A method and apparatus for an irreversible temperature sensor for measuring a peak exposure temperature. The apparatus is fabricated by printing an admixture of conductive nanoparticles on a dielectric substrate to form a film. The film has an electrical resistance that is inversely proportional to the exposure temperature. The electrical resistance also irreversibly decreases as the exposure temperature of the film increases. A portion of the film is exposed to a pulse of electromagnetic energy sufficient to render it substantially more electrically conductive than the portion that was not exposed. In use, the peak exposure temperature is determined by measuring the electrical resistance of the non-altered portion of the film and the electrical resistance of the portion that was exposed to the pulse of electromagnetic energy, and subtracting the electrical resistance of the altered portion from the electrical resistance of the portion that was not altered, to provide a difference value. The peak exposure temperature is then be calculated as a function of the difference value.