Table 1.
Main findings about the possibility of derogations for the different frequency ranges.
| Overexposure Conditions | Literature Indications | Possibility of Derogations | |
|---|---|---|---|
| Static magnetic fields | >8 T | For exposures > 8 T there is no human experience and therefore there is lack of knowledge [2]. | It is not possible to guarantee health and safety of workers. |
| Low-frequency electric and magnetic fields | >0.8 V/m * (1 Hz–3 kHz) >2.7 × 10−4 f V/m * (3 kHz–10 MHz) |
The stimulation threshold of peripheral nerve fibers falls in the 4–6 V/m * range of electric field strength [3]. Human studies reported increasing thresholds for effects ranging from the mere perception of induced electric currents to uncomfortable and intolerable painful stimulations [26]. Thresholds for potentially dangerous effects (up to the induction of ventricular fibrillation) are increasingly higher [3]. In the case of Magnetic Resonance Imaging (MRI) equipment, the EN 60601-2-33 [21] extends to workers the limits set for patients. The limit set to avoid cardiac stimulation incorporates a safety factor 3 below the 1-percentile threshold reported in literature [24,25]. Compliance to the limits set to minimize peripheral nerve stimulation implies the possibility that up to 50% of exposed subjects can sense the passage of induced electric currents and up to 1% can feel uncomfortable sensations, while confidently excluding the possibility of intolerable stimulation [26]. |
It is not possible to exclude stimulation effects at exposure levels below the identified threshold in specific individuals for their particular sensitivities. Protection of workers could be assured if it is guaranteed that just minor effects like not painful perception can occur. Derogations from the compliance to health effects ELVs could apply in the case of MRI equipment if the instructions for safe use provided by the manufacturer, which have to be followed, are in accordance with EN 60601-2-33 the provisions of which allow prevention of severe health effects like cardiac stimulation while allowing just the possibility of minor effects like perceptions. Similar provisions should be provided in specific technical standards for other sectors or specific activities different from MRI, in particular in the case of medical applications where a balance of health risks for workers with benefits to the patient could be considered as in the EN 60601-2-33. |
| Radiofrequency fields | >0.4 W/kg (Whole-body) |
Results of dosimetric studies suggest that a temperature rise < 0.5 °C can be assured by limiting the whole-body average SAR to about 2 W/kg, excluding the case of subjects with a reduced sweating rate [30]. | Considering that the body core temperature is not constant during the day, and can vary by some tenths of °C, it can be reasonably assumed that, in case of derogations, an increase of about 0.5 °C does not pose real risks for workers and a relaxation of whole-body SAR up to 2 W/kg could be considered acceptable. |
| >10 W/kg (Local SAR for head and trunk)>20 W/kg (Local SAR for limbs) |
The local SAR limits are obtained applying a reduction factor of 2 to the effect thresholds to account for scientific uncertainty [10]. | The reduction factor 2 for derivation of local SAR restrictions leaves a narrow margin of safety; therefore, it is not possible to guarantee health and safety of workers. |
* r.m.s. values (in the European Directive 2013/35/EU the low frequency ELVs are reported in terms of peak values).