It comes from the natural decay of uranium that is found in nearly all soils.

Diogenes Laërtius says that he died at a great age, and of natural decay.

While the world's natural decay works itself through, we sleep here.

Every 12.3 years, half of what exists disappears through natural decay, he said.

An example is the natural decay of potassium-40 to the argon-40 which forms most of the argon in air.

The samples show no traces of anything beyond natural decay.

The main isotope responsible is xenon-135, mainly produced by natural decay of iodine-135.

It was in a bad state of repair, having suffered from both natural decay and bomb damage.

Others have been destroyed by fire or by natural decay.

It is a measure of the natural decay within a social system.