Two years ago, when U.Va. Ph.D. student Thomas Talhelm was a Fulbright scholar in Beijing, he built his own air purifier after growing concerned about the city’s notorious pollution. To test his handiwork, he spent about $260 for a portable device that counts tiny particles in the air. “I always had the intuition that indoor air was cleaner than outdoor air, and that’s borne out in the data very clearly,” he said. Mr. Talhelm, who used a device made by a California company called Dylos, also noticed that cooking could cause indoor pollution to spike.