Comments on ICNIRP draft Guidelines for Limiting Exposure to Time-varying Electric and Magnetic Fields (1 Hz to 100 khz) This is a joint response from health care professional bodies within the UK. The response collates detailed information provided by the British Institute of Radiology (BIR), Institute of Physics and Engineering in Medicine (IPEM), the Royal College of Radiologists (RCR), the Society and College of Radiographers (CSoR), the British Chapter of the International Society of Magnetic Resonance in Medicine (BChISMRM) and the Institute of Physics (IOP). The comments provided below are in the context of occupational exposure in magnetic resonance imaging (MRI), and the basic restrictions referred to throughout are those for occupational exposure. Line Comment number 18-21 This is a circular reference. It is stated here that the revised static field guidelines (ICNIRP 2009) should be used for movement in a static field and fields up to 1 Hz. However, in the static field guidelines it is stated that Sensory effects due to the movement in the field can be avoided by complying with basic restrictions set in the ELF guidelines (ICNIRP 2009 p510)! We suggest that something that overrides the statement in the static field limits would be better, eg The static field exposure limits provided in that document will protect against any effects of electric fields induced by movements by humans in the static field. However this still leaves a gap in advice from 0-1 Hz for non motion induced ELF. Suggesting that the static field limits apply to this will lead to future confusion. It is proposed that this document should be extended to 0 Hz and a separate paragraph should be introduced to deal with that regimen. Note that extrapolating from the static field guidelines, and knowing that it is very easy to turn through 180 o in 1 second in a 2 T field, then the static field limits suggest that ICNIRP is suggesting that > 4T/s is acceptable for <1 Hz. This will cause a discontinuity in the limits at 1 Hz. 23, 40, 478 The claim that the guidelines are based on scientifically established adverse health effects is not borne out in the rest of the document, as is discussed in more detail below. It is important to recognise the specific inference of the words acute and adverse in English speaking counties. Adverse strictly refers to an event that results in harm, and therefore is considered to be harmful. It is clear that the effects referred to in the document represent, in the main transient responses and are not acute effects. To match the definition of the effets described in the body of the document it would be significantly clearer to use the term transient untoward effects. 65-67 This only allows future revision for other adverse health effects and apparently not for, say, the arbitrary reduction factors (see 451-453). 92 Adopting in situ electric field as the key dosimetric quantity, in place of current density, is appropriate and is a much more biologically-relevant quantity. 112 It would be helpful to explain briefly why the human body is more often exposed to vertical electric fields.
131-140 The hypothesis although not stated explicitly is based on the size of the induced current loop. The bullet point at line 137 could be moved to the top of the list, and then changed to:- for a given magnetic field strength and orientation, higher electric fields are induced in the bodies of larger people because the possible conduction loops are larger. 141-274 This is a useful and succinct review of the literature, essentially concluding that there is no consistent evidence for anything other than a limited set of neurophysiological effects as a basis for exposure limits that guard against acute effects. 275 This doesn t really seem to be a rational for the restriction levels. There is much general discussion and conjecture in this section, and little hard information that would allow us to draw specific conclusions. It would be helpful if this section could be arranged into the different frequency bands considered in the restrictions, and could have a conclusion for each frequency band, following from the text that preceded it and then linking to the final restriction levels. 279...a number of well established acute effects of exposure to ELF EMFs... Only two such effects are listed, whereas the expression a number suggests rather more. 299-356 This section describes excitation of nerve and muscle tissue due to electric fields induced by EMF. These are well-established effects. The minimum threshold for PNS perception (not in itself an adverse health effect in any meaningful sense) is stated as 2-6 Vm -1. Figure 1 (Matthes 2008) indicates a frequency dependent threshold ranging from about 3-30 Vm -1 over the frequency range approximately 100-10000 Hz. Uncomfortable PNS, which might be regarded as an adverse effect, would occur at considerably higher electric field strengths. 357-403 (and 35-36) This section describes effects of induced electric fields on the CNS, and much of it is speculative. Theoretical expectations (line 370) and in vitro evidence of neurophysiological effects at 100 mv m -1 (lines 372-374) do not constitute scientifically established adverse health effects, and so cannot be used as a basis for exposure limits consistent with the stated objective of the paper (lines 40-41). The assumption that induction of phosphenes in the retina represents a good but conservative model of the neurophysiological processes that occur in CNS tissue in general (line 384) is at best a hypothesis worthy of further scientific investigation. Saunders and Jefferys (2002), discussing phosphenes, state that the possibility that similar effects can be induced in other neuronal circuits of the CNS cannot be ruled out. In their 2007 paper the same authors conclude that consistent evidence for such effects on cognitive function is lacking. It is clear that these putative effects are certainly not scientifically established, and since they are purely hypothetical, there is clearly no evidence that they are adverse health effects. Speculation about the possibility of CNS effects other than phosphenes may be traced back to the work of the NRPB Weak Electric Fields Group (WEFG) in 2003. In evidence to a UK parliamentary inquiry (House of Commons Science and Technology Committee 2006, p Ev 19), Professor Colin Blakemore, who chaired the WEFG, described the brief of the group as to speculate to speculate about the possible levels of field strength at which there were detectable interactions with the human body, particularly the nervous system,
but without a clear instruction to think about or comment on possible hazards... we could see no clear evidence that those limits... indicate a hazard. The only scientifically established effects in the 10-100 Hz range are phosphenes, with a well established threshold at 50-100 mv m -1 (line 388 and see figure 1), but phosphenes have been acknowledged elsewhere by ICNIRP to be transient sensory effects rather than health effects (ICNIRP 2009 p508). 445-453 We disagree with the approach that ICNIRP has taken in the face of uncertainty. In providing a single set of basic restrictions, the uncertain status of much of the evidence is lost to key audiences such as legislators, who will not understand the details of the underlying science and the huge amount of uncertainty and speculation involved. The basic restrictions give the clear impression of being thresholds for the onset of effects, an impression reinforced by the statement that the restrictions are based on established adverse health effects and the recommendation against time averaging (lines 497-499). In fact, as discussed below, it is by no means clear what the restrictions are based on at best very cautious interpretation of a mixture of transient effects and electrophysiological speculation. This was a major drawback of the 1998 guidelines, and it is very disappointing that the problem has not been addressed in this revision. It would be useful to provide at this point a clear table summarising purported effects, the strength of evidence for their existence, the frequency range over which they occur, whether they are harmless transient effects or real adverse health effects, and the threshold at which they occur. For example: Effect Approx freq Evidence Status Threshold Range (Hz) (mv m -1 ) Phosphenes 10-50 Established Harmless sensation 50-100 PNS 60-6000 Established Harmless sensation 2000-6000 Painful/adverse 3000-9000* Effects on Unknown Theoretical Unknown 100? Neural nets Neuro- 10-50 Speculative Unknown 50-100? Physiological [*This represents an estimate, accurate figures may be available form the literature] 464-465 The improved modelling underlying the reference levels is welcome. 480 Here it is acknowledged that limits are based on possible CNS effects, not established adverse effects at all! 483-484 The 1998 guidelines were ambiguous as to what is meant by CNS tissue - purely the neural tissue of the CNS, or the broader collection of tissues making up the CNS anatomically? This ambiguity remains in the new draft. 482-495 ICNIRP has repeatedly stated (for example at the Milan meeting in 2007) that its approach to exposure limitation is to identify the relevant established critical effect and then set an appropriate basic restriction. This document fails to specify what critical effect these limits are aiming to protect against. (We do not believe that there is one). The limit in the 10-100 Hz range (lines 482-485) is based either on phosphenes (harmless) or on speculative effects (not established). It is not clear which, but in either case, the paper fails the key test of identifying an established adverse health effect. Based on the previous section discussing the rationale for the
limits, it does not relate to scientifically established adverse biological effects, and it clearly is not limited by the phosphene effect as it exceeds the limit at 20 Hz (line 485) (additionally, phosphenes are now allowed by the static field limits according to this document). Given the speculative nature of the limits in this frequency range, and particularly the reliance on phosphenes, a harmless transient sensory effect, apparently authoritative exposure limits seem inappropriate. We suggest that a more appropriate approach would be to refrain from setting a basic restriction and instead recommend an explicitly precautionary approach, for example requiring a risk assessment and a risk benefit analysis for exposure above 100 mvm -1. Above 100 Hz, the basic restriction... increases progressively, corresponding to the increase in the threshold for the effects considered in these frequency ranges (lines 492-494). What effects are these? The only established effect in this frequency range is PNS, with a threshold 3-30 Vm -1 over the frequency range 100-10000 Hz (figure 1). Yet the basic restriction over this range is 0.1-2 Vm -1. Why the discrepancy? These are well-established thresholds for onset of PNS perception, so a safety factor of 10-30 seems unjustified. What is the rationale behind the frequency dependence of the basic restriction above 100 Hz? The new proposed restrictions of course cannot be compared directly with those in the 1998 guidelines, since a different dosimetric quantity has been adopted. However, assuming a typical tissue conductivity of 0.2 Sm -1, it is clear that there has been a significant relaxation of the limits (for example at 1 khz 0.2 Vm-1 40 mam -2 as compared to 10 mam -2 in 1998). However, studies have shown that occupational exposure to switched gradients in MRI may result in induced current densities of over 200 mam -2 ( 1 Vm -1 ) (Crozier et al 2007), with no evidence of adverse effects. Therefore in reality, if enforced, the limits would still have a devastating effect on the practice of MRI. It is perhaps worth stressing again, that up to 500 million people have been exposed to MRI over the past 25 years, with no evidence of acute adverse health effects below the PNS threshold. Although this is in no way a formal scientific study, but it is at least as valid a piece of evidence as some of the speculation that is discussed after line 275; we hope that ICNIRP can make use of this information in their revision of the document. (There are a number of small studies that can be used to support this anecdotal evidence, for instance the papers from Chakeres et al on the safety of 7T MRI discuss the effect of 7T imaging, and therefore also exposure to imaging gradients on a limited number of subjects). 500-535 This section addresses the long-standing practical difficulty of defining the volume element over which dosimetric quantities are to be averaged. This was ambiguous in the 1998 guidelines, and it is helpful to have a firm recommendation in this revision, especially if these guidelines are to form the basis of legislation. However, another ambiguity remains: is the 5x5x5 mm 3 cube intended to be composed purely of neural tissue (physiologically and anatomically unrealistic), or is it intended as an actual cube of body tissue composed of a mixture of different tissues? 626-665 This approach to non-sinusoidal fields repeats that adopted in ICNIRP (2003), which has been shown by Heinrich (2007) to be over conservative by a factor of up to 100 or more, especially for the pulsed fields used in MRI.
Figure 1: thresholds for vertigo, phosphenes and PNS (Matthes 2008) References Crozier S et al (2007) J Magn Reson Imaging 26 1236-1254. House of Commons Science and Technology Committee (2006) 4 th Report of Session 2005-6. London: The Stationery Office. Heinrich H (2007) Health Physics 92 541-546. ICNIRP (2003) Health Physics 84 383-387. ICNIRP (2009) Health Physics 96 504-514. Matthes, R (2008) Recommendations for static and ELF fields. ICNIRP 6th International Non-Ionizing Radiation Workshop. Rio de Janeiro, October 14-17, 2008. Saunders RD and Jefferys JGR (2002) Health Physics 83 366-375. Saunders RD and Jefferys JGR (2007) Health Physics 92 596-603. Comments compiled 29 th October 2009