Overdose deaths among Washington heroin addicts -- a key barometer of heroin availability on the streets -- have increased dramatically, more than doubling from 15 in the first six months last year to 33 this year, city officials report.

Also, officials say, most other indicators of heroin use are up; new addicts are flooding treatment centers for help; heroin traces in the urine of criminal defendants in Superior Court have increased; the purity and potency of heroin seized by police on the street are up.

Now, the increase, which began more than a year ago with an influx of Iranian and other Middle East heroin into the United States, has a new wrinkle. Many of the addicts appear older than the average, and many are combining alcohol in large quantities with heroin, law enforcement officials say.

Seventeen of the 33 addicts who died of overdose so far this year, for example, also had high levels of alchol in their blood, according to D.C. Medical Examiner James L. Luke, and 13 of the dead addicts were 35 years old or older, much higher than the average addict age.

"It's really bad," said Kurt Brandt, medical director at the city's alcohol and drug abuse services administration who regularly interviews heroin addicts seeking treatment. "It [heroin] is increasing in quality. There is more of it. We are seeing more people coming in."

"I am seeing an appalling amount of heroin on the street," said Gen. Hassan Jeru-Ahmed, director of the Blackman's Development Center and a veteran observer of the city's drug scene. "It's extremely strong and volatile."

In the past, most heroin coming into the city has been impure brown Mexican heroin, called "Mexican mud," or white heroin originating from the poppy fields of the "Golden Triangle," a remote hilly area in Southeast Asia where the borders of Thailand, Laos and Burma meet.

More recently, according to Frank Pryal, intelligence supervisor for the Washington office of the Drug Enforcement Administration, additional supplies have been coming from the Middle East, especially Iran, Turkey, Pakistan and Afghanistan.

He said the heroin generally is smuggled through Europe by couriers and then into east coast U.S. cities, including Washington.

The marked upsurge in heroin availability here is evident in the growing narcotic treatment population, Brandt said.

Heroin addicts seeking treatment at city facilities for the first four months of this year totaled 983, more than one-third of the 2,849 treated in all of 1979. The monthly average of heroin addicts seeking addiction treatment at city facilities is up from 1,972 in 1979 to 2,063 in th first four months of this year.

Similarly, new admissions -- addicts seeking treatment at city facilities for the first time -- numbered 350 in the first four months of this year, more than one-third of the 897 new admissions in all of 1979 and more than half of the 623 new admissions in 1977.

Also, the average monthly percentage of criminal defendants arraigned in D.C. Superior Court showing heroin traces in urinalyses is up from the 10 percent in 1979 to 14 percent for the first four months of this year, according to official figures. This is the highest percentage in five years.

In addition to the 33 heroin-related deaths for this year, there has been 149 heroin-related emergency room "episodes" reported at Washington hospitals for the first quarter of this year. That figure is nearly half of the 380 heroin-related cases reported at the hospitals for all of 1979.

Brandt, Luke and others said they are still trying to determine the reasons for the increased heroin-related deaths. "We don't know the whys," Brandt said.

Luke said he has requested a joint investigation by the police department's homicide and narcotics divisions and the city's alcohol and drug abuses services administration into the deaths.

"I thought it would be productive to find out why this is happening," Luke said. "There is a group substantially much older than what we generally see. There is a high amount of alcohol in some of the victims."

Brandt said that many heroin users also are alcohol abusers. Hassan said younger heroin addicts often mix the two -- alcohol and heroin -- but the more experienced and older addicts generally refrain from drinking alcohol when using heroin. "It dulls the high," he said.

Pryal, of the Drug Enforcement Administration, said the purity of heroin seized from the Middle East has ranged from 2.8 percent to 8.7 percent, while the purity of heroin from Southeast Asia has been from 4 percent to 48 percent.

Pure heroin is heavily cut (diluted) with quinine, milk, sugar or other elements to make it palatable and nonlethal. The higher the purity, the stronger the euphoric "rush" effect for the addict.

City officials said the estimated purity of heroin on the streets was 3.5-to-4 percent, in the first three months this year, a little higher than the level for the same period last year.

The average monthly price for one milligram of heroin for the first three months this year was $2.83, according to police figures, which is close to the $2.64 average price for the same period last year but far lower than the $7.71 price in 1978. Police said increased availability of heroin on the street typically drives its price down.