Harvard Public Health Review
Spring 2005

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Student Profile

How Do You Solve Environmental Health Problems?
Do the Math.

Brisa Sanchez knows what counts--and not just because she's a whiz at inventing new biostatistical methods to explain links between environmental hazards and health. This doctoral student, now in her fourth year, is also deeply invested in applying what she's learned to the real world. Whether she is quantifying the relationship between children's neurological development and their exposure to lead in the womb, or exploring whether fast-food restaurants cluster around schools, Sánchez wants to give physician-scientists and policy makers the precision tools they need to keep people healthy.

"It takes most students a few years to understand the link between theory and application, and to grasp that the best research comes out of working on real-world problems," says Sánchez's adviser, Harvard School of Public Health Professor of Biostatistics Louise Ryan. "Brisa already understands that."

From a Mexican Border Town
The 26-year-old Sánchez--who in 2002 received a Howard Hughes Medical Institute Pre-doctoral Fellowship in Biological Sciences--is a natural at blending the abstract and the practical. Born and raised in the small Mexican border town of Palomas, Chihuahua, she says she learned to add and subtract while making change for customers at the family grocery store. Because the local school only went up to ninth grade, her parents had her bused to Deming, New Mexico, 36 miles away, for high school. Though in algebra class Sánchez hit a bump with word problems because she'd only just begun to learn English, her math skills flourished.

But it wasn't until 1999, between her junior and senior years as a theoretical math major at the University of Texas–El Paso, that Sánchez put the conceptual and the concrete together in terms of a career. That summer, she was one of 20 minority students selected to attend Cornell University's Mathematical and Theoretical Biology Institute, where she helped assess the usefulness of prostate-specific antigen ("PSA") in detecting prostate cancer. Her academic mentors not only showed her how to apply math to medicine, but also encouraged her to pursue research.

"That was a turning point. I thought, 'Maybe I have the aptitude to do pure math, but I want to be closer to the people,'" says Sanchez, who nixed thoughts of teaching secondary school math or attending a pure-math graduate program after that. "I wanted to be closer to real data, closer to public health."

At HSPH, Sánchez is working to make structural equation models (SEMs) used in social science "more robust"--that is, less dependent on the assumptions that define variables and relationships between them, such as the assumption that there's a linear relationship between environmental exposure and health outcomes. Her goal is to apply SEMs to questions in environmental health, where the sheer number of variables can make standard statistical analysis unwieldy. Social scientists, for example, use SEMs to derive intelligence quotients--to take the results of, say, five intelligence tests and compute a single score. Sanchez wants to adapt this same distillation process to environmental science.

Examining Fetal Lead Exposure
To that end, she's applying SEMs to data collected by the School's Metals Epidemiology Research Group in a large, 12-year study based in Mexico City of the physical and cognitive effects of maternal lead exposure on the unborn child. Researchers have measured lead in the blood, bones, and umbilical cords of pregnant women as well as the blood of their babies, first at birth, then every two years up to age four. The study, led by Howard Hu, HSPH professor of occupational and environmental medicine, will test the children at six-month intervals to assess their neurodevelopment.

"There are so many results out there. How can we synthesize them?" says Sánchez, with a smile that lights up her Cambridge living room. "That's where I come in."

Hu is impressed not only with Sánchez's quantitative and teaching skills--she's mentoring doctoral and post-doctoral trainees--but also with her commitment to what he calls "the broader social implications of research."
" We have sophisticated quantitative biological markers of lead exposure, lead dose, early neurotoxic effects, and years of follow-up on thousands of subjects. Brisa is figuring out how to incorporate all these markers into statistical models that accurately depict their interrelationships," Hu explains. "The result will be a much more precise understanding of how well different markers of lead exposure predict disease."

In the process, Sánchez also hopes to develop and apply statistical methods that correct for errors in measurement. "When your data is measured with a degree of error, you are less able to quantify the relationships between variables, and your conclusion becomes a lot weaker," she explains. "In the environmental setting, for example, we know that if there's measurement error for lead in the mother's bones, the relationship between fetal exposure to lead and neurodevelopment will be clouded."

What's enabled Sánchez to hit the ground running, notes Louise Ryan, is her grasp of the collaborative process, as demonstrated in another project that entails measuring exposure among newborns in intensive-care units to near-ubiquitous compounds called phthalates. "Brisa's willing to dive in and get her hands dirty," says Ryan. But to hear Sánchez tell it, the collaborative nature of biostatistics is just part of its attraction. "The best thing about being a statistician," she says, quoting the field's legendary John W. Tukey, "is that you get to play in everyone else's backyard."

"There's such a mix of things going on," Sánchez says, tallying up the advantages HSPH has to offer. "It's amazing, the number of people who are bringing biostatistics to all areas, from AIDS to cancer research to neurostatistics to computational biology. So many choices!"

Thea Singer has written about health, science, and the arts for the Boston Globe, the Washington Post, Natural Health and other publications.

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