Tobacco companies may have been able to downplay the amount of tar and carbon monoxide ingested by smokers in Europe because of a flaw in the testing process, according to a study.

The loophole means most cigarette exceed legal limits on their harmful contents in the EU, according to the Netherlands National Institute for Public Health (RIVM).

Tiny holes in cigarette filters that allow air to mix with tobacco fumes are closed up when held in smokers’ fingers. However, the common method for testing tobacco products under European Union rules, called ISO, leaves these holes uncovered.

Levels of tar, nicotine and carbon monoxide inhaled in normal conditions could be up to 20 times higher than indicated, according to RIVM, part of the Dutch ministry of health.

The results were obtained by testing 100 cigarette brands using the Canadian Intense testing method which covers over the holes in the filters. Under this system all but one of the brands tested exceed the legal limits with tar content between 2 and 26 times higher than using the European regime, nicotine between 2 and 17 times higher and carbon monoxide between 2 and 20 times higher.

“The results of this research support the conclusion that the prescribed ISO method underestimates the amounts of tar, nicotine and carbon monoxide that a smoker ingests,” the RIVM wrote on its website. “The committee that drew up this method is largely influenced by the tobacco industry. RIVM therefore recommends that an independent measurement method … be included in the law.”