I'm trying to find a short story where the protagonist notices that people who breathe at the same time shortly die together. This is the major theme of the story. It was probably a sci-fi or horror genre and I would have read it in an anthology or story collection book, rather than a magazine. I'd say would be an older story (50s, 60s, 70s) because it was written with the feel of early sci-fi stories. I'd love to read it again if anyone recognises it. Sorry I can't provide more detail.