Spencer Platt, Getty Images

Smartphones are the defining accessory of 21st century life. In spite of all the good work they have done, it appears they might be bad for our health.

A major multi-year study carried out by the US government and published on Thursday by the US National Toxicology Program found a potential link between smartphones and cancer. The study looked at how radio frequency radiation at frequencies used in US wireless networks affected rats. It has also reignited a debate that has raged on quietly for years as to whether phones are making humans ill.

Groups of rats used in the study were exposed to different intensities of radio frequency radiation. Some male specimens were found to exhibit low incidences of two different types of tumors, one in the brain and one in the heart. The more radiation they were exposed to, the more likely, it seemed, they were to develop cancer.

If you're among the 92 percent of Americans who own a mobile phone, your first instinct might be to become wary of your device. But there's no need to don your tinfoil hat just yet.

The study validates the concerns some scientists have expressed, but ultimately the question as to whether phones can cause cancer remains unresolved. This is far from the first time people have proposed the link. For every study claiming that phones are harmful, there is another suggesting just the opposite.

There are three main reasons that phones could be responsible for certain types of cancer, according to the US National Cancer Institute. Firstly, phones emits radio waves. It's thought that tissues near the antenna can absorb the energy from these waves. Then there is the exponential growth in smartphone use, which has raised concerns, as well as the increase in the number and length of calls people make.

"Given the widespread global usage of mobile communications among users of all ages, even a very small increase in the incidence of disease resulting from exposure to RFR [radio frequency radiation] could have broad implications for public health," reads the study.

The many studies that have been carried on this subject have ultimately been inconclusive, and experts like the International Agency for Research on Cancer, which is part of the World Health Organization, are yet to be persuaded definitively one way or another that phones are carcinogenic to humans.

The National Toxicology Program is still analyzing the findings of the study.

For now, it's important to keep in mind that brain tumors are rare and there is no upward trend that shows a correlation between the growth in smartphone use and the number of tumors seen by doctors. In fact, according to the National Cancer Institute, the rates for the number of new brain cancer cases have actually been falling slightly every year for the last ten years.