Study type: Medical/biological study (experimental study)

The detection threshold for extremely low frequency magnetic fields may be below 1000 nT-Hz in mice. med./bio.

Published in: Bioelectromagnetics 2011; 32 (7): 561-569

Aim of study (acc. to author)

Previous experiments (Prato et al. 2005) with mice have shown that a repeated 1 h daily exposure to a magnetic field shielded environment induced analgesia. This shielding reduced ambient static and extremely low frequency magnetic fields by approximately 100 times for frequencies below 120 Hz.
To determine the threshold of extremely low frequency magnetic field amplitude that would attenuate or abolish this analgesic effect, magnetic fields were introduced into the shielded environment at different peak amplitudes.

Background/further details

As it has been shown that the induced analgesia can be attenuated or abolished by simultaneous light exposure (Koziak et al. 2006), care was taken to eliminate light intensity and wavelength as confounders (dark conditions).
Four experiments were performed: 1) to study the threshold for attenuation at 30 Hz (n=120 mice in six groups), 2) to study the attenuation/abolishment for 120 Hz (n=120 mice in six groups), 3) to evaluate the attenuation of 10, 60, 100, and 240 Hz (n=60 mice in six groups), and 4) to evaluate if the effect changed if the product of amplitude and frequency remained constant but the amplitude and frequency were varied (n=180 mice in six groups).

Endpoint

Exposure

Exposure Parameters
Exposure 1: 30 Hz
Exposure duration: continuous for 1 h/day on 5 days
Exposure 2: 120 Hz
Exposure duration: continuous for 1 h/day on 5 days
Exposure 3: 10–240 Hz
Exposure duration: continuous for 1 h/day on 5 days
Exposure 4: 12.5–100 Hz
Exposure duration: continuous for 1 h/day on 5 days

Exposure 1

Main characteristics
Frequency 30 Hz
Type
Waveform
Exposure duration continuous for 1 h/day on 5 days
Additional info 120 mice were treated in six groups (each group n=20): i) sham exposure (fiberglass box, no shielding) ii) positive control (0 nT, mu-metal box providing static and EMF field shielding) iii) EMF with 25 nT (shielding plus reintroduced fields) iv) EMF with 50 nT v) EMF with 100 nT vi) EMF with 500 nT
Exposure setup
Exposure source
Chamber 33 cm x 38 cm x 20 cm mu-metal box made of 1.6 mm thick mu-metal with magnetically shielded holes of 2.5 cm diameter at each of the four corners on the top and bottom surface; 2.5 cm high mu-metal cylinders surrounding each hole for shielding of the ambient magnetic field and light; box laminated on the inside with black opaque polyethylene
Setup four coils in a Merrit-like configuration, spaced 9.7 cm apart, for a total length of 29 cm, placed inside the mu-metal box; coils made of 150 turns of AWG#28 magnetic wire (0.32 mm diameter with a thin enamel insulation) wound on 30 cm x 17.8 cm plastic formers; coils connected in parallel pairs and electrically shielded with conductive silver paint and copper foil; mice placed individually in a 26 cm x 16 cm x 12 cm transparent polycarbonate cage covered by a clear polycarbonate top with ventilation holes; cage inserted into the mu-metal box
Sham exposure A sham exposure was conducted.
Additional info for sham exposure: 33 cm x 38 cm x 20 cm box made of opaque fiberglass laminated on the inside with black opaque polyethylene
Parameters
Measurand Value Type Method Mass Remarks
power 12.57 µW effective value - - -
magnetic flux density 500 nT peak value measured - 25 nT, 50 nT, 100 nT, 500 nT

Exposure 2

Main characteristics
Frequency 120 Hz
Type
Waveform
Exposure duration continuous for 1 h/day on 5 days
Additional info mice were treated in the same six groups as in E1:
Exposure setup
Exposure source
Sham exposure A sham exposure was conducted.
Parameters
Measurand Value Type Method Mass Remarks
power 12.57 µW effective value - - -
magnetic flux density 500 nT peak value measured - 25 nT, 50 nT, 100 nT, 500 nT

Exposure 3

Main characteristics
Frequency 10–240 Hz
Type
Waveform
Exposure duration continuous for 1 h/day on 5 days
Additional info 60 mice were treated in six groups (each group n=10): i) sham exposure (fiberglass box, no shielding) ii) positive control (0 nT, mu-metal box providing static and EMF field shielding) iii) exposure with 60 Hz, 300 nT (shielding plus reintroduced fields) iv) exposure with 240 Hz, 300 nT v) exposure with 10 Hz, 500 nT vi) exposure with 100 Hz, 500 nT
Exposure setup
Exposure source
Sham exposure A sham exposure was conducted.
Parameters
Measurand Value Type Method Mass Remarks
power 12.57 µW effective value - - -
magnetic flux density 500 nT peak value measured - -
magnetic flux density 300 nT peak value measured - -

Exposure 4

Main characteristics
Frequency 12.5–100 Hz
Type
Waveform
Exposure duration continuous for 1 h/day on 5 days
Additional info 180 mice were treated in six groups (each group n=30): i) sham exposure (fiberglass box, no shielding) ii) positive control (0 nT, mu-metal box providing static and EMF field shielding) iii) exposure with 50 Hz, 120 nT (shielding plus reintroduced fields) iv) exposure with 100 Hz, 60 nT v) exposure with 25 Hz, 240 nT vi) exposure with 12.5 Hz, 480 nT for groups iii) to vi) the product of frequency and magnetic flux density was kept constant (6000 nT Hz)
Exposure setup
Exposure source
Sham exposure A sham exposure was conducted.
Parameters
Measurand Value Type Method Mass Remarks
power 12.57 µW effective value - - -
magnetic flux density 480 nT peak value measured - 60 nT, 120 nT, 240 nT, 480 nT

Reference articles

Exposed system:

Methods Endpoint/measurement parameters/methodology

Investigated system:
Investigated organ system:
Time of investigation:
  • before exposure
  • after exposure

Main outcome of study (acc. to author)

At 30 Hz (field 1), peak amplitudes of 50, 100, and 500 nT attenuated the analgesic effect in proportion to the amplitude magnitude. At 120 Hz (field 2) significant attenuation was observed at all amplitudes. Exposures of experiment 3 (field 3) also attenuated the induced analgesia. If the peak amplitude frequency product was kept constant at 6000 nT-Hz (field 4), the extent of attenuation was constant, indicating that the detection mechanism is dependent on the nT-Hz product. A plot of effect (inhibition of analgesia) versus the "nT-Hz" product suggests a threshold of extremely low frequency magnetic field detection in mice at or below 1000 nT-Hz and maximum attenuation was achieved at about 6000 nT-Hz.
In conclusion, mice can sense and response to ambient extremely low frequency magnetic fields.

Study character:

Study funded by

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