Risk of Cancer in Occupational Exposure to Radar Radiation: A Systematic Review

Background and aim: Microwave radiations are one of the most environmental workplace factors that exposed too many workers. In microwave spectrum, radar frequency with 1-300 GHz range have varied applications such as satellite, communications, military, Network, navigation, air-traffic Control, navigation, marine and weather. Uncontrolled occupational exposure to radar radiation caused to various disease and disorders such as oxidative stress, different kinds of cancers and other thermal and non-thermal adverse health effects. Regard to the importance of identification work related carcinogenic factors in prevention and decreasing cancer related costs and lack of similar studies, this study was conducted to systematically review the research on the risk of cancer in occupational exposure to radar radiation with frequency ranges 1 to 3000 GHz.  Methods: The present systematic search was carried out based on PRISMA guidelines and comprehensive search strategy was focused from March 2017 to September 2018 and update on September 2018 in PubMed, ISI Web of Science, Scopus and Google scholar, MAGIRAN and SID databases with English and Persian articles without time limits were searched .Keywords were selected based on PICO principle and collected from MeSH database. comprehensive search  were accomplishment by the following search  terms  : (worker OR technician OR occupation OR military OR airline OR navy OR police officer OR  Weather )  AND (occupational exposure OR  workplace OR  long-term exposure OR  exposure  OR  radar OR  microwave OR  wireless OR  high frequency range OR  radiofrequency OR radiation OR  electromagnetic) AND (control group, cohort OR prospective OR retrospective OR follow-up OR randomized control trial OR case-control) AND (cancer OR malignant OR melanoma OR metastatic OR non-thermal effect OR biological effect OR health effect OR Adverse Effect OR risk factor OR Sarcoma OR tumor or leukemia OR neoplasm OR Carcinoma OR Hepatoma OR lymphoma OR mortality ) as single or complex terms in titles, abstracts and keywords. Then, taking into inclusion and exclusion criteria, the process of reviewing, screening and limiting the repeated and unrelated articles was carried out. In addition, manual references checking were done to retrieve the related articles. Also, any disagreements were resolved by consensus between reviewers. The inclusion criteria for selection studies were in design of case–control, cohort and randomized control trial studies, with control group and referring to the association between occupational exposure to radar radiation and all types of cancer in workers. The exclusion criteria were as follows: 1) studies without control group 2) reviews, case and field studies, 3) studies with inhumane population such as in vitro, in vivo and animal studies. 4) Studies that radar frequencies were out of considered ranges (1-300 GHz) .5) studies with other occupational or non-occupational carcinogenic risk factors (such as solvents, workplace air pollution, environmental air pollution, smoking and etc). After implementation of inclusion and exclusion criteria, data were extracted after preparation of the full text of included articles. The Results: A total of 533 studies was found. After removal of duplicated references, 272 studies were included for the title, keywords and abstract screening. Then, 219 studies were excluded since they did not meet inclusion criteria. Hence, 53 studies were selected for the eligibility assessment. At the end of selection process and after the quality assessment, 7 studies remained in the systematic review that including 3 case-control studies and 4 cohort studies with no randomized control trial study. A manual search of the reference lists added no more articles in this review.  All included studies were from English language and external databases in various countries including USA, Germany, and France. In this systematic review, a study of clinical trials and Persian studies were not found. Finally, based on inclusion and exclusion criteria, 7 articles (4 cohorts and 3 case-control) were included, which were conducted during the 1950 to 2005 with 51898 sample size and range of ages 15–69 years that published from 1993 to 2016. Included studies examined relationship between occupational exposure to radar radiation and cancer strength among workers. Whole of the studies put determination of cancers and risk estimates in military workers. The great majority of included studies reported risk estimates with 95% confidence interval. Most included studies were conducted on testicular cancer (4 from 7 included studies) and brain cancer (4 from 7 included studies). In included studies, Relative risk were estimated, particularly about testicular, leukemia and brain cancers and mortality ratio in various types of cancers has been reported. The included studies reported no significant increase in mortality ratio about testicular cancer but increasing in relative risk were significant in two studies. There was also a significant increase in the relative risk of brain cancer in workers who occupationally exposed to radar radiation. Increase in mortality ratio due to brain cancer were significantly reported in two included studies. Conclusion: The current systematic review was carried out to investigate the cancer among workers with occupational exposure to radar radiation. It is noticeable that previous related studies focused on both environmental and occupational exposure in worker and public population and it clearly did not determine the relationship between occupational radar exposure and cancer risk in workplace. Whilst, our study just focused on occupational exposure to radar radiation that lead to small number of included studies. The results of this study can be useful to prepare the occupational health policies in related to radar radiation and provide some information to conduct associated meta-analysis in future studies. On the limitations of the included studies, the continuation of experimental studies on humans with control groups, and focusing on the study of cancer-causing radar radiation in occupational exposure to workers is necessary.in this line, World Health Organization propose to conduct further differences studies  especially case-control studies. However, it should be mentioned that these results are yielded by a few numbers of available studies with no report in occupational dose and time exposure to radar frequency range. also, considering to some limitation such as few numbers of included studies, lack of data about exposure characterizations (exposure time, dose-response, average of exposure level) and demographic characterizations (average of age, average of experience, radar frequency range) it is better to continue further studies about this topic and future review studies include the congress publications without limitation in language. It is also suggested that other published articles in congresses also be used and the limitation in language of publications in the search process be removed. It is noticeable that, slight researches about occupational cancers in Iran were conducted in yet, such as studies in related to asbestosis, aromatic amines and chromium. Therefore, regarding the fast growing of cancer in developing countries like Iran, determination of occupational cancer risk factors could be useful to provide the cancer prevention and control program. So in this line, designing epidemiologic studies to focus on occupational cancers are very important especially about microwave and radar radiation.