Introduction No studies of health effects from long-term exposure to strong MRI-related static magnetic fields exist.

Methods Within an industrial cohort of employees from an MRI producing plant longitudinal blood pressure data from a medical surveillance scheme were used to study the association between cumulative exposure to static magnetic fields (SMF) and development of hypertension during employment.

The cohort consisted of all employees employed for at least one year between 1984 and 2010 in the business units Magnetic Resonance and X-Ray (reference population). Employees with at least two years between two complete blood pressure measurement records were selected for this analysis.

Systolic and diastolic blood pressure were measured with a manual sphygmomanometer. Hypertension was defined as a systolic pressure > 140 mm Hg and/or a diastolic blood pressure > 90 mm Hg. Development of hypertension was defined as hypertension at final exam among individuals with no hypertension at first exam (117/538 male employees). Statistical models were adjusted for age, BMI, blood pressure levels at first examination and having undergone voluntary MRI scans.

Exposure to SMF (Tesla-minutes) was estimated by combining employment records with a historical job-SMF exposure-matrix. Cumulative exposure between blood pressure measurements was categorised as low or high using median exposure as cut-point.

Results Only employees with high cumulative exposure to SMF (≥7413 T-min) showed an increased odds ratio for developing hypertension (OR = 2.24, 95% CI: 1.25–4.00) compared to non-exposed non-volunteering employees from both departments. Ever being scanned as a volunteer resulted in a significantly reduced odds ratio (OR = 0.57, 95% CI: 0.37–0.90), reflecting the policy of selecting healthy volunteers. Age, body mass index and systolic blood pressure at first exam were, as expected, positively associated with developing hypertension.

Conclusion Long-term high exposure to MRI-related SMF during MRI manufacturing may results in developing hypertension during employment.