But the bottom line of the report is that people should not worry that they will get cancer from using their cellphones.
To see if radiation itself - not heat from cell phones - could cause cancer, they exposed male and female rats and mice to full-body RFR at levels far higher than we experience from our phones.
The FDA disagreed. "The current safety limits for cell phone radiofrequency energy exposure remain acceptable for protecting the public health", said Shuren, the Food and Drug Administration official.
"We agree that these findings should not be applied to human cell phone usage", the FDA's Dr. Jeffrey Shuren said in a statement.
Cancer experts say rates of brain tumors in humans haven't increased in 40 years. Other animal studies have found less dire effects at lower cell radiation levels, or more extreme effects in small animal studies using high radiation levels.
Researchers were able to control the amount of radio frequency the animals received - something that isn't possible with humans.
And, "the exposures used in the studies can not be compared directly to the exposure that humans experience when using a cellphone", NTP senior scientist John Bucher noted in an NIEHS news release.
Research has found that high exposure to 2G and 3G cell phone radiation is linked to cancer....
.... If the results of this study still makes you feel a little concerned about putting your smartphone to your ear, you can always opt to use earbuds to make your calls instead.
Interestingly, the researchers found that the male mice that were exposed to cellphone radiation actually lived longer than those that were not exposed.
The National Toxicology Program concluded there's "clear evidence" that prolonged exposure to high levels of radio-frequency radiation, like that used in 2-G and 3-G cellphones, triggers cancerous brain and heart tumors in lab rats. As scientists often say, more study is needed. Malignant schwannomas are similar to acoustic neuromas, benign tumors that can develop in people in the nerve that connects the ear to the brain. "In addition, the exposure levels and durations in our studies were greater than what people experience", said Bucher. Ming Yeung/Getty Images Entertainment/Getty ImagesCurrent 4G and 5G cell phones do emit much higher frequencies than second generation cell phones, says The New York Times, but scientists say those radio waves aren't as capable of penetrating the human body as older models.
"From what we now understand, it likely differs dramatically from what we studied".
The National Toxicology Program is an inter-agency program headquartered at the National Institutes of Health.
The NTP researchers are planning future studies on the effects of newer technologies, and these studies will use different methods so that they will be completed in weeks to months, rather than years.
Scientists who reviewed the NTP study are calling for the Federal communications Commission to warn the public, but the FDA's response suggests the findings are over-hyped as it tries to quell consumer's fears.
"My family and I won't change our mobile-phone habits based on this news", said George Gray, co-author of the book "Risk: A Practical Guide for Deciding What's Really Safe and What's Really Dangerous in the World Around You".