TOKYO (Asahi News Service) - Smokers may be inhaling up to five times the amount of nicotine and seven times the amount of tar than is claimed on cigarette packaging, according to a health ministry study. But the ministry, fearful of hurting sales, won't say which companies are falsely labeling their products, which include even supposedly light cigarettes.

The closest the ministry would come to identifying the biggest offender was to say the package most removed from reality read: "Tar 1 milligram and nicotine 0.1 milligram."

Japan Tobacco Inc., the dominant force in the market, refused to comment on the study.

A lab in Canada, commissioned by the Ministry of Health, Labor and Welfare to analyze the contents of seven cigarettes selected from the top sellers in Japan, measured the presence of about 30 substances normally found in cigarette smoke, including tar and nicotine-known carcinogens.

The 20-million-yen study showed the amounts of each substance a person would inhale while smoking a single cigarette.

The analysis was conducted in accordance with two separate models of smoking patterns. Manufacturers currently base their claims on the first model, a previous international standard of measurement that dates back to 1965. Health authorities in the United States and Canada, on the other hand, base their measurements on the second, more recent model.

Based on the newer model, the analysis found that smokers of the brand with the most misleading packaging inhaled 6.7 times more tar and 4.8 times more nicotine than advertised. Even for the brand whose labeling was closest to the truth, levels of both tar and nicotine inhaled were 2.2 times that shown on the package.

Brands claiming to have the least concentration of noxious substances were found in the lab tests to have the greatest disparity between the claim and the reality. In some cases, levels of the carcinogens benzene and benzopyrene were greater in the so-called light brands than in regular cigarettes.

Yumiko Mochizuki, a senior researcher at the National Institute of Public Health, explained the rationale behind using the more recent model of smoking patterns in the test.

"The lighter cigarettes become, the more smokers crave nicotine and the more strongly and frequently they inhale," Mochizuki said.

But manufacturers' labeling does not take this into account.

In measuring the level of noxious substances in a cigarette, the smoke is drawn into a device that simulates inhalation from when the cigarette is lit until it has burned down to 3 millimeters from the filter. Manufacturers base their claims on levels found in 35 milliliters of smoke drawn in on two-second inhalations once a minute through vents placed at the end of the filter. Levels based on the more recent model of smoking patterns, however, are taken from 45 milliliters of smoke drawn through the vents, which are half-closed, in two-second inhalations made once every 30 seconds.

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