The Charleston Gazette-Mail reports that police in Huntington, W.Va., responded to 26 heroin overdose cases in a span of four hours on Monday evening.

To get a sense of the scale of the outbreak, consider this: Huntington is a small city with a population of about 49,000 people, according to the Census Bureau. An overdose outbreak of similar magnitude in New York City (population 8.4 million) would affect more than 4,400 people.

The cases overwhelmed first responders in Huntington. “Every ambulance in the city of Huntington went out in 10 minutes on these overdoses,” Cabell County EMS director Gordon Merry told 13 News. He said all of the overdose victims were able to be revived using naloxone, a drug that reverses the effects of an overdose.

In 2014, West Virginia led the nation in drug poisoning deaths, with 36 for every 100,000 residents. That’s head and shoulders above the next-highest state death rate – 28 deaths per 100,000 in New Mexico – and more than double the national rate of 15 deaths per 100,000, according to the Centers for Disease Control and Prevention.

In terms of both rates and raw numbers, drug overdose deaths have exploded in recent decades. Since 1982, the drug overdose mortality rate has risen by 425 percent, adjusting for population.

They have eclipsed motor vehicle fatalities as a leading cause of death in the United States, according to a new working paper from Christopher J. Ruhm from the University of Virginia. In 1982, motor vehicle deaths were seven times more common than drug overdose deaths, according to Ruhm’s analysis of CDC data. By 2014, overdose deaths had become considerably more common than vehicle fatalities.

Authorities in Huntington said the heroin involved in Monday’s overdoses was laced with another, as yet unknown substance. “Right now we don’t know what it’s been cut with,” Merry told the Huntington Herald-Dispatch. These combination overdoses, involving heroin and other powerful drugs such as the painkiller fentanyl or the sedative carfentanil, are becoming more common.

Ruhm’s paper underscores the dangers posed by these powerful opiate cocktails. One challenge in tracking the drug overdose epidemic is incomplete data on the types of drugs used. The CDC tallies this using death certificates issued by local authorities, but the accuracy and completeness of these records varies.

Nearly half of overdose death certificates included mentions of “unspecified” drugs in 2014. In nearly 20 percent of all overdose deaths, an “unspecified” drug is the only drug mentioned, meaning that authorities classified the death as an overdose but were not able to determine what drug caused it.

Ruhm attempted to fill in these blank spots statistically. Some counties are much better about including complete drug information on death certificates than others, and he used their information to extrapolate what caused overdoses in counties that were similar in demographic and drug use patterns.

The resulting analysis suggests that deadly combinations of drugs are a much bigger factor in overdose deaths than the raw death certificate data shows. In the CDC’s raw numbers, for instance, 34.4 percent of overdoses involve the use of more than one drug. But in Ruhm’s adjusted numbers, that figure rises to 49.3 percent of overdose deaths.

The numbers underscore a key fact about buying drugs on the black market: Users often don’t know exactly what they’re going to get. Some users may seek out more powerful drugs like fentanyl and carfentanil for the more powerful high. But others may have no idea that these drugs may be included in the heroin they’re getting from dealers. And dealers may not even know it themselves.

Places like Huntington are at the leading edge of these trends. For residents of the city, these overdose statistics are not abstract numbers. They represent colleagues, friends and family members. Merry issued a plea to residents on Monday.

“If you have heroin please see what is going on and don’t use it,” he told the Huntington Herald-Dispatch. “It could be your last time.”