What the NTP cell phone study found in rats and mice

The National Toxicology Program's two-year studies, finalized in November 2018, found clear evidence that the radio-frequency radiation used by 2G and 3G phones caused cancer in male rats, based on malignant heart tumors called schwannomas. NTP also found some evidence for brain gliomas and adrenal tumors in male rats, and only equivocal results in female rats and in mice. The animals were exposed over their whole bodies at 1.5 to 10 watts per kilogram, many times the public limit, and the FDA disagreed with parts of the reading. Two replication studies published in 2026 found no tumor increase in male rats at 4 W/kg.

Why the government ran the study

The FDA nominated cell phone radiation for testing in 1999, when long-term animal and human data were thin. The work took about 10 years and cost $30 million, according to NTP's cell phone research page. It was run at the Illinois Institute of Technology Research Institute in Chicago, and pathologists examined 40 tissues from nearly 3,000 animals. The rodents lived unrestrained in reverberation chambers designed to expose them evenly, so nobody had to hold animals still for hours.

NTP picked power levels in pilot studies so that body heating stayed under 1 degree Celsius. That matters because the FDA calls a rise in temperature the only established biological effect of radio-frequency energy on tissue.

How the animals were exposed

SAR, the specific absorption rate, measures how much radio-frequency energy tissue absorbs, in watts per kilogram. The federal rule at 47 CFR 1.1310 sets two limits for the general public: 0.08 W/kg averaged over the whole body, and 1.6 W/kg averaged over any 1 gram of tissue.

The two agencies compare these numbers differently, and each comparison is accurate as arithmetic. The FDA set whole-body against whole-body: 6 W/kg is 75 times the 0.08 W/kg public limit. NTP's FAQ describes the top rat dose as "four times higher than the maximum human exposure," which matches a comparison with the 1.6 W/kg local limit (6 divided by 1.6 is 3.75). A phone at the ear exposes part of the head, so neither ratio translates cleanly to a person on a call.

What NTP concluded, tumor by tumor

NTP grades each finding on a fixed scale: clear evidence, some evidence, equivocal evidence or no evidence of carcinogenic activity. The counts below come from TR-595 and TR-596, and the grades from those reports and NTP's own summary of them. Counts run from the sham control group to the highest dose.

No group got a "no evidence" grade, and only male rats got anything stronger than equivocal. The mouse lymphoma counts come with a caveat from NTP itself: the shared control group had unusually few lymphomas compared with control mice in earlier studies.

The DNA tests were mixed too. Comet assays found more DNA damage in some tissues, including the hippocampus of male rats on the CDMA signal and the frontal cortex of male mice. A second test, for chromosome damage in blood cells, was negative in both species.

The survival gap

The oddest result was that exposed male rats lived longer. At two years only 25 of 90 male control rats were alive, or 28%. In the exposed male groups, 43 to 60 of 90 survived, between 48% and 67%. NTP attributed the gap to chronic kidney disease in the control males, and its FAQ calls the longer lifespans the study's main limitation.

A short-lived control group has less time to develop tumors, so the gap complicates the rat comparisons above. NTP said its statistics accounted for the survival differences, and the FDA called the public health meaning of the finding unclear. Mice did not show the same gap: survival was similar across most groups, with two exposed male groups doing better than controls.

The limits NTP and the FDA pointed out

NTP's own FAQ says the findings "should not be directly extrapolated to human cell phone usage." The FDA went further. Its February 2020 literature review states that the agency "does not agree with all the conclusions" and lists specific problems:

• The control group had zero heart schwannomas. Historical controls in NTP's 2016 partial report had about 9 in 699 rats, or 1.3%, with a range of 0 to 6%. An NIH reviewer noted that a single control case would have erased the statistical significance.
• The pathology review was only partly blind, since pathologists knew which slides came from the control group. NTP's account is that its process ended with a blinded review of the target lesions by NTP and outside pathologists.
• Old, large male rats regulate body heat less well, so the FDA saw possible heat effects at 6 W/kg. NTP's temperature checks came from short pilot studies in younger animals, and no temperatures were taken during the two-year study.
• The brain glioma increase was not statistically significant, and control rats in other NTP studies had gliomas at about 2% to 4%.
• The adrenal tumors showed no clear dose response; the highest dose did not differ significantly from controls (P = 0.472).
• Whole-body exposure of a rat does not predict what happens with local exposure of a human head.

The heart finding was graded "some evidence" in the draft and raised to "clear evidence" after an outside peer review panel met in March 2018. In a February 2018 statement on the drafts, the FDA said it still considered the current phone limits "acceptable for protecting the public health."

The study also aged in a technical sense. It used 2G and 3G signals. NTP didn't test WiFi, and its page notes that much of the energy at the higher frequencies planned for some 5G networks is absorbed in the skin.

What came after

NIEHS built a small exposure system to probe the findings, finished those follow-up studies and says it has no plans for further radio-frequency exposure studies. An Italian study at the Ramazzini Institute, run at far lower doses of 0.001 to 0.1 W/kg for 19 hours a day, also reported more heart schwannomas in male rats at its top dose; the NCI fact sheet notes that key details needed to interpret it were missing.

The direct replication came from Japan and Korea, which ran matched two-year studies on male Sprague Dawley rats with a 900 MHz CDMA signal at 4 W/kg, on the same 10-minute cycle. Both were published in Toxicological Sciences in 2026. The Japanese study found no significant increase in tumors in any major organ, including the brain, heart and adrenal glands, and again saw better survival in the exposed rats. The Korean study found no significant change in tumor rates or survival. Both tested one dose, one signal and males only.

A WHO-commissioned systematic review of animal studies, published in 2025 before those two papers, rated the certainty of the evidence as high for gliomas and heart schwannomas in male rats (Mevissen et al.). Its authors also wrote that carrying animal results over to people is "particularly complex" for this kind of exposure.

None of this research tested consumer products. If you're weighing something like the EMF stickers for phones and tablets that EMFCALM sells, questions about what a product is meant to do go to the seller, for instance through EMFCALM's contact page.

The finding that started all this was 5 and 6 heart schwannomas per 90 male rats at 6 W/kg, against none in the controls. At 4 W/kg, two labs following a similar design have now found no such increase, and IARC's advisers have said they want the results of bioassays like these before they re-rate radio-frequency fields.