"Hair" is a word that is used both ways. If I'm talking about a lot of hair ("I combed my hair this morning"), then "hair" is appropriate. On the other hand, if I'm talking about just a few individual strands of hair ("I noticed a few grey hairs"), then "hairs" is correct.
So, "I noticed a few more grey hairs while I was combing my hair this morning" would be correct usage of both senses.
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