A marking element (1) for indicating whether a pre-defined temperature condition has been maintained. The element comprises a first material (7) capable of flowing above a predetermined temperature separated from a second absorbent material (5, 10) by a heat disruptable barrier layer (4). The first and second materials are such that when the barrier layer is punctured and the predetermined temperature is exceeded the first material flows in the second material to produce a detectable change. The heat disruptable barrier layer (4) is comprised of a heat disruptable material (8) associated with an element (9) capable of being inductively heated by electromagnetic energy to effect disruption of said material thereby to activate the marking element.
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