Mayor Mark Farrell has said, repeatedly, in recent weeks that the problem of discarded syringes on city streets has become a sticking point for him, and the city promised millions of dollars to curb the problem of hazardous waste on sidewalks and streets.

Meanwhile, San Francisco Chronicle’s Matier and Ross chimed in Wednesday with an uncomfortable observation: Most of the needles littering streets in downtown neighborhoods came by way of the city itself, as part of the Department of Public Health’s 25-year-old needle exchange program.

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According to the paper, the city hands out millions of syringes each year but collects roughly 60 percent of them back.

While not all of them end up on the streets—and indeed, it’s not possible to source where every single needle in the citywide haystack comes from—the numbers make it hard to conclude anything except that San Francisco is helping to supply the materials for a pressing public health problem.

As for why the city does this in the first place, here’s a rundown of the relevant numbers:

SF Director of Health Barbara Garcia estimated in 2016 that San Francisco has about 22,000 intravenous drug users, about one per every 38.49 residents based on a rough 2016 population of 846,816.

The city distributes approximately 400,000 syringes per month, about 18 per person, or one every one and a half to two days, between 4.45 to 4.8 million annually.

The reason the city does this is because in 2016 San Francisco had roughly 16,000 residents living with HIV and some 13,000 people with hepatitis C. The Center For Disease Control consistently reports that free needle programs significantly reduce transmission rates for blood-born diseases and that they’re cheaper than the additional public health costs from more infections.

Of the 400,000 or so monthly distributed needles, the city collects approximately 246,000 back on a monthly basis, and the Department of Public Works estimates about 12,640 it nets during monthly cleanups.

The Department of Public Health hopes that proposed safe injection sites will further reduce the number of discarded needles, estimating in 2016 that some 85 percent of the city’s injection drug users would utilize such services at least some of the time if available.

Although discarded needles pose a serious health risk to the general public, the Chronicle does note that “there are no known cases of disease from needle sticks in San Francisco.”