Just under two miles stretch between the Boston subway stops for Back Bay and Roxbury Crossing, but the distance might as well separate two worlds. In less time than it takes for commuters to scan their newspaper headlines, the train has left one of the city's healthiest neighborhoods and pulled into one where residents are more likely to suffer from preventable diseases, such as tuberculosis and HIV/AIDS.

The federal government and every state health department grapple with such stark differences in health, hampered in the quest to understand their causes by inadequate data and analytic methods. But now, a team of HSPH researchers has come up with a reliable tool for health officials to monitor inequalities by focusing on the pervasive power of poverty to undermine health. The effort should help untangle why some groups of Americans thrive while others languish.

"Clearly, poverty creates health disparities," says HSPH associate professor Nancy Krieger, who led the team. "Our work paints a vivid picture of powerful socioeconomic gradients across a wide range of health measures. It also shows that deprivation exacerbates racial/ethnic health disparities and helps account for a large fraction of the total burden of disease, disability, and premature death."

Missing in Action
Public health researchers have long known that poverty erodes health. As early as 1826, French physician Louis René Villermé showed that poorer neighborhoods in Paris had higher mortality rates. In the U.S., epidemiologist Edgar Sydenstricker broke ground in the 1930s with a 10-city study of how the Depression impacted people's health. Since then, scores of health studies in this country and elsewhere have demonstrated the influence of poverty and other socioeconomic factors, such as education levels, incomes, and occupations.

Yet, for all that evidence, major U.S. public health surveillance systems consistently fail to collect meaningful socioeconomic data. That's because the sources from which they draw, such as cancer registries and death certificates, include little or no such information.

"The purpose of collecting public health data is to monitor the health of Americans," says Krieger. "But when those surveillance systems do not gather fundamental data on socioeconomic factors, we end up with a very incomplete picture."

To round out that picture by adding socioeconomic influences, the HSPH team established the Public Health Disparities Geocoding Project. In addition to Krieger, the team consists of project director Pamela Waterman, HSPH assistant professor S.V. Subramanian, research scientist Jarvis Chen, and doctoral student David Rehkopf.

The goal was to figure out how to link health and socioeconomic information in order to find a cost-effective way for health departments and researchers alike to routinely monitor socioeconomic disparities in health, so that improvements--or setbacks--in reducing them could readily be known.

The team set to work, combing through more than 760,000 department of public health records from Massachusetts and Rhode Island. They gathered information on 18 commonly documented health outcomes that span the life course, from low birth weight to premature death.

The researchers sent addresses from the health records to a private "geocoding" company, which identified the ZIP code, census tract, and census block group into which each address fell. Established by the U.S. Census Bureau, census tracts are subdivisions of counties containing about 4,000 people; block groups are smaller areas nested within census tracts. All three geographic areas are gateways to information about an area's average income, education level, and typical jobs, as well as other demographic characteristics.

By matching census data to health statistics, the researchers found that the single best socioeconomic predictor of health inequalities turns out to be poverty, measured at the level of the census tract. Nearly all 18 health outcomes worsened as poverty in census tracts increased.

"Seems like a 'no duh' deduction, doesn't it?" says Pam Waterman. "When I told my uncle in Brooklyn about it, he said he didn't need to go to Harvard to figure out that poverty affects people's health. But the fact is, research has been clouded by not having an accurate, easy-to-interpret way of measuring the impact on health of socioeconomic factors."

The team's work also extended the capacity of surveillance systems to explain, and not simply describe, racial/ethnic health disparities. For nearly all 18 health outcomes, Blacks and Hispanics had higher risks than Whites. When the researchers controlled for poverty in census tracts--conceptually putting all of the racial/ethnic groups on the same economic footing--the excess health risks faced by Blacks and Hispanics for nearly all of the outcomes dropped substantially. For some conditions, like childhood lead poisoning, the risk was halved. But the differential did not disappear.

These results, published in the American Journal of Public Health in February 2005, did not surprise the team. Numerous studies have shown that populations of color in the U.S. suffer worse health than Whites, owing to factors as varied as economic deprivation; hazardous conditions at work and in neighborhoods; deteriorated housing; less availability of affordable, healthy foods; targeted marketing of unhealthy products, such as tobacco; lack of health insurance, and less access to quality health care--all of which may be exacerbated by racial discrimination.

While recognizing that the project's methods cannot address all of these complexities, Krieger says they "show what would be possible if we had much less poverty."

Spreading the Word
The Public Health Disparities Geo-coding Project's research method can be used by local, state, and national agencies across health outcomes, for any racial/ethnic group, and for both genders. Every state health department and cancer registry in the country has received a how-to guide, which is also available free of charge on the web at http://www.hsph.harvard.edu/thegeocodingproject/ webpage/monograph/.
Word of the tool is spreading fast. Krieger has given dozens of talks around the country. And with a grant from the National Institutes of Health, the HSPH team will conduct training sessions for professionals from public health departments, the Centers for Disease Control and Prevention, universities, and cancer registries.

In a pilot project for Mayor Thomas Menino's Task Force to Eliminate Health Disparities, the Boston Public Health Commission used the tool and concluded that the higher the poverty rate in Boston's census tracts, the higher the rate of premature death. Says Director of Research Mary Ostrem: "Using poverty as an indicator for poor health helps point to the fact that, regardless of anything you do personally to be healthier, where you live can affect your health."

Says Krieger, "The field of public health cannot, by itself, improve health and prevent disease. Effort across all of society is required. But it is the public health profession's singular task--and fundamental responsibility--to provide the data needed to identify, and ultimately reduce, inequalities in health that persist across this country."

Christina Roache is the editor of Harvard Public Health NOW, a newsletter published biweekly for students, faculty, and friends of HSPH. NOW can be read online at www.hsph.harvard.edu/now.