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A wristwatch-sized air-quality monitor that tells you whether the air you’re
breathing is putting your health at risk has been developed by Da-Ren Chen at
Washington University in St Louis, Missouri. The monitor continually takes air
samples and subjects them to an electric field. Any particles in the air move in
the field, and the smaller they are the faster they move. The monitor is set to
detect the level of particles less than 2.5 micrometres across—the size
liable to irritate the airways. If the level exceeds safety limits, a warning
light on the monitor flashes.