Assessment of radiofrequency exposure from cellular telephone daily use in an epidemiological study: German Validation study of the international case-control study of cancers of the brain--INTERPHONE-Study

OBJECTIVE: The objective of the study is to validate self-reported cellular phone use information by comparing it with the cumulative emitted power and duration of calls measured by software-modified cellular phones (SMP). The information was obtained using a questionnaire developed for the international case-control study on the risk of the use of mobile phones in tumours of the brain or salivary gland (INTERPHONE-study). METHOD: The study was conducted in Bielefeld, Germany. Volunteers were asked to use SMPs instead of their own cellular phones for a period of 1 month. The SMP recorded the power emitted by the mobile phone handset during each base station contact. Information on cellular phone use for the same time period from traffic records of the network providers and from face-to-face interviews with the participants 3 months after the SMP use was assessed. Pearson's correlation coefficients and linear regression models were used to analyse the association between information from the interview and from the SMP. RESULTS: In total, 1757 personal mobile phone calls were recorded for 45 persons by SMP and traffic records. The correlation between the self-reported information about the number and the duration of calls with the cumulative power of calls was 0.50 (P<0.01) and 0.48 (P<0.01), respectively. Almost 23% of the variance of the cumulative power was explained by either the number or the cumulative duration of calls. After inclusion of possible confounding factors in the regression model, the variance increased to 26%. Minor confounding factors were "network provider", "contract form", and "cellular phone model". DISCUSSION: The number of calls alone is a sufficient parameter to estimate the cumulative power emitted by the handset of a cellular telephone. The cumulative power emitted by these phones is only associated with number of calls but not with possible confounding factors. Using the mobile phone while driving, mainly in cities, or mainly in rural areas is not associated with the recorded cumulative power in the SMP.     

Radiofrequency electromagnetic fields,screen time,and emotional and behavioural problems in 5-year-old children

Background

Little is known about the exposure of young children to radiofrequency electromagnetic fields (RF-EMF) and potentially associated health effects. We assessed the relationship of RF-EMF exposure from different sources and screen time exposure with emotional and behavioural problems in 5-year-old children.

Validation of self-reported cellular phone use

BACKGROUND: In recent years, concern has been raised over possible adverse health effects of cellular telephone use. In epidemiological studies of cancer risk associated with the use of cellular telephones, the validity of self-reported cellular phone use has been problematic. Up to now there is very little information published on this subject. METHODS: We conducted a study to validate the questionnaire used in an ongoing international case-control study on cellular phone use, the "Interphone study". Self-reported cellular phone use from 68 of 104 participants who took part in our study was compared with information derived from the network providers over a period of 3 months (taken as the gold standard). RESULTS: Using Spearman's rank correlation, the correlation between self-reported phone use and information from the network providers for cellular phone use in terms of the number of calls per day was good (r=0.62, 95% CI: 0.45-0.75), while that of the average duration of each call was rather moderate (r=0.34, 95% CI: 0.11-0.54). Similar results were found when Kappa coefficients were estimated. A value of r=0.56 (Spearman's correlation, CI: 0.38-0.70) was found for cumulative cellular phone use. CONCLUSION: Our study suggests that cellular phone use is easier to recall in terms of number of calls made than in terms of cumulative phone use and should thus be used as the basis for the dose-response analysis.     

Mobile phone consultation for community health care in rural north India

We conducted a study to ascertain the acceptability and feasibility of consultation by mobile phone in a rural area of northern India. The mobile phone number of a community physician was advertised to the general public and people were invited to telephone at any time for a medical consultation. Details of the calls received were recorded. During a seven-month study, 660 calls were received. The mean call duration was 2.7 min. Eighty percent of calls were made by men. Forty-eight percent of calls were made during office hours. A total of 417 (63%) calls were for seeking advice, 146 (22%) were for outpatient follow-up, 23 (4%) were for seeking appointments and the remaining 74 (11%) for other reasons. The most common problems were skin, respiratory, mental health and sexual problems. Of the 387 callers who were interviewed at follow-up, 302 (78%) stated that they had followed the advice provided. Of these, 91% found the advice very helpful in managing their health problems. About 96% of users wished to continue to use the service in future. The majority of calls made were of a primary care nature which could easily be dealt with by phone. The concept of using mobile phones for medical consultation seemed to be acceptable to people in rural Haryana.     

Instruments to assess and measure personal and environmental radiofrequency-electromagnetic field exposures

Radiofrequency-electromagnetic field (RF-EMF) exposure of human populations is increasing due to the widespread use of mobile phones and other telecommunication and broadcasting technologies. There are ongoing concerns about potential short- and long-term public health consequences from RF-EMF exposures. To elucidate the RF-EMF exposure-effect relationships, an objective evaluation of the exposures with robust assessment tools is necessary. This review discusses and compares currently available RF-EMF exposure assessment instruments, which can be used in human epidemiological studies. Quantitative assessment instruments are either mobile phone-based (apps/software-modified and hardware-modified) or exposimeters. Each of these tool has its usefulness and limitations. Our review suggests that assessment of RF-EMF exposures can be improved by using these tools compared to the proxy measures of exposure (e.g. questionnaires and billing records). This in turn, could be used to help increase knowledge about RF-EMF exposure induced health effects in human populations.     

Validation of short term recall of mobile phone use for the Interphone study

Aim

To validate short term recall of mobile phone use within Interphone, an international collaborative case control study of tumours of the brain, acoustic nerve, and salivary glands related to mobile telephone use.

Methods

Mobile phone use of 672 volunteers in 11 countries was recorded by operators or through the use of software modified phones, and compared to use recalled six months later using the Interphone study questionnaire. Agreement between recalled and actual phone use was analysed using both categorical and continuous measures of number and duration of phone calls.

Results

Correlations between recalled and actual phone use were moderate to high (ranging from 0.5 to 0.8 across countries) and of the same order for number and duration of calls. The kappa statistic demonstrated fair to moderate agreement for both number and duration of calls (weighted kappa ranging from 0.20 to 0.60 across countries). On average, subjects underestimated the number of calls per month (geometric mean ratio of recalled to actual = 0.92, 95% CI 0.85 to 0.99), whereas duration of calls was overestimated (geometric mean ratio = 1.42, 95% CI 1.29 to 1.56). The ratio of recalled to actual use increased with level of use, showing underestimation in light users and overestimation in heavy users. There was substantial heterogeneity in this ratio between countries. Inter‐individual variation was also large, and increased with level of use.

Conclusions

Volunteer subjects recalled their recent phone use with moderate systematic error and substantial random error. This large random error can be expected to reduce the power of the Interphone study to detect an increase in risk of brain, acoustic nerve, and parotid gland tumours with increasing mobile phone use, if one exists.     

Symptoms experienced in connection with mobile phone use

Many people in Norway and Sweden reported headaches, fatigue, and other symptoms experienced in connection with the use of a mobile phone (MP). Therefore, we initiated a cross-sectional epidemiological study among 17,000 people, all using an MP in their job. Thirty-one percent of the respondents in Norway and 13% of those in Sweden had experienced at least one symptom in connection with MP use. Next to the sensations of warmth on the ear and behind/around the ear, burning sensations in the facial skin and headaches were most commonly reported. Most symptoms usually began during or within half an hour after the call and lasted for up to 2 h. Relatively few had consulted a physician or been on sick leave because of the symptoms, but about 45% among those with an MP attributed symptom had taken steps to reduce the symptom. These results suggest an awareness of the symptoms, but not necessarily a serious health problem.     

Cellular phone use and risk of benign and malignant parotid gland tumors--a nationwide case-control study

The objective of this nationwide study was to assess the association between cellular phone use and development of parotid gland tumors (PGTs). The methods were based on the international INTERPHONE study that aimed to evaluate possible adverse effects of cellular phone use. The study included 402 benign and 58 malignant incident cases of PGTs diagnosed in Israel at age 18 years or more, in 2001-2003, and 1,266 population individually matched controls. For the entire group, no increased risk of PGTs was observed for ever having been a regular cellular phone user (odds ratio = 0.87; p = 0.3) or for any other measure of exposure investigated. However, analysis restricted to regular users or to conditions that may yield higher levels of exposure (e.g., heavy use in rural areas) showed consistently elevated risks. For ipsilateral use, the odds ratios in the highest category of cumulative number of calls and call time without use of hands-free devices were 1.58 (95% confidence interval: 1.11, 2.24) and 1.49 (95% confidence interval: 1.05, 2.13), respectively. The risk for contralateral use was not significantly different from 1. A positive dose-response trend was found for these measurements. Based on the largest number of benign PGT patients reported to date, our results suggest an association between cellular phone use and PGTs.     

An international prospective cohort study of mobile phone users and health (COSMOS): Factors affecting validity of self-reported mobile phone use

This study investigates validity of self-reported mobile phone use in a subset of 75 993 adults from the COSMOS cohort study. Agreement between self-reported and operator-derived mobile call frequency and duration for a 3-month period was assessed using Cohen’s weighted Kappa (κ). Sensitivity and specificity of both self-reported high (≥10 calls/day or ≥4 h/week) and low (≤6 calls/week or <30 min/week) mobile phone use were calculated, as compared to operator data. For users of one mobile phone, agreement was fair for call frequency (κ = 0.35, 95% CI: 0.35, 0.36) and moderate for call duration (κ = 0.50, 95% CI: 0.49, 0.50). Self-reported low call frequency and duration demonstrated high sensitivity (87% and 76% respectively), but for high call frequency and duration sensitivity was lower (38% and 56% respectively), reflecting a tendency for greater underestimation than overestimation. Validity of self-reported mobile phone use was lower in women, younger age groups and those reporting symptoms during/shortly after using a mobile phone. This study highlights the ongoing value of using self-report data to measure mobile phone use. Furthermore, compared to continuous scale estimates used by previous studies, categorical response options used in COSMOS appear to improve validity considerably, most likely by preventing unrealistically high estimates from being reported.     

Mobile phone use and the risk of acoustic neuroma

BACKGROUND: Radiofrequency exposure from mobile phones is concentrated to the tissue closest to the handset, which includes the auditory nerve. If this type of exposure increases tumor risk, acoustic neuroma would be a potential concern. METHODS: In this population-based case-control study we identified all cases age 20 to 69 years diagnosed with acoustic neuroma during 1999 to 2002 in certain parts of Sweden. Controls were randomly selected from the study base, stratified on age, sex, and residential area. Detailed information about mobile phone use and other environmental exposures was collected from 148 (93%) cases and 604 (72%) controls. RESULTS: The overall odds ratio for acoustic neuroma associated with regular mobile phone use was 1.0 (95% confidence interval = 0.6-1.5). Ten years after the start of mobile phone use the estimates relative risk increased to 1.9 (0.9-4.1); when restricting to tumors on the same side of the head as the phone was normally used, the relative risk was 3.9 (1.6-9.5). CONCLUSIONS: Our findings do not indicate an increased risk of acoustic neuroma related to short-term mobile phone use after a short latency period. However, our data suggest an increased risk of acoustic neuroma associated with mobile phone use of at least 10 years' duration.     

大学生手机使用行为与抑郁症状双向关联的随访研究

目的 描述大学生手机使用行为和抑郁症状的流行现状及双向关联,为促进大学生健康行为养成提供循证依据。方法 于2019年4—5月选取安徽省合肥市和江西省上饶市2所高校的1 135名学生进行基线问卷调查,每6个月随访1次,于2021年4—5月进行最后一次问卷随访,匹配后获得有效人数999人。运用自评问卷调查大学生手机使用时长和手机功能使用情况,运用青少年手机使用依赖自评问卷和患者健康问卷-9项(PHQ-9)评估大学生手机依赖情况和抑郁症状。采用Pearson相关分析检验基线和2年后手机使用行为和抑郁之间的相关性;采用线性回归模型分析手机使用行为与抑郁症状评分之间的线性关联;采用自回归交叉滞后模型分析大学生手机使用行为和抑郁症状之间的双向关联。结果 基线调查中大学生手机依赖和抑郁症状的检出率分别为24.3%和42.4%。基线和2年后随访中大学生平均手机使用时长分别为[(2.84±0.90)(2.02±1.05)h/d];手机依赖平均得分分别为[(23.30±9.00)(23.29±10.45)分];手机功能使用平均得分分别为[(30.12±6.66)(29.12±7.27)分];抑郁症状平均得...     

Biological monitoring of smoke exposure among wildland firefighters: a pilot study comparing urinary methoxyphenols with personal exposures to carbon monoxide, particular matter, and levoglucosan

Urinary methoxyphenols (MPs) have been proposed as biomarkers of woodsmoke exposure. However, few field studies have been undertaken to evaluate the relationship between woodsmoke exposure and urinary MP concentrations. We conducted a pilot study at the US Forest Service-Savannah River Site, in which carbon monoxide (CO), levoglucosan (LG), and particulate matter (PM(2.5)) exposures were measured in wildland firefighters on prescribed burn days. Pre- and post-shift urine samples were collected from each subject, and cross-shift changes in creatinine-corrected urinary MP concentrations were calculated. Correlations between exposure measures and creatine-adjusted urinary MP concentrations were explored, and regression models were developed relating changes in urinary MP concentrations to measured exposure levels. Full-shift measurements were made on 13 firefighters over 20 work shifts in winter 2004 at the US Forest Service Savannah River site, a National Environmental Research Park. The average workshift length across the 20 measured shifts was 701+/-95 min. LG and CO exposures were significantly correlated for samples where the filter measurement captured at least 60% of the work shift (16 samples), as well as for the smaller set of full-shift exposure samples (n=9). PM(2.5) and CO exposures were not significantly correlated, and LG and PM(2.5) exposures were only significantly correlated for samples representing at least 60% of the work shift. Creatinine-corrected urinary concentrations for 20 of the 22 MPs showed cross-shift increases, with 14 of these changes showing statistical significance. Individual and summed creatinine-adjusted guaiacol urinary MPs were highly associated with CO (and, to a lesser degree, LG) exposure levels, and random-effects regression models including CO and LG exposure levels explained up to 80% of the variance in cross-shift changes in summed creatinine-adjusted guaiacol urinary MP concentrations. Although limited by the small sample size, this pilot study demonstrates that urinary MP concentrations may be effective biomarkers of occupational exposure to wood smoke among wildland firefighters.     

Follow-up phone calls increase nutrient intake estimated by three-day food diaries in 13-year-old participants of the Raine study

Aim: Our primary objective was to determine the effect of follow‐up phone calls on estimated nutrient intakes obtained by three‐day food diaries from 13‐year‐old adolescents. Methods: Food diaries were recorded using household measures and entered into a dietary analysis software program, before and after follow up by telephone. A sample of 340 participants aged 13 years born into the Western Australian Pregnancy Cohort (Raine) Study, a population‐based longitudinal cohort followed from 16 to 20 weeks' gestation to 13 years of age (current follow up). After face‐to‐face instruction, participants completed three‐day food diaries at home and returned them by post. Follow‐up telephone calls were made to each participant to improve data collection response and to verify missing details in the food diaries. Nutrient intakes before and after telephone follow up were compared using Student's t‐tests in spss . Results were also compared with those of the Child and Adolescent Physical Activity and Nutrition survey. Results: Follow‐up phone calls significantly increased the estimated intake of total kilojoules, water, total carbohydrates, sugars and magnesium (P < 0.05). Conclusion: These results indicate the importance of follow‐up phone calls to obtain missing details in three‐day food diaries completed by adolescents.     

Radiofrequency electromagnetic field exposure and non-specific symptoms of ill health: a systematic review

This article is a systematic review of whether everyday exposure to radiofrequency electromagnetic field (RF-EMF) causes symptoms, and whether some individuals are able to detect low-level RF-EMF (below the ICNIRP [International Commission on Non-Ionizing Radiation Protection] guidelines). Peer-reviewed articles published before August 2007 were identified by means of a systematic literature search. Meta-analytic techniques were used to pool the results from studies investigating the ability to discriminate active from sham RF-EMF exposure. RF-EMF discrimination was investigated in seven studies including a total of 182 self-declared electromagnetic hypersensitive (EHS) individuals and 332 non-EHS individuals. The pooled correct field detection rate was 4.2% better than expected by chance (95% CI: −2.1 to 10.5). There was no evidence that EHS individuals could detect presence or absence of RF-EMF better than other persons. There was little evidence that short-term exposure to a mobile phone or base station causes symptoms based on the results of eight randomized trials investigating 194 EHS and 346 non-EHS individuals in a laboratory. Some of the trials provided evidence for the occurrence of nocebo effects. In population based studies an association between symptoms and exposure to RF-EMF in the everyday environment was repeatedly observed. This review showed that the large majority of individuals who claims to be able to detect low level RF-EMF are not able to do so under double-blind conditions. If such individuals exist, they represent a small minority and have not been identified yet. The available observational studies do not allow differentiating between biophysical from EMF and nocebo effects.     

不同手机功能对高职学生压力和手机成瘾的影响

探讨不同手机功能的使用对高职学生感知到的压力和手机成瘾倾向的影响,为指导高职生科学使用手机提供参考.方法 使用方便抽样法在武汉、聊城2所高职院校抽取911名学生,使用智能手机使用问卷(SU)、大学生手机成瘾倾向量表(MPATS)、简易应对方式问卷(SCSQ)和抑郁-焦虑-压力量表简版中的压力分量表(DASS-S)进行施测.结果 高职学生最常使用的手机功能是社交网站[(4.77±1.06)分],女生在手机社交功能上的得分高于男生(t=2.05,P=0.04),男生在MPATS和DASS-S、消极应对方式分量表上的得分高于女生(P值均<0.01).手机的社交功能与积极应对方式呈正相关(r=0.17,P<0.01),手机的游戏功能与消极应对方式、压力和手机成瘾呈正相关(P值均<0.01).消极应对方式能够正向预测压力和手机成瘾(β值分别为0.53,0.50,P值均<0.01),消极应对方式和压力在非社交功能与手机成瘾中的链式中介效应有统计学意义(95%CI=0.06~0.24,P<0.01).结论 经常使用手机非社交功能的高职生感知到的压力更大.手机的游戏功能对高职学生身心健康有不利影响,消极应对方式是压力和手机成瘾的一个强有力的预测变量.     

The influence of the call with a mobile phone on heart rate variability parameters in healthy volunteers

It is possible that electromagnetic field (EMF) generated by mobile phones (MP) may have an influence on the autonomic nervous system (ANS) and modulates the function of circulatory system. The aim of the study was to estimate the influence of the call with a mobile phone on heart rate variability (HRV) in young healthy people. The time and frequency domain HRV analyses were performed to assess the changes in sympathovagal balance in a group of 32 healthy students with normal electrocardiogram (ECG) and echocardiogram at rest. The frequency domain variables were computed: ultra low frequency (ULF) power, very low frequency (VLF) power, low frequency (LF) power, high frequency (HF) power and LF/HF ratio was determined. ECG Holter monitoring was recorded in standardized conditions: from 08:00 to 09:00 in the morning in a sitting position, within 20 min periods: before the telephone call (period I), during the call with use of mobile phone (period II), and after the telephone call (period III). During 20 min call with a mobile phone time domain parameters such as standard deviation of all normal sinus RR intervals (SDNN [ms]--period I: 73.94+/-25.02, period II: 91.63+/-35.99, period III: 75.06+/-27.62; I-II: p<0.05, II-III: p<0.05) and standard deviation of the averaged normal sinus RR intervals for all 5-mm segments (SDANN [ms]--period I: 47.78+/-22.69, period II: 60.72+/-27.55, period III: 47.12+/-23.21; I-II: p<0.05, II-III: p<0.05) were significantly increased. As well as very low frequency (VLF [ms2]--period I: 456.62+/-214.13, period II: 566.84+/-216.99, period III: 477.43+/-203.94; I-II: p<0.05), low frequency (LF [ms(2)]--period I: 607.97+/-201.33, period II: 758.28+/-307.90, period III: 627.09+/-220.33; I-II: p<0.01, II-III: p<0.05) and high frequency (HF [ms(2)]--period I: 538.44+/-290.63, period II: 730.31+/-445.78, period III: 590.94+/-301.64; I-II: p<0.05) components were the highest and the LF/HF ratio (period I: 1.48+/-0.38, period II: 1.16+/-0.35, period III: 1.46+/-0.40; I-II: p<0.05, II-III: p<0.05) was the lowest during a call with a mobile phone. The tone of the parasympathetic system measured indirectly by analysis of heart rate variability was increased while sympathetic tone was lowered during the call with use of a mobile phone. It was shown that the call with a mobile phone may change the autonomic balance in healthy subjects. Changes in heart rate variability during the call with a mobile phone could be affected by electromagnetic field but the influence of speaking cannot be excluded.     

Off-the-cuff cellular phone consultations in a family practice

In Israel the public tends to make use of informal medicine alongside organized health services, and cellular phones now allow contact with physicians at almost any time or place. For three months in 1999 a family physician documented all consultations on medical subjects conducted by cellular phone, the phone being available 24 hours a day. There were 94 cellular phone medical consultations, of mean duration 5.8 min (range 2-18). Only 11 took place over the weekend, and 63 took place while the clinic was open. The most common reasons for consultation were advice on treatment (29%) and a second opinion (28%). In 48 cases the consultation was for a close relative rather than the caller. In 42 cases the request for consultation came while the physician was busy with other patients. The results of this small personal study confirm that the practice of informal consultations now extends to the cellular phone. Technologies of this sort demand new rules of conduct, if we are to avoid the various hazards of off-the-cuff medicine.     

Output power levels from mobile phones in different geographical areas; implications for exposure assessment

Background: The power level used by the mobile phone is one of the most important factors determining the intensity of the radiofrequency exposure during a call. Mobile phone calls made in areas where base stations are densely situated (normally urban areas) should theoretically on average use lower output power levels than mobile phone calls made in areas with larger distances between base stations (rural areas).     

Towards 5G communication systems: Are there health implications?

The spread of radiofrequency electromagnetic fields (RF-EMF) is rising and health effects are still under investigation. RF-EMF promote oxidative stress, a condition involved in cancer onset, in several acute and chronic diseases and in vascular homeostasis. Although some evidences are still controversial, the WHO IARC classified RF-EMF as “possible carcinogenic to humans”, and more recent studies suggested reproductive, metabolic and neurologic effects of RF-EMF, which are also able to alter bacterial antibiotic resistance. In this evolving scenario, although the biological effects of 5G communication systems are very scarcely investigated, an international action plan for the development of 5G networks has started, with a forthcoming increment in devices and density of small cells, and with the future use of millimeter waves (MMW). Preliminary observations showed that MMW increase skin temperature, alter gene expression, promote cellular proliferation and synthesis of proteins linked with oxidative stress, inflammatory and metabolic processes, could generate ocular damages, affect neuro-muscular dynamics. Further studies are needed to better and independently explore the health effects of RF-EMF in general and of MMW in particular. However, available findings seem sufficient to demonstrate the existence of biomedical effects, to invoke the precautionary principle, to define exposed subjects as potentially vulnerable and to revise existing limits. An adequate knowledge of pathophysiological mechanisms linking RF-EMF exposure to health risk should also be useful in the current clinical practice, in particular in consideration of evidences pointing to extrinsic factors as heavy contributors to cancer risk and to the progressive epidemiological growth of noncommunicable diseases.     

Cellular phones: are they detrimental?

The issue of possible health effects of cellular phones is very much alive in the public's mind where the rapid increase in the number of the users of cell phones in the last decade has increased the exposure of people to the electromagnetic fields (EMFs). Health consequences of long term use of mobile phones are not known in detail but available data indicates the development of non specific annoying symptoms on acute exposure to mobile phone radiations. In an attempt to determine the prevalence of such cell phones associated health manifestations and the factors affecting their occurrence, a cross sectional study was conducted in five randomly selected faculties of Alexandria University. Where, 300 individuals including teaching staff, students and literate employee were equally allocated and randomly selected among the five faculties. Data about mobile phone's users and their medical history, their pattern of mobile usage and the possible deleterious health manifestations associated with cellular phone use was collected. The results revealed 68% prevalence of mobile phone usage, nearly three quarters of them (72.5%) were complainers of the health manifestations. They suffered from headache (43%), earache (38.3%), sense of fatigue (31.6%), sleep disturbance (29.5%), concentration difficulty (28.5%) and face burning sensation (19.2%). Both univariate and multivariate analysis were consistent in their findings. Symptomatic users were found to have significantly higher frequency of calls/day, longer call duration and longer total duration of mobile phone usage/day than non symptomatic users. For headache both call duration and frequency of calls/day were the significant predicting factors for its occurrence (chi2 = 18.208, p = 0.0001). For earache, in addition to call duration, the longer period of owning the mobile phone were significant predictors (chi2 = 16.996, p = 0.0002). Sense of fatigue was significantly affected by both call duration and age of the user (chi2 = 24.214, p = 0.0000), while burning sensation was only affected by frequency of calls/day (chi2 = 5.360, p = 0.020). According to the 95% confidence interval of frequency and duration of calls, the study recommended not to increase the call duration more than four minutes and limit their frequency to less than seven calls/day with total duration of exposure less than 22 min./day.