An accurate monitoring of the conditions of a perishable product is achieved by integrating two indicator types - a primary indicator which develops a color change as a result of cumulative time-temperature exposure and a threshold second indicator - into a single device. The combination of the two indicators in the same indicator device provides a human readible signal that gradually and irreversibly develops color as a function of time and temperature and more closely monitors the actual condition of a deteriorative product than does a single indicator. Important features of this integrated monitoring device are reliability and printability. The primary indicator in the system is assisted in color development by the secondary indictor that is set to trigger at a predetermined temperature. When this temperature is reached or exceeded both indicator mechanisms function in an additive mode to cause a break in the typical Arrhenius curve, which a primary indicator alone produces. This can be used to signal actual rather than an apparent the end of a product life. The secondary indicia can be formulated to change very rapidly as would be needed to detect the thawing of a frozen product or modified so as to act more slowly, i.e. over at time-temperature range, and may comprise a substance that melts or changes some physical property at a given temperature.