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What hypothesis, if any, for this association between exfoliation glaucoma and latitude?

http://www.ophthalmologyweb.com/1317-Journal-Updates/37147-Demographic-and-Geographic-Features-of-Exfoliation-Glaucoma-in-2-United-States-Based-Prospective-Cohorts/

http://online.wsj.com/article/SB10001424052970204331304577144853241744454.html

Geographical glaucoma: People in the U.S. who spend most of their lives below 41 degrees north latitude, which passes through Chicago, have a significantly lower risk of developing exfoliation glaucoma than residents in more northerly locations, according to research published in the journal Ophthalmology.
Exfoliation glaucoma is characterized by protein deposits in the eye, which clog drainage channels and cause vision loss. Exfoliation glaucoma is the leading cause of a secondary condition and a common form of the disease, open-angle glaucoma.

Boston researchers analyzed data on U.S. residency for 78,955 women and 41,191 men from two national health studies. Participants completed biannual questionnaires from 1980 to 2008. During the study period, 5,022 women and 1,848 men had a physician-confirmed glaucoma diagnosis such as elevated eye pressure, open-angle glaucoma or another type of glaucoma. Exfoliation glaucoma was confirmed in 5.7% of women and 3.3% of men.

Living below 41 degrees north latitude was associated with a 47% reduced risk of suspected or confirmed exfoliation glaucoma, and below 37 degrees north latitude, which passes through San Francisco, with a 75% reduced risk. Men were 68% less likely to develop the condition than women.

Researchers determined that iris color and ancestry weren't risk factors. Identifying environmental factors in glaucoma could reduce vision loss, researchers said.

Caveat: The relationship of eye color and glaucoma was examined only in men. Iris color was self-reported by the subjects so classification errors are possible, the researchers said.