A large multinational research effort overseen by the World Health Organization has concluded that heavy mobile phone use significantly increases the risk of brain and salivary gland tumors.

The Interphone studies surveyed 12,800 people in 13 countries between 2000 and 2004. Although the final findings have not yet been released, they have been accepted for publication in a scientific journal and will see print before the end of 2009.

The conclusions are particularly surprising given that the industry-funded effort has been widely criticized for designing its studies to minimize the apparent risks of cell phone use.

The studies examined the relationship between cell phone use and the risk of three different types of brain tumor and one tumor of the salivary gland. They concluded that “use of mobile phones for a period of 10 years or more” was associated with a “significantly increased risk” of the tumors.

Six of eight studies found up to a 39 percent increase in the risk of glioma, the most common type of brain tumor. Gliomas can be either benign or malignant. The risk of acoustic neurinoma, a benign tumor of the nerve between the brain and the ear, was found to increase up to 3.9 times in two of seven studies, but problems with participants’ memories interfered with these findings. Another study found a 50 percent increase in the risk of salivary gland tumors.

Some researchers have suggested that the Interphone study probably understates the risks of cell phone use, due to flaws in its methodology. The study has been criticized for including people who made as little as one call per week yet excluding children and young adults (considered the most at-risk population), non-cellular cordless phones (which also emit radiation), several kinds of tumors, and participants who either died before the study concluded or became too sick to answer questions.

Some of the Interphone studies found that short-term cell phone use decreased the risk of cancer, further suggesting research flaws.