A little smaller than a deck of cards, it’s designed to clip on the side of a backpack or bag, so you can track changing pollution levels as you walk down the street. A tiny fan inside the gadget pulls in air and pushes it through chambers with sensors that measure pollutants like particulate matter, microscopic particles of soot, and other pollution that can worsen asthma and cause other health problems. The sensors measure three sizes of particulate matter—PM 1, PM 2.5, and PM 10. The smaller the particle, the more likely it will enter the bloodstream. The device also measures volatile organic compounds, gases that are also linked to disease. On a connected app, it tracks pollution levels in real time—with alerts if pollution gets particularly bad—along with temperature, humidity, atmospheric pressure, and altitude.