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For much of its horrific history, the faces of the deadly opioid epidemic that ravaged the Rust Belt and killed more than 350,000 Americans since 1999 were largely white.

That is rapidly changing as the plague spreads to the big cities on the East Coast, according to a study released Friday by a quartet of researchers led by Stanford University’s Mathew Kiang.

“Although opioid-related mortality has been stereotyped as a rural, low-income phenomenon concentrated among Appalachian or Midwestern states, it has spread rapidly, particularly among the eastern states,” the researchers reported in the study that was published in JAMA.

The result is “a wider range of populations being affected, with the spread of the epidemic from rural to urban areas and considerable increases in opioid-related mortality observed in the black population.”

For example, the death rate from opioids in the District of Columbia, where nearly half the residents are black, has tripled every year since 2013.

Why is it spreading?

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“Heroin,” Kiang told NBC News.

Specifically, fentanyl-laced heroin which is 25 to 50 times more powerful than straight heroin and fast becoming the fix of choice for big city addicts.

“The heroin on the East Coast is much more lethal than the heroin on the West Coast,” Kiang said.

Mike, a heroin addict who wants to get help, prepares to inject himself in the Kensington section of Philadelphia which has become a hub for heroin addicts in Philadelphia on July 21, 2017. Spencer Platt / Getty Images file

Because heroin is so cheap, drug smugglers have taken to lacing it with fentanyl to boost profits and boost the high and often the street-level dealers don't even know they're selling fentanyl-spiked heroin, NBC News reported earlier.

In the west, junkies use black tar heroin that looks more like molasses than the white or brownish powder that predominates on the East Coast. As a result, the heroin that addicts inject in Los Angeles or Seattle is less likely to be laced — or even replaced — with fentanyl, which is also a white powder.

“The worry is fentanyl making its way into other drugs that unsuspecting people use like cocaine,” said Kiang. “That is probably the one people are most concerned about and it has real implications for places like California.”

In an earlier study published last year, Kiang and two other researchers took note of what was already widely accepted — that the epidemic was driven in the 1990s and 2000s “by prescription opioids as part of aggressive marketing techniques by the pharmaceutical industry and changing medical standards in diagnosis and treatment of chronic pain.”

People would get hooked on prescription painkillers and then gravitate to heroin, they said.

But African-Americans were less affected because there were “much lower opioid prescription rates in the black population than in the white population,” the study stated.

In recent years, however, “rates of heroin use and prescription opioid misuse in blacks and whites have begun to converge,” that study said.

From 1999 to 2016, according to the study by Kiang and his collaborators which was released Friday, 231,264 men and 120,366 women “died from opioid-related causes across the whole United States.”

The median death age for men was 39.8 and for women it was 43.5, the researchers found.

"Opioid-related mortality, particularly mortality associated with synthetic opioids, has increased in the eastern United States," they concluded.