A Danish study found no evidence of a link between cell phone use and cancer, adding fuel to the debate over whether mobile devices are carcinogenic.

Scientists from the Institute of Cancer Epidemiology in Copenhagen followed more than 350,000 adults 30 years of age and older for more than a decade, making this study the largest ever to examine a possible cell phone-cancer relationship.

The study concluded there was no difference in cancer rates between people who used a cell phone and those who did not, including no risk of developing a brain tumor at the site where a cell phone is held close to the head.

The paper, which was published in the British Medical Journal, supports other recent reports disputing increased cancer rates among mobile users, including a smaller study conducted by European, Scandinavian and American experts in July of this year that came to the same conclusion.

The U.S. Food and Drug Administration and Federal Communications Commission have also found no evidence that cell phones cause malignant brain tumors.

Yet, some agencies and groups still take a cautious stance. Advocacy group MobileWise, for example, said the Danish study’s decade of research was not long enough, considering many brain tumors take years to develop.

But the strongest cautionary advice comes from the World Health Organization, which in June classified cell phones as “possibly carcinogenic,” to humans, putting them in the same group as the pesticide DDT, gasoline engine exhaust, and even coffee.

The Danish study also cautioned that results may not apply to the heaviest users or to children.

Conflicting reports may add to confusion among cell phone users over how much cell phone use is too much, and, even more seriously, may leave brain cancer patients looking for answers and potential avenues of recourse that simply aren’t there.

The U.S. Supreme Court is debating whether to allow health-related lawsuits against cell phone companies, a move heavily opposed by handset manufacturers and wireless carriers. This Danish study, because of its size and scope, may lend fuel to the opposition.

Since three-quarters of the world’s population are now cell-phone users, large-scale studies are difficult, since scientists need a “control group” of people who have never used a cell phone in order to conduct comparative research. Some studies are also limited by “recall bias,” the tendency for cancer sufferers to over-report behaviors such as cell-phone use as they search for the reasons behind their condition.

Challenges like these make the Danish study all the more significant, although when it comes to cell phone use and cancer, there is still no absolute certainty about which side of the debate the evidence may ultimately land heaviest on.