WASHINGTON (AP) -- A new federal study says that teenagers' perception of the dangers of marijuana has fallen to the lowest level in more than 20 years.

The survey released by the National Institutes of Health found that only 41.7 percent of eighth graders believe that occasional use of marijuana is harmful, while nearly 67 percent regard it as dangerous when used regularly.

Both rates are the lowest since 1991.

Teens' perception of marijuana risks diminished even more as they got older. About 20.6 percent of 12th graders said occasional use of pot is harmful. Roughly 44.1 percent believe that using it regularly was detrimental, the lowest since 1979.

The study said teens' dwindling concerns about the dangers of marijuana, despite the risks, "can signal future increases in use."