Would one cm nodule in breast be a health risk?

Nodule: A one cm nodule can be a variety of things. A breast ultrasound is needed to diagnose what the nodule is. It could be a benign cyst or benign solid lesion, such as a fibroadenoma. It could also be a small cancer. Ultrasound imaging will determine whether a biopsy will be needed or not.

It depends: on its clinical and imaging features. It could be completely benign, or a cancer. In this respect, size doesn't really matter.