Static Fields

4. How may static magnetic fields interact with the body?

• 4.1 Through what mechanisms may static magnetic fields affect the body?
• 4.2 What determines the strength of static magnetic fields inside the body?

4.1 Through what mechanisms may static magnetic fields affect the body?

The source document for this Digest states:

The following three classes of physical interactions of static magnetic fields with biological systems are well established on the basis of experimental data:

(1) Electrodynamic interactions with ionic conduction currents. Ionic currents interact with static magnetic fields as a result of Lorentz forces exerted on moving charge carriers. These effects lead to the induction of electrical (flow) potentials and currents. Flow potentials are generally associated with ventricular contraction and the ejection of blood into the aorta in animals and humans. The Lorentz interaction also results in a magnetohydrodynamic force opposing the flow of blood. The reduction of aortic blood flow has been estimated to reach about 10% at 15 T.

(2) Magnetomechanical effects, including the orientation of magnetically anisotropic structures in uniform fields and the translation of paramagnetic and ferromagnetic materials in magnetic field gradients. Forces and torques on both endogenous and exogenous metallic objects are the interaction mechanism of most concern.

(3) Effects on electronic spin states of reaction intermediates. Spin-correlated radical pair chemistry has long been a consideration for magnetic field effects in chemistry and biology. Several classes of organic chemical reactions can be influenced by static magnetic fieldsin the range of 10 to 100mT as a result of effects on the electronic spin states of the reaction intermediates. A spin- correlated radical pair may recombine and prevent the formation of a reaction product if two conditions are met: (a) the pair, formed in a triplet state, must be converted into a singlet state by some mechanism and (b) the radicals must physically meet again in order to recombine. Step (a) can be sensitive to magnetic fields. Most research has been on the use of radical pair magnetic field effects as a tool to study enzyme reactions. However, neither physiological effects on cellular functions, nor long-term mutagenic effects from magnetic-field induced changes in free radical concentrations or fluxes appear possible.

Source & ©: WHO "Environmental Health Criteria 232: Static Fields" (2006)
 Chapter 1, Section 1.1.2 Interaction Mechanisms

For more information on	See EHC 232
Interaction mechanisms	Chapter 5

4.2 What determines the strength of static magnetic fields inside the body?

The source document for this Digest states:

Dosimetry

To understand the biological effects of electric and magnetic fields, it is important to consider the fields directly influencing cells in different parts of the body and tissues. A dose can then be defined as an appropriate function of the electric and magnetic fields at the point of interaction. The establishment of a relationship between the external non-perturbed fields and internal fields is the main objective of dosimetry. Computational studies using voxel-based models of humans and animals, and experimental studies of exposure are important aspects of dosimetry.

The interactions of tissue with static magnetic fields are likely to be parametric of physical properties of the field including the magnetic field vector, the gradient of the magnetic field, and/or the product of those quantities, often termed the ‘force product’. Some of the larger interactions are characterized by motion through these field quantities, such as body motion or blood flow.

Appropriate dosimetric parameters depend on the physical mechanism for the safety concern. Clearly, ferromagnetic objects must be restricted from the vicinity of the magnet. Screening for such objects and for implants that may move either due to forces or torques is imperative. Measures of peak magnetic induction vector and peak magnetic force product are appropriate. Field maps may be used to estimate these at various locations near the magnets where workers may be exposed, but personal dosimetry may be more useful.

Movement of the whole or part of the body, e.g. eyes and head, in a static magnetic field gradient will also induce an electric field and simetric calculation suggests that such induced electric fields will be substantial during normal movement around or within fields > 2 - 3 T, and may account for the numerous anecdotal reports of vertigo and occasionally magnetic phosphenes experienced by patients, volunteers and workers during movement in the field.

There are many sources of exposure and one of the most prolific is that of magnetic resonance imaging(MRI) equipment. In the past decade, there has been a concerted effort to enable MRI to operate at very high field strengths. The most common system in current clinical use has a 1.5 T central field. However, 3.0 T systems are now accepted for routine clinical work and more than 100 systems were operational worldwide by 2004. Research systems from 4 - 9.4 T are now being developed for clinical imaging. As the field strength of the MRI system increases, so does the potential for a variety of types of tissue/field interactions. Understanding the interactions between the electromagnetic fields generated by MRI systems and the human body has become more significant with this push to high field strengths.

Source & ©: WHO "Environmental Health Criteria 232: Static Fields" (2006)
 Chapter 1, Section 1.1.2 Interaction Mechanisms

For more information on	See EHC 232
Dosimetry	Chapter 6