EFFECTS OF ELECTROMAGNETIC FIELDS ON HUMANS
Electric and magnetic fields that vary in time interact with matter consisting of electrically charged particles and, in particular, interact with matter constituting biological systems such as cells or complex organisms such as plants and animals. Dosimetric quantities are used to properly quantify the energy absorbed by a given matter, more specifically human tissue. Such dosimetric quantities express current density, power density and energy absorbed per unit of surface or mass.
Then the following are defined:
- CURRENT DENSITY ‘J’: defined as the current flowing through a unit cross-section in a conducting volume — such as the human body or part thereof — and running perpendicular to its direction. It is expressed in A/m2.
- POWER DENSITY ‘S’: used in the case of very high frequencies with modest body depth penetration; it is the radiant power incident perpendicular to a surface, divided by the area of that surface and is expressed in W/m2.
- SPECIFIC ENERGY ABSORPTION ‘SA’: defined as the energy absorbed per unit mass of biological tissue and is expressed in Joules/kg.
- SPECIFIC ENERGY ABSORPTION RATE ‘SAR’: This is the value of the energy absorption rate per unit body tissue mass, averaged over the whole body or certain parts thereof. Both whole-body averaged SAR and local values are used to assess and limit the deposition of excessive energy in small parts of the body consequent to particular exposure conditions. It is measured in W/kg.
CURRENT DENSITY j [mA/m2] |
Effects |
|---|---|
| J > 1000 | Extrasystole and fibrillation: well-determined risks |
| 100 < J <1000 | Tissue stimulation: possible risks |
| 10 < J < 100 | Possible effects on the nervous system |
| 1 < J < 10 | Minor biological effects |
The above-mentioned quantities are used as points of reference to quantify effects on the human body and to define exposure limits. However, these cannot be measured directly on the exposed individual to assess the intensity of radiation; therefore directly measurable physical quantities such as magnetic field and induction are used. In fact, the action limits are defined in terms of the magnetic induction module and magnetic field, derived through mathematical models simulating the behaviour of the human body.
At low frequencies, as the frequency increases, the body can attenuate the electric field because the tissue’s dielectric constant relative to air increases; the body is then effectively shielded. Conversely, the magnetic field — i.e., the magnetic induction — remains almost constant because tissues do not possess magnetic properties and thus their magnetic permeability matches that of air; consequently, the body does not attenuate the magnetic field. Thus, we can see that, for the purpose of biological effects occurring at low frequencies, the magnetic field is the predominant pollutant. Direct, short-term or acute effects due to EMF are well represented by current density (A/m2).
Another category of health effects is that of long-term effects that can result from prolonged exposure (even years), even to field levels much lower than those associated with short-term effects.
All known effects due to time-varying electric and magnetic fields stem from the induction of fields and currents within the exposed organism.
Electric fields exert forces on any electrically charged particle such as the ions in liquids. Consequently, all particles struck by an electric field move until they reach an arrangement of such electrostatic surface equilibrium that, within the human body, the field is nil.
When the electric field is time-varying, the charges change position according to the sign of the field while constantly trying to reach equilibrium; consequently this creates an alternating motion of surface charges (electric current induced by the varying electric field) and this increases in intensity with an increase in the frequency with which the inducing field varies.
On the other hand, in the presence of a time-varying magnetic field, a different mechanism is activated since this field generates a time-varying electric field in the surrounding space. If the variable electric field is produced directly inside the human body, it generates an electric current according to Ohm’s law: J = σE .
While, as the main source, the electric field generates surface currents to the body, the magnetic field causes currents to circulate within the body itself, thus affecting much more delicate parts.
The electric field generated by a variable magnetic field has a spatial distribution that can be visualised by force lines closed in on themselves and concatenated with the force lines of the magnetic field (see Fig. 1).
Within the human body, the induction of electric fields and currents gives rise to two biological effects, both of which can potentially affect health: those related to the electrical stimulation of muscle and nerve tissues, and the thermal effects related to heating by the Joule effect.
When the effects of these two phenomena occur immediately after exposure to the fields, we can speak of short-term effects; on the other hand, when they occur after a number of years — due to prolonged exposure to lower value fields — we speak of long-term effects.
Electric field
Magnetic field
Fig. 1 – Currents induced in the human body by exposure to an E field (vertical) or an H field (vertical or horizontal).
EFFECTS OF ELECTROMAGNETIC FIELDS ON ELECTRONIC EQUIPMENT
The magnetic field immunity levels established in the BS EN 61000 series of standards specify electromagnetic immunity levels for environments where various types of equipment are required to operate.
MAGNETIC FIELD EMISSION
Class 1 – 1A/m (1.26μT)
- Environmental level in places where sensitive devices may be used. Computer CRT and electron microscope monitors are examples.
Class 2 – 3A/m (3.78μT)ù
Well-protected environment
- Areas not subject to bus bars and voltage transformers, for example electrical transformer substations.
- Protected areas of homes, laboratories, offices and hospitals away from protective earth conductors and industrial plants are examples of such an environment.
Class 3 – 10A/m (12.6μT)
Protected environment
- Electrical appliances and cables that can cause flux leakage or magnetic fields within the environment, including in the vicinity of protection system earth conductors.
- Medium voltage (MV) circuits and high voltage (HV) busbars located at a distance (a few hundred meters) from affected equipment.
Class 4 – 30A/m (37.8μT)
Typical industrial environment
- Short branch power lines such as busbars, etc.
- High power electrical equipment that can cause flux leakage.
- Earthing of protection system conductors.
- Medium voltage circuits and high voltage busbars located tens of meters away from affected equipment.
Electric transformer stations and high voltage substation control rooms are examples of this environment.
Class 5 – 100A/m (126μT)
Heavy-duty industrial environment
- Conductors, busbars on medium and high voltage lines carrying tens of kA.
- Protection system earthing conductors.
- Proximity of M.V. and H.V. busbars.
- Proximity of high power electrical equipment.
Control panel areas of heavy duty, medium and high voltage industrial plants and power plants are examples of this environment.
Class X
Special environment
- Minor or major emissions from sources of interference from circuits, conductor cables, electronic circuits, equipment cables and the type of installation may require the use of ambient levels higher or lower than those described above.
It is worth noting that higher level equipment lines may penetrate into a lower severity environment.