The other day I was talking to a friend whose PhD work (many years ago) involved experiments to determine whether it was safe to expose humans to ultrasound, for example in sonography. The hypothesis was that heating effects are the primary way that ultrasound causes tissue damage. One of the experiments involved determining the length of time required to create lesions in soft tissue when it is heated to various temperatures. As expected, a nice curve emerged. One of the students in the lab had the bright idea of extrapolating this curve to see how long it took for tissue damage to occur at body temperature; the result was not far from the average human lifespan. The implication is that at 98.6 degrees, we are slowly cooking ourselves to death.