The level of purity of heroin available for sale on the streets in Washington has more than tripled in two years, resulting in a surge in drug overdose deaths in the last 12 months.

Medical authorities say the new potency and its unpredictability threaten longtime heroin addicts as well as new heroin users who are attracted to the narcotic as a way of easing the crash from crack.

Although complete statistics are not available, the D.C. medical examiner's records show that at least 100 people died last year with lethal amounts of both cocaine and heroin in their bodies -- twice the number who died of heroin-cocaine combinations the previous year and a 2,500 percent increase over the four deaths in 1985.

Most of last year's deaths occurred in the year's final five months. No figures are available for 1990, but sources say preliminary indications are that the high level of overdose deaths is continuing.

"Instead of the crash they normally get from crack, addicts are learning that if they do some heroin they can float down," said Beverly Coleman-Miller, a doctor with the D.C. Department of Public Health.

"Then they find that if they do more and more heroin, the crack lasts longer and longer," Coleman-Miller said.

Law enforcement officials say the deadly heroin is the result of a glut of the drug on the international market that has allowed a new breed of drug entrepreneurs to bypass traditional international networks and import the drug directly from Turkey, China and other producer countries.

Heroin, which used to be "cut" or diluted many times as it passed through numerous middlemen in the drug network, is now reaching Washington wholesalers in the full strength that it left the processing lab -- 80 to 98 percent purity.

The purity of heroin sold on the street is now routinely found to be between 30 and 40 percent.

The emergency medical personnel in the District, where the area's heroin use is centered, are seeing the effects of the stronger heroin.

"They don't wake up the way they used to," one worker said of the efforts to revive addicts who have taken the purer heroin.

The medical protocol calls for administering 2 milligrams of Narcan, the main antidote to heroin, but many addicts here now require much stronger doses, sources said.

Capt. Collin Younger, commander of the D.C. police department's narcotics branch, said the purity of street-sale heroin "started to creep up" several years ago from the longtime level of 2 to 4 percent.

Then, about a year ago, narcotics officers began finding street-sale heroin in the 30 percent purity range, a level that two years earlier was the purity of heroin at the wholesale level.

Law enforcement officials fear the purer heroin, which is strong enough and inexpensive enough to be smoked or sniffed, may also be attracting a new group of users who had previously avoided the drug because of their aversion to needles -- the traditional way to ingest heroin -- and the threat of AIDS from shared needles.

Authorities say they haven't documented a major rise in the number of heroin users, but they say that heroin addiction has been a significant and continuing problem in the District for two decades.

While a recent study estimated that the number of regular cocaine users in the city has grown to 20,000, officials of the District's Alcohol and Drug Abuse Services Administration have, since the early 1970s, estimated the number of heroin addicts in the District at 16,000.

George McFarland, a private consultant who was the prime statistical researcher for the agency, said the 16,000 figure is based largely on the number of addicts seeking treatment at city clinics and private facilities and is probably much too conservative.

McFarland and others say that most of the addicts here have been using heroin since the late 1960s and early 1970s.

Since its inception in 1984, the drug testing program of the District's Pretrial Services Agency has recorded wide swings in the popularity of drugs such as PCP and cocaine.

However, the percentage of adults arrested on criminal charges who test positive for heroin has remained relatively steady in the 15 to 20 percent range.

However, other statistics reveal some temporary volatility in heroin use, showing an unexplained rise in popularity in 1988, when the number of Washington area emergency room visits because of heroin and the number of heroin overdose deaths increased sharply.

Statistics collected by the National Institute for Drug Abuse from the District and suburban Maryland and Virginia show that there were 1,719 emergency room visits for heroin in 1988, compared with 1,379 in 1987 and 1,349 in 1989.

Several researchers cautioned that although the institute figures can be used to show trends, not all area hospitals report to the agency. Although the list of participating institutions is secret, sources said one hospital that doesn't participate is D.C. General, where many of the city's overdose victims are treated.

The institute's Drug Alert Warning Network recorded 169 heroin overdose deaths in the Washington area in 1986; local jurisdictions say that number climbed to 242 in 1988.

According to the D.C. Medical Examiner's Office, 171 persons died of heroin overdoses in 1988. The Maryland Medical Examiner's Office reported 38 heroin overdose deaths in Prince George's County in 1988; 17 heroin overdose deaths in Montgomery County the same year and three heroin overdose deaths in Howard County.

The medical examiner's office that covers 19 Northern Virginia counties reported 13 heroin overdose deaths in 1988. Officials said that the deaths occurred primarily in suburban Washington.

Complete overdose statistics for 1989 are not available, but statistics for Maryland show that the number of heroin overdose deaths was about half the 1988 levels.

The Drug Enforcement Administration's statistics on arrests and drug seizures show a sharp increase in heroin activity here.

In 1988, the DEA made 36 heroin arrests in the District and seized two kilograms (4.4 pounds) of the drug here. Last year DEA agents made 31 arrests and seized 2 1/2 kilograms of heroin in the District.

In the first three months of this year, the DEA has made 12 heroin arrests in the District and seized more than 3 kilograms here.

Local DEA spokesman Mario Perez said heroin arrests have jumped as a percentage of all arrests in this region, which includes all of Virginia, Maryland, West Virginia and the District.

In 1988, heroin arrests were about 2 percent of the total; so far this year, they are about 10 percent.

Still, federal and local law enforcement agencies have concentrated on crack in recent years because of the violence it causes and because changes in drug laws made it easier to obtain severe penalties against dealers with relatively small amounts of crack.

Under federal law, trafficking in five milligrams of crack -- about one-fifth of an ounce -- carries the same penalty as trafficking in 100 milligrams of heroin -- about a quarter pound.

Perez and the D.C. police department's Younger say the higher arrest figures are even more significant considering that heroin dealers are more secretive than cocaine and crack dealers, who are often brazen in their street sales.

D.C. police efforts to clean up the city's longstanding heroin market at 14th and W streets NW has scattered dealers throughout the city, according to Younger, who said prime heroin markets are now the 3200 block of Georgia Avenue NW, the area at Eighth and O streets NW and the 600 block of Division Avenue SE.

"But it's the purity that concerns us," Younger said. Narcotics officers believe that many addicts are taking advantage of the high-purity heroin and splitting a quarter-spoon bag, which sells for $25 to $40, to provide three injections.

"But a person has to be pretty knowledgeable to find out what the purity is," Younger said.