Think of it like thin fishing line. If it gets loose, it forms loops that will not break or chew thru. This is the dangerous part. A small finger or toe gets caught in that loop and can't break it. I am not usually that concerned about cotton vs. poly cotton thread except when making baby quilts. Then I use 100% cotton thread for this very reason. If after repeated washings a thread breaks, I can repair it. But in reality, the quilts we make today, while treasured, are not kept as heirlooms.