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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?

Exposure to a static electric field does not produce a significant field inside the body, but instead leads to the build up of electric charges on the body surface. In contrast, magnetic fields are virtually as strong inside the body as outside, which can lead to interactions within the body.

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

Experimental data have established that static magnetic fields can interact with different components of the body in the following ways:

Static magnetic fields may generate currents around the heart

• By exerting forces on moving charges in the blood, such as ions, they generate electrical fields and currents around the heart and major blood vessels, and slightly impede the flow of blood.
• By exerting forces that result in changes in the orientation or position of biological molecules and cellular components according to their magnetic properties, although these forces are very small, and by exerting large forces on implanted metallic devices such as pacemakers present inside the body.
• By interfering with certain types of chemical reactions that take place in the body, though it does not appear that this could cause measurable changes in cellular functions, or result in mutations in the long term.

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

Electric current induced in the body by a magnetic field
Source : BBEMG

The interactions of biological tissue with a static magnetic field depend on the physical properties of the field such as the strength and direction of the field at a given place inside the body.

The interactions with biological tissue that are likely to be of most consequence for health occur when there is movement in the field, such as body motion or blood flow. Movement in a static magnetic field can induce an electric field and current in the body, which may account for the reports of vertigo and light flashes (phosphenes) from people moving in fields stronger than 2000 or 3000 mT.

Computer models and direct measurements are used in order to estimate the strength of the internal electric fields in humans or animals that are induced by a given magnetic field. Induced electric fields are estimated to be substantial during normal movement around or within a magnetic field of more than 2 000 or 3 000 mT.

There are many sources of exposure to static magnetic fields, but one of the strongest and most common is Magnetic Resonance Imaging (MRI) equipment. In the past decade, the field strengths used in MRI equipment has increased significantly. The most common system in current clinical use has a field of 1 500 mT, but more than one hundred 3 000 mT -systems were already operational worldwide in 2004. Systems ranging from 4 000 to 9 400 mT are now being developed for clinical imaging.

With this move towards high field strengths, understanding the interactions between the fields generated by MRI systems and the human body has become more important since the potential for interactions increases accordingly. This can be studied through further computer modelling as well as experimental observations. More...