Toenail clippings from over 100,000 people are among the 3.5 million samples of blood, plasma, urine, hair, and other specimens donated by participants in the nearly 40-year-old Nurses’ Health Study and several other large cohort studies that continue to provide new clues about diseases to scientists today. The toenails are housed in the BWH/Harvard Cohorts Biorepository, jointly run by Harvard T.H. Chan School of Public and the Channing Division of Network Medicine at Brigham and Women’s Hospital.

The toenail samples, used in at least 400 research projects on 40 diseases, and pony tail hair samples were the subject of a STAT news story on November 12, 2015. The samples are valuable because they enable scientists to analyze the average level of hormones and trace elements such as selenium and arsenic over a period of months.

“When you take the edge of the nail of all five toes, you have a measure that stretches over the past year,” Shelley Tworoger, associate professor of epidemiology at Harvard Chan School and executive director of the biorepository, who is using the toenails to research ovarian cancer, told STAT.

Meir Stampfer, professor of epidemiology and nutrition, also conducted a study that used the toenails to compare the mercury levels in people who had heart attacks to those who did not. In the end, the researchers concluded mercury levels didn’t have a role in the cardiac issues, he told STAT.

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