They're called "heroin tourists" the hundreds of people traveling to Philadelphia every month because of its reputation for having purer heroin with better highs than other metropolitan areas.

Consequently, overdoses in public places have surged, including public libraries, where librarians have begun to stock Narcan to counteract overdoses that occur there.

Librarians have been called into duty so often to revive overdose victims, they have learned to tell the difference between a regular heroin overdose and a more deadly fentanyl overdose, by the sound the victim makes while collapsing, a story in Philly.com reports.

But they are not just using bathrooms, overdoses occur on the lawns and property of public libraries, and other public places, one of which has been dubbed "needle park" for its use by addicts to shoot up.

The Philadelphia Department of Health reported in February about an "alarming increase" in overdose deaths in the city.

During the first five days of December, the city reported 35 deaths from heroin overdoses, most included an amount of fentanyl, a synthetic type of heroin that can be as much as 50 times more dangerous.

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The 12 drug deaths reported on December 1, 2016 were more than had ever been reported on a single day to the Medical Examiner’s Office.