Scientists have long been worried about the possible harmful effects of regular cellular phone use, but studies so far have been largely inconclusive.
Researchers said currently, radio-frequency electromagnetic fields, such as those produced by cell phones, are classified as possibly carcinogenic to humans (Group 2B) by the International Agency for Research on Cancer (IARC).
To further explore the relationship between cancer rates and cell phone use, Dr Yaniv Hamzany from Tel Aviv University and colleagues looked for clues in the saliva of cell phone users.
Since the cell phone is placed close to the salivary gland when in use, researchers hypothesised that salivary content could reveal whether there was a connection to developing cancer.
Comparing heavy mobile phone users to non-users, they found that the saliva of heavy users showed indications of higher oxidative stress - a process that damages all aspects of a human cell, including DNA - through the development of toxic peroxide and free radicals.
More importantly, it is considered a major risk factor for cancer.
Researchers examined the saliva content of 20 heavy-user patients, defined as speaking on their phones for a minimum of eight hours a month. Most participants speak much more, Hamzany said, as much as 30 to 40 hours a month.
Their salivary content was compared to that of a control group, which consisted of deaf patients who either do not use a cell phone, or use the device exclusively for sending text messages and other non-verbal functions.
Compared to the control group, the heavy cell phone users had a significant increase in all salivary oxidative stress measurements studied.
"This suggests that there is considerable oxidative stress on the tissue and glands which are close to the cell phone when in use," he said. The damage caused by oxidative stress is linked to cellular and genetic mutations which cause the development of tumours, researchers said.
This field of research reflects longstanding concerns about the impact of cell phone use, specifically the effects of radio-frequency non-ionising electromagnetic radiation on human tissue located close to the ear, researchers said.
Researchers said although these results don't uncover a conclusive "cause and effect" relationship between cellular phone use and cancer, they add to the building evidence that cell phone use may be harmful in the long term, and point to a new direction for further research.
The findings have been reported in the journal Antioxidants and Redox Signalling.