Increasing supply, diminishing price, and greater purity have contributed to a dramatic increase in deaths and emergency room admissions for heroin overdoses in the past three years, according to Drug Enforcement Administration officials.

Paul D. Newland, chief of the strategic heroin unit of DEA's office of intelligence, said yesterday that the average price of a milligram of heroin has dropped nationally from $2.34 in 1980 to $2.22 in the first quarter of this year, despite a 19 percent rise in consumer prices overall.

At the same time, he said, the average purity of heroin sold on the street has increased from 4 percent to 5.6 percent.

"There is an increase in availability," Newland said. "We've seen a big movement towards increasing purity, which means greater supply."

Like any commodity, heroin is subject to price changes based on supply and demand. In the heroin economy, officials said, a drought in the poppy fields of Afghanistan, a drug crackdown in Mexico, or more efficient laboratory processing methods can influence the supply and therefore the price and average purity of the bitter, white, odorless powder.

Over the years, the chief sources of supply of the opium poppy needed to make the morphine base that, in turn, is made into heroin have varied. Currently, according to DEA officials, roughly 55 percent of heroin used in the United States comes from Southwest Asia, primarily Pakistan and Afghanistan.

About 85 percent of heroin bought and sold in the Washington area comes from the Southwest Asia region, officials said.

The DEA's Newland attributed those percentages to diminshed availability of heroin from Mexico because of that government's program of spraying the poppy crop with poisons. Also, there has been a drought in Southeast Asia, and there was a crackdown on farming of the crop by Turkey beginning in the 1970s.