Volunteer Patrick Pezzati searches yards in Turners Falls, Mass., for discarded heroin needles. Karen Brown/WFCR hide caption

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Uzuri Pease-Greene, right, leads a walk through the public housing complex in the Potrero Hill neighborhood of San Francisco where her family lives. She is working to have the old buildings replaced. Talia Herman for NPR hide caption

itoggle caption Talia Herman for NPR

Closing loopholes in background checks for gun purchases would reduce the risk of death and injury, doctors' and attorneys' groups say. Alexa Miller/Getty Images hide caption

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The Pseudomonas stutzeri bacterium, commonly found in soil, was the most prevalent subway microbe. Lower Manhattan was its prime hangout. Mason/Cell Systems 2015 hide caption

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The UASK app helps sexually assaulted college students in D.C. access a range of services, from rides to the hospital to phone numbers for counselors. The information is personalized to their school. Another version of the app, ASK, provides the same resources to non-students. Emily Jan/NPR hide caption

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Vapor from an e-cigarette obscures the user's face in a London coffee bar. Dan Kitwood/Getty Images hide caption

itoggle caption Dan Kitwood/Getty Images