emphasized that in the communities studied, poverty was more closely linked to HIV than was behavior traditionally seen as driving the epidemic.

Smoking crack cocaine and exchanging sex for money, for example, "are probably symptoms of the socioeconomic conditions in these areas rather then independent risk factors," Paul Denning, lead author of the report, said in an interview.

The report did not look at specific prevention strategies. By coincidence, the Philadelphia Department of Public Health on Thursday announced that, four months after introducing its custom-labeled Freedom Condom, it had distributed more than one million free prophylactics this year and was on target to give out 2.5 million, up from one million last year.

The condom campaign is aimed at adolescents, whose high rates of sexually transmitted diseases and low rates of safe sex put them at particular risk of contracting HIV. Most of the condoms have been distributed through local community and health centers, barber shops, and other retailers; the city's new youth-focused website www.takecontrolphilly.org has handled more than 1,700 condom mail orders.

Although the condoms are a prevention strategy based on behavior, poverty plays a role there as well, said Kathleen A. Brady, medical director of the city's AIDS Activities Coordinating Office.

"Condoms cost money. Distributing free condoms is a way to overcome an access barrier," she said.

Data collected by the city independently of the new federal study show that Philadelphia's overall infection rate - based on a far broader population than included in the new report - was about 1.2 percent, and much higher in zip codes where high percentages of residents live in poverty.

Amy Nunn, an assistant professor of medicine at Brown University, said the new report seemed to represent a significant change in federal emphasis on prevention strategies. She pointed to the seemingly subtle wording in the first paragraph: ". . . HIV prevention efforts targeted at heterosexuals in urban areas with high AIDS prevalence should be focused on those with lower SES" - socioeconomic status.

"The resources need to be allocated to the communities that are hardest hit, and that is not always the case," said Nunn, who has studied community-based approaches to prevention in Philadelphia.

"Once HIV takes root in a community, it puts everyone in that community at risk even if people are not necessarily engaging in higher-risk behaviors. For example, a white woman in Center City who is in a monogamous sexual relationship is less likely to come in contact with the virus than a monogamous black woman who is living in a poor area like Germantown," she said, "and that is simply because there is more virus in that community, and people are more likely to choose sexual partners who live close to their home."

The new analysis, published in the CDC's Morbidity and Mortality Weekly Report, is an expansion of findings presented at last year's international AIDS conference in Vienna. It is based on interviews with 14,837 heterosexual adults in 24 urban areas with high HIV prevalence in 2006 and 2007.

Men who said they had sex with men and people who acknowledged ever injecting drugs - the two major means of transmission besides heterosexual relations - were excluded from the study so researchers could focus on the heterosexual epidemic that is far greater in urban areas than nationally.

Overall, about 2 percent of those surveyed were infected with HIV - between 10 and 20 times the national rate for heterosexuals who do not inject drugs.

"Two percent is what you see in some of sub-Saharan Africa or places like Haiti," Denning said.

Within that group, rates were two to three times higher for those with various higher measures of socioeconomic status compared to lower - for example, HIV prevalence was 2.8 percent among participants with less than a high school education vs. 1.2 percent among those who went beyond high school.

These differences proved more significant than not only certain behaviors but also race and ethnicity. The high rates of HIV among blacks and Latinos compared with whites disappeared when researchers controlled for poverty.

Although this study focused on heterosexuals, other data from the National HIV Behavioral Surveillance System confirm the importance of poverty as a risk factor among injection drug users and men who have sex with men, said Denning, a member of the team that collects that data.

The findings were no surprise to people who work with HIV in urban areas. Still, they help explain what is behind the high rates in poor communities, said Brady, of the city's AIDS office.

"In those areas there is not just poverty but crack cocaine use and higher crime, but it is not those things" that drive the infection, said Brady. "It is mainly poverty."

Contact staff writer Don Sapatkin at 215-854-2617 or dsapatkin@phillynews.com.