Boston - Researchers at the Harvard School of Public Health say they have confirmed a study by the state that found nicotine levels in cigarettes increased from 1997 until 2005.

The analysis, based on data submitted to the Massachusetts Department of Public Health by cigarette manufacturers, found that increases in smoke nicotine yield per cigarette averaged 1.6% each year, for a total of about 11% over a seven-year period.

"Cigarettes are finely tuned drug delivery devices, designed to perpetuate a tobacco pandemic," said Howard Koh, an associate dean for public health practice who worked on the analysis. "Yet precise information about these products remains shrouded in secrecy, hidden from the public."

The health department study released last October examined nicotine levels in more than 100 brands over a six-year period. The study showed a steady climb in the amount of nicotine delivered to the lungs of smokers.

Gregory Connolly, head of the Tobacco Control Research Program at the Harvard School of Public Health, said the increase found in Harvard's study is due primarily to an increase in nicotine in the raw tobacco used in the cigarettes.

"There's something going on either with the type of tobacco they're using or the addition of more nicotine to the reconstituted tobacco. We just don't know," Connolly said.

He also said the findings call into question whether the tobacco industry is living up to its 1998 agreement with states that it would launch a campaign to reduce smoking by young people.

"If that same industry turns around and advances the availability of nicotine in the product, you may not get fewer kids smoking," he said.

Cigarette manufacturers disputed the findings of both studies. Cigarette manufacturer Philip Morris USA said data reported to the state by the company shows nicotine yields for the Marlboro cigarettes were the same in 2006 and 1997.

The company said that the data reflect random variations in cigarette nicotine yields, both upward and downwards, and that variations are not consistent.

Health department officials have defended the report, which said that the higher nicotine levels made it easier to get hooked on cigarettes and harder to quit.

Massachusetts is one of three states to require tobacco companies to submit information about nicotine testing according to its specifications and the only state with data going back to 1997.

Harvard did a more in-depth analysis of the state's data and looked at one additional year. Work on the report was supported by funds from the anti-smoking advocacy group The American Legacy Foundation and the National Cancer Institute.