A seal assembly for use in a pressure containment device for containing a fluid pressure in an annular space around a drill string whilst allowing the drill string to rotate in the pressure containment device. The seal assembly includes a seal with an inner surface and a tubular body, and a wear sensor embedded in the tubular body of the seal. The wear sensor emits an alert signal when the inner surface of the seal has been worn away by a first preset amount. The alert signal includes a pressure signal and/or a chemical signal.