The National Toxicology Program (NTP) has published the results of a long-term study in mice and rats designed to identify the possible hazards of high whole-body exposure to cell phone radio frequency radiation. The Federal Office for Radiation Protection presents the results and evaluates them.
The National Toxicology Program (NTP) has published the results of a long-term study in mice and rats designed to identify the possible hazards of high whole-body exposure to cell phone radio frequency radiation. The Federal Office for Radiation Protection presents the results and evaluates them.
The NTP (National Toxicology Program) study is a long-term study in mice and rats designed to identify the possible hazards of high whole-body exposure to cell phone radio frequency radiation. The study was conducted according to standardised toxicological protocols. The NTP authors conclude that there is clear evidence (highest level of evidence) for the exposure-related occurrence of heart tumours as well as some evidence for the occurrence of brain tumours and diseases of the adrenal medulla in male rats.
In contrast, the Federal Office for Radiation Protection, after careful analysis of the various results, sees indications but neither a clear nor some evidence for a carcinogenic effect at high whole-body exposures - which were clearly above the limit values. Methodological weaknesses and inconsistencies in the study results clearly limit the meaningfulness of the study. The carcinogenic activity was limited to male rats (absent in female rats and both sexes of mice). The incidences of the preneoplastic lesions to be expected in comparison with the observed tumours were too low to correspond with the current models of tumour development. A particular striking feature of the study was the high mortality rate of the control animals compared with the exposed animals; this complicates the direct comparison of tumours occurring in old age. Methodological peculiarities of this toxicological study (no blinding in the initial pathological assessment and no correction for multiple testing) may have also led to biased or random results. In addition, body temperature was not measured. With the high whole-body exposures, it cannot be ruled out that thermal stress – with body temperature increases occurring above the limits and known to lead to health effects – led to the striking results, especially in male rats.
The whole-body exposures used in the animal experiments are about 20 times (and more) above the limit value for whole-body exposures of the general population. They can therefore not be transferred to the cell phone exposures that occur in people’s everyday lives.
The Federal Office for Radiation Protection therefore continues to assume that no negative health effects are to be expected from electromagnetic fields with the frequencies used by cell phones if the limit values are adhered to.
Because – independent of the NTP study – there are still uncertainties in the evaluation of the possible long-term risks of intensive mobile phone use, precautionary recommendations are an effective means of minimising possible (but not proven) risks. In the area of mobile communications, the precautionary recommendations of the Federal Office for Radiation Protection concentrate on terminal devices: manufacturers should design their products in such a way that users are exposed to the lowest possible field strengths. Users can keep their personal exposure low by selecting devices with low SAR values and taking simple behavioural measures. The Federal Office for Radiation Protection gives precautionary recommendations for this.
The NTP study [1, 2] was conducted in three phases. In the 1st and 2nd phase, the authors determined the (maximum) specific absorption rates (SAR) suitable for whole body exposure that do not lead to increased mortality or excessive warming (above 1 °C) in laboratory rodents (Harlan Sprague-Dawley rats and B63F1 mice) after a short time. Based on the results from the 1st and 2nd phase, the whole body SAR values for the chronic exposure of the laboratory rodents were determined in the 3rd phase, the actual main study (detailed information on the three study phases can be found in Annex 1: Study design and results of the first and second study phase and in Annex 2: Study design and results of the NTP main study).
In the main study, the individually kept, freely moving animals (90 animals/group) were subjected to whole body exposure (for 18 h and 20 min per day, 10 min field off/10 min field on) with GSM and CDMA modulated 900 MHz (rats) and 1900 MHz (mice) signals at SAR values of 1.5, 3, and 6 W/kg for rats and 2.5, 5, and 10 W/kg for mice. The SAR averaged over the entire body of the animals was determined using numerical computer simulations and kept as constant as possible throughout the study. A sham-exposed control group for both modulations (GSM, CDMA) was used as comparison group. The following endpoints were investigated at the end of the study after two years: body weight and survival rate of animals, haematological and clinical-chemical variables, and neoplastic and non-neoplastic changes in organs and tissues.
Heart: In male rats, the incidence of malignant schwannomas of the heart increased with increasing GSM exposure (0, 2, 1, 5 cases at 0, 1.5, 3, and 6 W/kg) and CDMA exposure (0, 2, 3, 6* cases at 0, 1.5, 3, and 6 W/kg). This exposure-response trend was statistically significant for both modulations and for CDMA in the highest exposure group (6 W/kg) compared with sham exposure. In the female rats, there was neither a trend nor a significantly increased incidence for individual exposure levels (GSM: 0, 0, 2, 0 cases and CDMA: 0, 2, 0, 2 cases at 0, 1.5, 3, and 6 W/kg respectively). The incidence of schwannomas in other parts of the body was unremarkable.
Significantly (*) increased incidences of right ventricular cardiomyopathies occurred at the highest CDMA and the two highest GSM exposure levels in male rats (GSM: 54, 62, 72*, 74* cases and CDMA: 54, 45, 62, 74* cases at 0, 1.5, 3, and 6 W/kg) as well as in female rats (GSM: 4, 9, 14*, 15* cases at 0, 1.5, 3, and 6 W/kg) compared with sham-exposed controls.
Increases in the incidence of Schwann cell hyperplasias (possible precancerous stage of schwannomas in the heart) in the male (GSM: 0, 1, 0, 2 cases and CDMA: 0, 0, 0, 3 cases at 0, 1.5, 3, and 6 W/kg) and female rats (only for CDMA: 0, 1, 1, 1 cases at 0, 1.5, 3, and 6 W/kg) were observed compared with controls. However, these were not significant.
Brain: In male rats, increased incidences of malignant gliomas were found in all exposed groups compared with the control group at GSM modulation (0, 3, 3, 2 cases at 0, 1.5, 3, and 6 W/kg). There was also a significant trend with increasing exposure to CDMA modulation (0, 0, 0, 3 cases at 0, 1.5, 3, and 6 W/kg). In female rats, increases in the incidence of malignant gliomas (GSM: 0, 3, 0, 0 cases and CDMA: 0, 0, 0, 1 cases at 0, 1.5, 3, and 6 W/kg) occurred in only a few exposure levels, all of which were not significant.
Increased incidences of glial cell hyperplasia (possible precancerous stage of malignant glioma) occurred in most exposed groups of both sexes. However, these were not significant.
A detailed list of further results from the main study can be found in Annex 2: Study design and results of the NTP main study.
[1] NTP TECHNICAL REPORT ON THE TOXICOLOGY AND CARCINOGENESIS STUDIES IN Hsd:SPRAGUE DAWLEY SD RATS EXPOSED TO WHOLE-BODY RADIO FREQUENCY RADIATION AT A FREQUENCY (900 MHz) AND MODULATIONS (GSM AND CDMA) USED BY CELL PHONES; NTP TR 595, November 2018
[2] NTP TECHNICAL REPORT ON THE TOXICOLOGY AND CARCINOGENESIS STUDIES IN B6C3F1/N MICE EXPOSED TO WHOLE-BODY RADIO FREQUENCY RADIATION AT A FREQUENCY (1,900 MHz) AND MODULATIONS (GSM AND CDMA) USED BY CELL PHONES; NTP TR 596, November 2018
The final evaluation by the 19-member NTP team took place after a three-day peer review process of a pre-publication in March 2018. Scientists as well as members of the public participated and discussed the results.
According to their specific toxicological classification criteria, NTP sees
The Federal Office for Radiation Protection sees indications in the extensive data but no proof that the tumours in the hearts of male rats could be a consequence of the whole body exposure under the toxicological test conditions of (partly extremely) high and long-lasting exposure. This is due to a number of methodological weaknesses and inconsistencies in the study results as well as particularities in toxicological study design, which are described below:
A significantly increased incidence of malignant schwannomas in the heart was found only in male rats. This result is therefore not consistent with the results in female rats or mice within the NTP study. However, they are consistent with another animal study recently published by Falcioni et al. [3]. However, gender differences in tumour incidence are often observed in NTP carcinogenicity studies. Possibly study-dependent, tumours mostly occur more frequently in male rats [4].
The number of possible preneoplastic lesions in both exposed and non-exposed control animals was smaller than the tumour incidence in almost all groups. The reverse case would have been expected from the current models of carcinogenesis. However, it is unclear whether the hyperplasias observed (Schwann cell and glial cell hyperplasia) are actually preneoplastic lesions of the malignant schwannomas or gliomas, which is why NTP authors also refer to them as "putative/potentially" preneoplastic lesions.
In the male rats, the control animals had a significantly lower life expectancy than the exposed animals (28 % survival rate in control animals versus 50–68 % in GSM- and 48–62 % in CDMA -exposed groups). The control animals showed no malignant schwannomas of the heart or malignant gliomas compared with the exposed animals. Although this is within the incidence range of historical controls (malignant schwannomas, 0–2 % incidence; malignant gliomas, 0–4 % incidence), it cannot be excluded that the animals died before developing a tumour. This may have led to an under-representation of late-onset heart schwannomas in control animals and thus to an overestimation of the associated risk. The low life expectancy of the control animals might have had an even greater effect on the malignant gliomas, which usually only occur in older animals. The differences in the survival rates were taken into account in the statistical evaluation through appropriate corrections. It is unclear whether the assumptions on which the corrections are based apply to the yet underexplored schwannomas of the heart.
Exposing freely moving animals while simultaneously ensuring meaningful dosimetry is a great challenge. However, the researchers were able to keep the specific absorption rate of whole-body exposure comparatively constant at a fixed value over the entire duration of the study per exposure group (time-averaged). In long-term toxicological studies, it is crucial that the dose (SAR) is kept constant. However, the specific absorption rate alone is not representative of the expected temperature increase of the animals, which, if too high ( > 1 °C), can lead to proven health effects.
A major weakness of the study is that body temperature was not recorded in the main study. Nor were any parameters collected to record the metabolism of the animals (e.g. water consumption, CO₂ production) and thus to obtain indications of increased body temperatures. In the first phase of the study (Annex 1), adult male rats had significant body temperature increases (some measurements significantly > 1 °C) from a whole body SAR of 6 W/kg onwards. There was also a clear correlation between body weight and temperature increase. This can be explained by the fact that large, heavy animals have a lower thermally radiated power (cooling effect) than small animals because of their small surface area compared to their body mass. This is also compatible with the small temperature increases measured in phase 2 in younger and thus lighter male rats. Because of the lack of temperature measurement in the main study, it cannot be excluded that an excessive warming of > 1 °C was observed in male (older and heavier) rats. These increases in body temperature above the limits are known to have health effects. It is therefore possible that the increased tumour incidences observed in male rats were the result of thermal stress.
Statistical tests were performed for a number of endpoints: separately for male and female animals, GSM and CDMA, intermediate results and two-year study as well as three exposure groups. Because of the large number of tests, significant results of a purely random nature can be expected. For continuous variables (e.g. body weight), the researchers corrected for multiple testing. However, discrete variables (e.g. incidences of tumours) were not corrected for multiple testing according to standardised NTP protocols. There is thus a higher likelihood of false positive results. In the statistical sense, none of the significant results can be individually evaluated as actually "significant" (i.e. higher than random).
The histopathological examinations were carried out in a multi-stage process by several pathologists. However, the initial evaluation of the tissue sections to identify possible lesions in mice and rats was unblinded. This means that the pathologists in the initial evaluation were always aware of whether the tissue section to be evaluated came from an exposed or sham-exposed mouse or rat. This is unusual in scientific research but corresponds to the recommendations of the American Society for Toxicological Studies [5]. A lack of blinding can lead to a distortion of the results because individual – also unconscious – expectations of the analysing scientists can influence the number and severity of the tissue changes found in the respective groups.
Malignant cardiac tumours of the heart are extremely rare in humans. However, the results of the NTP study are relevant insofar as benign acoustic neuroma (tumours of the auditory nerve), which are more frequent in humans, belong to the group of schwannomas. Epidemiological studies have revealed isolated indications of a possible connection between acoustic neuroma and mobile phone use. In this context, it is worth mentioning that in the NTP study, no significantly increased incidence rates were found in any of the schwannomas (of all organs) examined. However, a histopathological examination for acoustic neuroma was not performed in the NTP study.
Previous animal studies (e.g. the multi-generational studies carried out in the German Mobile Telecommunication Mobile Radio Research Programme (DMF)) had the aim of verifying the existing limit values. These were therefore carried out at lower radiation intensities. A carcinogenic effect of radio-frequency fields with cell phonemobile radio -relevant frequencies and modulations was not seen in most DMF studies and some other animal studies [6] (exception: Falcioni et al. (2018) [3]).
The results of the NTP study thus differ from previous study results and raise the question of whether thermal stress may have led to the findings observed. This should be clarified in future research.
The SAR distributions in human and animal tissues vary widely: significant differences exist with regard to body sizes, organ sizes, and organ arrangements, each of which affects the SAR distributions. In mice and rats, internal organs are reached more strongly by radio-frequency electromagnetic fields acting from outside than in humans.
The whole-body SAR values used in the main study (≥ 1.5 W/kg) are significantly (approx. 20 times) higher than the nationally and internationally recommended limit value of 0.08 W/kg for whole-body exposure for the general population to protect against proven health effects. This value is used to derive the limit values applicable in Germany for fixed radio-frequency systems; these are far from being exhausted in everyday situations. For example, studies conducted by the DMF (German Mobile Telecommunication Research Programme) have found that in the proximity of GSM and UMTS base stations the limit value for the electric field strength has been exhausted to a maximum of 10 %. Under these conditions, it can be estimated that the whole-body SAR values induced in the human body do not exceed the order of 0.001 W/kg and are therefore at least three orders of magnitude (factor 1000) below the whole-body SAR values of the exposed rats in the NTP study.
Only the lowest exposure level used for rats (1.5 W/kg) is below the recommended local exposure limit (for the general population, the recommended SAR limit for the local exposure of head or torso is 2 W/kg). However, the dosimetric analyses [7] carried out by the authors at organ level show that the exposures of the hearts of male rats that were striking in the histopathological examinations were on average 1.9 times higher than in the body average (i.e. in the lowest exposed group at 2.8 W/kg and thus already above the recommended limit value for head and trunk).
[3] Falcioni, L., et al., Report of final results regarding brain and heart tumors in Sprague-Dawley rats exposed from prenatal life until natural death to mobile phone radiofrequency field representative of a 1.8GHz GSM base station environmental emission. Environ Res, 2018. 165: p. 496-503.
[4] Kadekar, S., et al., Gender differences in chemical carcinogenesis in National Toxicology Program 2-year bioassays. Toxicol Pathol, 2012. 40(8): p. 1160-8.
[5] Crissman, J.W., et al., Best practices guideline: toxicologic histopathology. Toxicologic Pathology, 2004. 32(1): p. 126-131.
[6] SCENIHR 2015. Potential health effects of exposure to electromagnetic fields (EMF). 27 January 2015.
[7] Gong, Y., et al., Life-Time Dosimetric Assessment for Mice and Rats Exposed in Reverberation Chambers for the Two-Year NTP Cancer Bioassay Study on Cell Phone Radiation. IEEE transactions on electromagnetic compatibility, 2017. 59(6): p. 1798-1808.
For future animal studies we recommend a continuous recording of the following parameters:
We recognize that experiments can never control for every parameter due to limiting factors such as funding. Especially the monitoring of heart rates might be challenging if one deals with free roaming animals. In recent years, image processing has developed rapidly allowing to extract heart and breath rates from video data [Zhao et al., 2013]. For future studies we therefore suggest to continuously videotape animals in order to gain information on behavior, heart and respiratory rate.
In the first phase, a five-day pilot study [1], the influence of different whole-body SARs on the body temperatures of young and adult B63F1 laboratory mice and young, adult, and pregnant Harlan Sprague-Dawley laboratory rats was investigated. To this end, five animals of each group were exposed to either GSM- or CDMA-modulated radio-frequency electromagnetic fields used by cell phones leading to whole-body SAR of 0, 4, 6, 8, 10, and 12 W/kg. The exposure was switched on and off every 10 minutes. In the exposure-free time, the temperature was measured using subcutaneously implanted temperature microchips.
There was a significant increase in body temperature in pregnant rats and in adult male and female rats aged 5.5 (CDMA) or 9 months (GSM) starting from a SAR of 6 W/kg compared with sham-exposed control animals. At 6 W/kg, the average body temperature increase was below one degree Celsius. Higher SARs resulted in higher body temperature increases; these exceeded one degree Celsius at a SAR of 8 W/kg and led to death in adult male rats at a SAR of 12 W/kg within the first day. The smaller and lighter female rats showed a lower mean temperature increase. On the other hand, the smaller and lighter mice showed only sporadic increases in body temperature.
Based on the results from the pilot study, young rats and mice were exposed at SAR values of 0, 3, 6, and 9 W/kg (rats) and 0, 5, 10, and 15 W/kg (mice) for 28 days in the second phase of the study. For rats and mice each, a shared control population (sham exposure, 0 W/kg) was used for GSM and UMTS modulation. The 28-day study was designed to determine the cumulative effects of repeated exposure to GSM- and CDMA-modulated radio-frequency fields at SAR values that did not result in increased mortality or excessively elevated animal body temperatures in the pilot study as well as to determine adequate SAR values for the two-year main study. In order to investigate a possible toxicity of radio-frequency fields in the case of in utero exposure as well as at an early postnatal time, pregnant female rats were exposed from the sixth day of gestation. The mice were between 5–6 weeks old at the beginning of the exposure. The effects of in utero or early postnatal exposure were not investigated in mice. The body weights and body temperatures of young rats and mice were measured on different days during the study. Body weights and temperatures were also measured in pregnant rats before birth and after birth during the lactation period.
The investigations in the 28-day study at 6 and 9 W/kg showed an increased mortality of young rats between Day 1 and 4 after birth, a reduced body weight of the mothers and young male and female rats, and increased body temperatures in the mothers. In mice, no effects on mortality and body weight were observed. At some time points, significant body temperature increases in male mice and significant body temperature decreases in female mice were observed at 5, 10, and 15 W/kg.
At the end of the 28-day study, histopathological examinations were performed on the control animals as well as on the highest exposed animals on all relevant organs (approx. 40 organs).
[1] Wyde, M.E., et al., Effect of cell phone radiofrequency radiation on body temperature in rodents: Pilot studies of the National Toxicology Program's reverberation chamber exposure system. Bioelectromagnetics, 2018. 39(3): p. 190-199.
The group size at the beginning of the main study was 105 animals per exposure level and control group for both species (mice and rats) for GSM and CDMA modulation (exception: here a common control group) and for both sexes. Almost 1900 rats and 1500 mice were used in the study. In rats, the mothers were exposed from the fifth day of gestation; the mice were exposed from the age of 5–6 weeks.
The freely moving animals were kept individually under well-known laboratory conditions, which differed only in exposure; the control group was sham exposed. Any concomitant effects of exposure (e.g. sounds) were identified and adjusted in the experiment between the exposure groups in order to ensure good comparability between the exposed and non-exposed animals. The intermittent whole body exposure (10 minutes on/10 minutes off) of the animals was 18 h and 20 min daily; this resulted in a total daily exposure duration of 9 h and 10 min.
An intermediate analysis was performed 14 weeks after the start of the study. Ten female and ten male rats and mice per group were euthanised, dissected, and examined for different endpoints (e.g. organ weights, microscopic examination of main tissues, sperm mobility and concentration, and vaginal cytology). In order to investigate genotoxicity, blood and tissue samples were taken from five male and five female rats and mice per group.
This left 90 animals per chronically long-term exposed exposure group and control group. After two years (roughly corresponding to the average life expectancy of rats and mice), any animals still living were euthanised. All animals, including those that were found dead during the study or had to be euthanised because of illness, were dissected and examined.
To test whether sham-exposed and exposed animals differ in terms of possible long-term effects, two types of statistical tests (separate for mice and rats, male and female animals, and GSM and CDMA modulation) were performed for the aforementioned endpoints (intermediate analysis and end of study). In the first test, one of the three exposure groups was compared with the control group (pairwise comparison). In the second test, it was examined whether there was a trend with increasing intensity of exposure. The significance level for the statistical tests was 0.05 (pathology).
Weight and weight gain and survival
Neoplastic and non-neoplastic lesions of the heart, brain, and adrenal medulla have already been listed in the statement but are listed again here for completeness:
Heart: Increased incidence of malignant schwannomas in all GSM- (0/2/1/5) and CDMA- (0/2/3/6*) exposed groups of male rats. Both modulations show a significantly positive trend. In female rats, incidences increased only in the 3 W/kg GSM (GSM 0/0/2/0) and the 1.5 and 6 W/kg CDMA-exposed groups (0/2/0/2). Increased incidence of Schwann cell hyperplasia (possible preneoplastic Schwann cell lesion) in male (GSM, 0/1/0/2; CDMA 0/0/0/3) and female rats (CDMA 0/1/1/1 only).
Increased incidences of right ventricular cardiomyopathies in all groups of male (GSM, 54/62/72*/74*; CDMA, 54/45/62/74*) and female rats (GSM, 4/9/14*/15*; CDMA, 4/7/9/9).
Brain: Increased incidence of malignant gliomas in all GSM-exposed groups of male rats (0/3/3/2) and in the 6 W/kg exposed group of female rats (0/0/0/1). For CDMA increased incidences in male (0/0/0/3, significantly positive trend) and female rats (0/3/0/0).
Increased incidence of hyperplasia of glial cells (possible preneoplastic lesion that may develop into a malignant glioma) in most exposed groups of both sexes. However, this was not significant.
Other organs: Lesions in various organs (e.g. tumours of the prostate gland, pituitary gland, or pancreas) were observed. However, it was not possible to assess whether these resulted from exposure or were accidental findings. The exposure-dependent reduction of the severity of chronic nephropathy in male rats was reflected in an exposure-dependent reduction of the incidence of secondary lesions such as mineral deposits and chronic inflammations in blood vessels and hyperplasia of the parathyroid gland.
Weight and weight gain and survival
Neoplastic and non-neoplastic lesions
The extraordinary transparency of the NTP is a positive aspect. A considerable amount of data (e.g. incidence tables) is freely available on the NTP servers to verify the validity of the statistical evaluation and others. The tissue sections can also be viewed in the archives of the NTP.