HEROIN looks like it is here to stay. Since the 1960s, millions of Americans have used heroin. Most stopped, with or without treatment, at some point.  But there are an estimated half-million Americans addicted to heroin and a comparable number using it with some regularity.  Heroin use grew rapidly during the 1960s, leveled off during the 1970s and 1980s, and now appears to be increasing again. Hospital emergency rooms saw an 80 percent increase in heroin-related episodes between 1990 and 1993. The average purity of heroin purchased on the street has increased from about 5 percent during the mid 1980s to 40 percent today—and in New York City, Philadelphia, and Newark, New Jersey, it’s over 60 percent.