Exposure Misclassification in the Danish Mobile Phone Subscriber Cohort and its Influence on International Radiofrequency (RF) Radiation Cancer Risk Assessments

Abstract

The Danish mobile phone subscriber cohort, updated in 2011, is one of the most frequently cited epidemiological studies concluding that mobile phone use is not associated with increased risk of brain tumors. Regulatory bodies routinely rely on this cohort as part of the evidence for null associations. However, the Danish cohort exhibits structural exposure misclassification and control group contamination that limit its ability to give information on health risks. The objective of this study was to critically examine methodological features, evaluate how they constrain inference regarding brain tumor risk, and document how the study was assessed in several international risk assessments. Methodological aspects, data on numbers of subscribers, minutes of mobile phone use over the cohort study period and incidence statistics of central nervous system (CNS) tumours were examined. We analyzed how the cohort’s findings have been evaluated, critically analyzed and incorporated into major international risk assessments from the WHO, the EU and national expert groups. Several structural limitations were identified: exposure classification restricted to pre-1996 private subscribers; misclassification of 200 507 corporate subscribers and all post-1995 users classified as “unexposed” leading to severe contamination of the unexposed group with exposed individuals; absence of individual usage data; DECT phone users classified as unexposed; insufficient window for slow-growing tumors. These factors bias the risks toward unity. Data from NORDCAN show increasing CNS tumour incidence in Denmark, which contradicts the findings in the cohort. Several expert evaluations have assessed the cohort as high quality evidence of absence of risk. The WHO commissioned review published in 2024 appears to be the assessment in which the cohort contributed most substantially to the overall results. Due to structural exposure misclassification and control group contamination, the Danish cohort cannot test associations between mobile phone use and brain tumor risk. The continued use of the study in international risk assessments bias overall results towards null. The study is scientifically flawed and is uninformative on health risks from mobile phone use.