Patent ID: 6991674

Claim:
In a combination of a closed compartment associated with an air circulation system including at least one blower associated with an air duct for effecting a flow of air between an inlet located exteriorly of said compartment and an outlet located interiorly of said compartment, with this flow passing through at least one air filter element, and at least one interior compartment sensor for detecting the degree of contamination of the air and being coupled to a control arrangement, the improvement comprising: said control arrangement including a memory; a performance map of the normal operation of the air circulation system being stored in said memory and containing information indicating a pre-determined decrease over time of the degree of contamination of the air by a known airborne contaminant; and said control arrangement including a comparing circuit for comparing the actual degree of contamination of said air over time with the values contained in said map, and for issuing a warning signal upon finding a deviation from said values contained in said map.