Linsey Marr, an environmental engineer, stood next to a pair of clear plastic boxes packed with tubes, nozzles and electronics, an odd-looking prototype that one day might serve to protect children in day care from airborne pathogens.

A nozzle filled the right-hand box with a faint silvery mist. A pump pulled some of that air into the left-hand box, where a sampler trapped floating particles and droplets. Soon, a digital screen bolted to the box turned red: “Detected! Dust mite allergen Der f 1.”

A protein shed by dust mites, Der f 1 can trigger asthma attacks when inhaled. Dr. Marr’s device had detected 843 picograms of Der f 1 per cubic meter. A single grain of salt is about 10 million times as heavy.