Is a bmi of 15 dangerous?

I'm underweight but I still look kinda stocky and don't really like the way my body looks. I don't think 4kg are going to make a difference. I know a bmi of 13 is dangerous but a bmi of 16 is fine. so I'm not sure. just wondering if It's safe to be that weight. I'm not exactly anorexic or anything by the way.

I'm currently 52kg for 5ft8, want to be 45kg or so. not lower.
Is a bmi of 15 dangerous?
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