“Period syncing” is the occasional punchline, but it’s an issue for women who live and work together in various stages of their cycles. Studies have shown that odorless compounds from the armpits of women in various stages of their cycles could affect the cycle length of other women. And it doesn’t require proximity: Pumping these chemicals into the air synced the cycles of rats housed separately. So if you can smell your coworker’s lunch over the cubicle wall, you could be close enough to hop on her menstrual calendar.
MORE: Ask a Scientist: Is Period Syncing for Real?

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