The NTP cell phone study in rats, explained

By the EMFCALM team · Updated November 9, 2026

The US National Toxicology Program (NTP) exposed rats and mice to 2G and 3G cell phone signals over their whole bodies, about nine hours a day for up to two years, at 1.5 to 10 watts per kilogram (W/kg). Its final reports, published in November 2018, found clear evidence of heart tumors in male rats and some evidence of brain and adrenal tumors in male rats; the results in female rats and in mice were unclear. NTP and the FDA both say the findings shouldn't be applied directly to people, because the doses were higher and longer than a phone delivers and covered the whole body.

What did the NTP study do?

The FDA nominated cell phone radiofrequency (RF) radiation for testing in 1999, when long-term animal and human studies were scarce. NTP says the work cost $30 million and took about 10 years. Its scientists examined 40 tissues from nearly 3,000 animals.

The results are in two Technical Reports: TR-595 for rats and TR-596 for mice.

• Rats (Hsd:Sprague Dawley) got a 900 MHz signal from gestation day 5, in the womb, then through nursing and for up to 104 weeks after weaning. The levels were 1.5, 3 or 6 W/kg.
• Mice (B6C3F1/N) got a 1,900 MHz signal from 5 to 6 weeks of age, for 106 weeks (males) or 108 weeks (females), at 2.5, 5 or 10 W/kg.
• Every level ran with both GSM and CDMA modulation, two common ways of sending phone signals in the US and Europe.
• The signal cycled 10 minutes on and 10 minutes off across 18 hours and 20 minutes a day. That adds up to 9 hours and 10 minutes of exposure, 7 days a week.

The animals moved freely inside shielded chambers designed with help from the National Institute of Standards and Technology and Switzerland's IT'IS Foundation. Sham control animals lived in identical chambers with the RF off. NTP set the power levels so that heating stayed under 1 degree Celsius.

What did NTP find, and how strong is the evidence?

NTP grades each finding on a four-step scale. From strongest to weakest: clear evidence, some evidence, equivocal evidence, no evidence. Each organ gets its own grade.

Source: NTP's cell phone page and the TR-595 and TR-596 conclusions, read November 9, 2026. The GSM label on the adrenal row is from FDA's 2020 review.

In mice, exposure didn't increase any non-cancer lesions.

The heart result rests on small counts. None of the sham control males had a malignant heart schwannoma:

Source: TR-595 summary table.

For CDMA, the report calls only the 6 W/kg increase statistically significant. An outside peer review panel met March 26 to 28, 2018. According to the FDA, it raised the heart finding from some to clear evidence, and the brain and adrenal findings from equivocal to some.

Why did the exposed male rats live longer?

Only 25 of the 90 sham control males were alive at two years. In the exposed groups it was 43 to 60. TR-595 traces the gap to chronic progressive nephropathy, a kidney disease, in the control males. NTP's FAQ says the exposure "appeared to lessen chronic kidney disease in aging male rats" and calls the longer lifespans the study's main limitation. Female rats survived at similar rates, except for a small but significant increase in the CDMA 6 W/kg group.

Read the tumor table with this in mind: in the comparison group, most males died before the two years were up. The FDA's 2020 review repeats NTP's statement that its statistics accounted for the survival gap, and calls the public health impact unclear.

Did the study find DNA damage?

NTP checked DNA in animals removed partway through the study, using the comet assay on three brain regions, the liver and blood cells. It found significant increases in DNA damage in the frontal cortex of male mice, in blood cells of female mice (CDMA only) and in the hippocampus of male rats (CDMA only). Results in several other tissues were judged equivocal. A second test, the micronucleus assay for chromosome damage in blood, showed no increase in rats or mice.

NTP adds two cautions. It didn't track the cells over time, so it can't say whether they repaired the damage. It also doesn't know how RF would cause it. The DNA results also came out as a separate paper in Environmental and Molecular Mutagenesis in late 2019.

How do 1.5 to 10 W/kg compare with the FCC limits?

The FCC rule, 47 CFR 1.1310, sets two limits for the general public: 0.08 W/kg averaged over the whole body, and a peak of 1.6 W/kg averaged over any 1 gram of tissue. Hands, wrists, feet, ankles and the outer ear get 4 W/kg over 10 grams.

The two agencies compare differently. NTP's fact sheet uses the local limit. It says the lowest level, 1.5 W/kg, equaled the most a phone may put into local tissue (the FCC figure is 1.6), a level that rarely occurs in typical use. The highest rat level was four times that. The FDA uses the whole-body limit, since the animals were dosed over their whole bodies. Its 2020 review puts 6 W/kg at 75 times the 0.08 W/kg limit. By the same arithmetic, the top mouse level of 10 W/kg is 125 times.

The body area matters too. A phone exposes one localized spot, NTP notes, while the animals were "exposed evenly across their entire bodies."

What does NTP say about applying this to people?

NTP's FAQ says the findings "should not be directly extrapolated to human cell phone usage." Its fact sheet gives two reasons: the higher, longer doses and the whole-body exposure.

NTP doesn't wave the results away. It says some of the tumor types have also been seen in humans, "so they may have relevance." And the fact sheet says the studies "question the long-held assumption" that RF is harmless as long as it doesn't significantly heat tissue.

NIEHS says the work doesn't apply to 4G or 5G, which use different modulation, and didn't test Wi-Fi; our 5G guide picks that up. If you want less exposure, NTP points to FDA tips: less time on the phone, and speaker mode or a headset. We cover those in how to reduce EMF from your phone.

How did the FDA respond?

The FDA is the agency that asked for the study. In a February 2, 2018 statement on the draft reports, it said its early reading was that the evidence of cancer was "mostly equivocal." It flagged the longer-lived rats as an unusual finding and said current phone limits remained acceptable.

On November 1, 2018, after the final reports, the FDA said it disagreed with the "clear evidence" conclusion. In its reading, effects on animal tissue begin only at exposures 50 times the FCC whole-body limits. Only the highest dose produced a statistically significant number of heart schwannomas, and the FDA saw "no true dose response." It agreed the findings should not be applied to human phone use, and noted the study wasn't designed to test that. That statement is no longer on fda.gov; we read the Internet Archive copy.

The FDA's 2020 literature review says the agency doesn't agree with all of NTP's conclusions but that the study added to the collective data and pointed to paths for more research. Our overview, is cell phone radiation harmful?, covers how the agencies read the wider evidence.

What did the Ramazzini Institute rat study add?

In 2018, Italy's Ramazzini Institute published its own rat study in Environmental Research (Falcioni et al.). It exposed 2,448 Sprague-Dawley rats, from before birth until natural death, to a 1.8 GHz GSM field of 0, 5, 25 or 50 V/m for 19 hours a day. It was meant to mimic far-field exposure from a cell tower antenna, not a handset.

The authors reported a statistically significant rise in heart schwannomas in males at 50 V/m, and a rise in malignant glial tumors in females that was not statistically significant. They called this consistent with NTP and asked IARC to re-evaluate its classification of RF; our IARC 2B explainer covers that label. The FDA's 2020 review lists the reported doses as 0.001 to 0.1 W/kg and calls the way they were estimated inaccurate. That top figure is 1.25 times the FCC whole-body limit, by our arithmetic.

Has NTP published anything since 2018?

Yes, a little. Besides the 2019 DNA paper, NIEHS published a peer-reviewed report in August 2025 on follow-up work with a new small-scale exposure system. NIEHS says that work was not done as part of NTP. In five-day studies, the rodents showed no change in behavior, and there was no DNA damage at up to 9 W/kg in rats and 15 W/kg in mice.

The team set out to learn whether tissue heating explained the original results, and couldn't. Neither the implanted microchips nor the data loggers measured internal body temperature. NTP's cell phone page, last updated November 9, 2026, says NIEHS has no further plans for RF exposure studies.

Every conclusion in TR-595 opens with the same words: "Under the conditions of this 2-year whole-body exposure study." We'd read the whole project through that phrase. The heart finding in male rats earned NTP's top grade, and those rats were dosed at about 19 to 75 times the FCC whole-body limit, nine hours a day, from before birth through two years after weaning.

This article explains published research. It isn't medical advice.

General information about radio-frequency exposure from everyday devices. Not medical advice.