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Seattle and King County set a new record for fatal overdoses with 359 deaths in 2016, according to new data from the King County Medical Examiner’s office.

The bulk of those deaths – 61 percent – involved opioids, both in synthetic form such as oxycodone or methadone, and opiates derived from poppies such as heroin and morphine. The number of overdose deaths exceeds the 2015 totals by 14 additional fatalities and marks the seventh straight year of increases.

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And hidden within the data is a particularly worrying record for health experts: The medical examiner’s office last year linked 22 deaths to the powerful, synthetic opiate fentanyl – up from a total of five in 2015 and far more than has ever been recorded in a single year in King County. The previous high count of fentanyl deaths was eight in 2010.

“Fentanyl is a big concern,” said Dr. Richard Harruff, the King County Medical Examiner. Harruff said the reasons for the spike in fentanyl deaths isn’t clear. But, he added, synthetic opioids still remain far less prevalent in the Puget Sound than elsewhere in the country.

“The problem here (with synthetic opiates) is less than in the Midwest, for example,” he said. “We see much more of the natural opiates such as heroin.”

The preliminary 2016 numbers – not yet finalized – show a 27 percent increase in fatal overdoses since 2010. But while both synthetic and natural forms of opium remain fully or partly responsible for the majority of the fatalities, they have remained, on average, a steady 63 percent of the overall total for the past seven years.

For example, 220 people died fully or in part from opioids in 2016. The remaining 139 fatal overdoses that year included causes ranging from methamphetamine and alcohol to carbon monoxide, which typically occurs from inhaling automotive exhaust — often as a suicide.

Suicides are included in the overdose totals. According to medical examiner’s office data, suicide typically accounts for 6-to-8 percent of the fatal overdoses each year.

In fewer than half of the fatal overdoses, only one substance is involved, autopsies show. This, Harruff said, can make it difficult or impossible to pin down a single cause.

Heroin and morphine alone remain the top single factor in fatal overdoses, with 113 (31 percent) in King County. (Harruff added that morphine and heroin get grouped together because the human body converts heroin into morphine. This means an autopsy often will test positive for morphine, which actually entered the body as heroin.)

In Washington state, heroin almost exclusively is Mexican Black Tar, not Chinese heroin, which typically is found 140 miles to the north in Vancouver, B.C. Harruff said the source of the increase in fentanyl remains unknown, but authorities suspect domestic illicit labs or Chinese synthetic opioids imported into Canada and smuggled across the border.

For all of the problems associated with synthetic opioids in the Midwest and southern U.S., it has been in steady decline locally. The number of deaths related specifically to synthetic painkillers peaked in 2009 and has continued a downward trend since. But, as Harruff pointed out, that decline was completely offset by a near exact rise in heroin and morphine-related deaths.

“Heroin has always had a grip here,” he said.