ELECTRICAL SAFETY
communications equipment or computers, 10 milligauss (mG) is the acceptable limit. For any patient care area, GOS stipulates a 5 mG limit. Finally, 2 mG is recommended in
any area where medical equipment is to be used, such as a biomedical workshop, or where sensitive medical electrical equipment is to be used, such as an electroencephalograph or an electrocardiograph. Some diagnostic instruments, such as a magnetoencephalography (MEG) or a magnetocardiograph (MCG), need an even lower level of interference and a completely magnetically shielded room is likely be specified by the equipment manufacturer. The maximum permissible magnetic field strength would be dictated by the manufacturer. Apart from interference with sensitive
electronics, the potential health effects of power frequency magnetic fields have raised questions for some time. In 2005, the World Health Organization (WHO) asked a task group of scientific experts to assess these concerns and evaluate any risks resulting from short and long term exposure to power frequency magnetic fields.1
Further epidemiological studies on
long term exposure risks from extremely low frequency (ELF) magnetic fields suggest a link between the increased occurrence of childhood leukaemia and field exposure of a little as 4 mG. In 2002, the International Agency for Research on Cancer (IARC) published a report that classified ELF magnetic fields as “possibly carcinogenic to humans”.2
Guidelines At present, there are no provincial or federal standards that aim to limit human exposure to power frequency magnetic fields. A Health Canada guideline known as Safety Code 63
regulates human
exposure to radio frequency (RF) electromagnetic fields spanning from 3 kilohertz to 300 gigahertz, and as such it is not directly applicable to power frequency electromagnetic fields. For the lowest frequency spectrum – (from 3 kHz to 1 MHz) – Safety Code 6 stipulates an exposure limit of 12.6 mG. With the aim of addressing the
concerns stemming from epidemiological studies, many jurisdictions around the world, including the City of Toronto, have established guidelines. The City of Toronto’s prudent avoidance policy mandates limiting children’s exposure to EMF in public places in the vicinity of hydro corridors with transmission lines to less than 4 mG. Prudent avoidance is to reduce human exposure to ELF magnetic fields with moderately priced measures. When private developers propose plans for recreational facilities or developments within or adjacent to
IFHE DIGEST 2018
Before
After
Source of electromagnetic field Shielding wall
Electromagnetic field levels (mG) High mG
Low mG
Figure 1. Graphical representation of electromagnetic field finite element analysis simulation of the effectiveness of passive field mitigation solutions. Passive shielding mounted on walls, floors or ceilings effectively eliminates magnetic field radiation from internal and external sources.
electrical transmission corridors, the City of Toronto’s initiative requires them to include an EMF management plan in support of their application, which is then assessed by Toronto Public Health.4 In newly constructed facilities, many
EMF concerns are usually addressed during the design stage. At the preliminary design stage, finite element analysis (FEA) software can model the magnetic field from the main power distribution equipment, providing architects, consultants and engineers with an accurate model of the extent and strength of the magnetic field. The extent and strength of the electromagnetic field from individual sources is superimposed onto architectural plans. Predicting and planning in this way eliminates unanticipated hazards and unaccounted for design errors that can jeopardise the performance of sensitive medical equipment, the confidentiality of precious patient data and ultimately the efficiency of the entire medical facility.
Conflict resolution Should a magnetic field analysis reveal a conflict with the GOS standard at any
location, floor layout arrangements and space utilisation can be modified to mitigate high field exposure situations. A design stage mitigation effort is the most cost efficient, as patient care areas or the rooms containing extremely sensitive medical equipment can sometimes be relocated away from the source of EMF radiation. However, this effort is quite often in direct contradiction to the effort to maximise space utilisation and efficiency within hospitals and medical facilities. When it is impractical to relocate critical areas, electromagnetic shielding is implemented to mitigate field exposure. Passive electromagnetic shielding is
the preferred approach to field mitigation (Figure 1). Passive shielding effectively eliminates magnetic field radiation from sources that are enclosed in rooms or closets where there are available surfaces on which to mount it. It can be mounted on the walls of the electrical room or closet itself, a ventilated enclosure can be designed for bus ducts or the shielding can be installed behind drywall, which is invisible to the end user. This method cannot be used, for example, on windows
In the case of extremely sensitive medical and laboratory equipment such as scanning electron microscopes and electron lithographs, additional protection alongside a passive shielding system may be required to successfully mitigate intrusive magnetic fields
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