# How strong is the magnetic fields generated in the human body?

How strong is the magnetic field produced by the human body? I have researched the question but all I find is how strong does a magnetic field have to be to kill a human.

• Depends on how strong the external field is, we are not particularly good at shielding magnetic fields. :-) – CuriousOne Aug 6 '16 at 22:23
• I think he means the magnetic field generated by the very small currents that control muscular contraction. – hebetudinous Aug 6 '16 at 22:25
• @hebetudinous Yes, and of course by the nerves that carry information to the brain, as electrical impulses. – Sigma6RPU Aug 6 '16 at 22:27
• That depends how close one can get to the axons, I would say. If we assume a 1nA current and a distance of e.g. 1um, then the field should be on the order of 100pT. For a more distant measurement on e.g. the human brain en.wikipedia.org/wiki/Magnetoencephalography gives an order of magnitude of 10-1000fT. That's hard to measure. – CuriousOne Aug 6 '16 at 22:37

Neuromagnetic signals are of the order of 50 to 500 femtoteslas in the brain. (Measured by a SQUID). Split the difference, say 300 fT.

Now you have to guesstimate how much output there is from ordinary cells and their connections.

Say 1/10th of what braincells have, then deduct the 1.4 kg of the highly connected braincells.

Average body weight say 90 kg, so you have (1.4 × 300 fT + 88.6 × 30 fT) and you have a 420 + 2658 = 3000 fT magnetic field.

Is this a very rough (probably wrong) estimate. Yes

• Magnetic field decreases with distance, so we cannot just add up magnetic fields from different parts of the body. I guess the magnetic field of the brain cells is measured close to the head. – akhmeteli Aug 6 '16 at 23:08
• @akhmeteli Absolutely right beside it, for surgical research. I mean on the brain tissue itself. The problem I have is I don't know what surface area the probe is, that why it's always going to be order of magnitude. By coincidence, I had a head MRI last month, and chatted to a technician , who had done research in the past, but it's still guessing. – user108787 Aug 6 '16 at 23:15

According to http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1024491/pdf/brheartj00038-0001.pdf, the magnetic field at the surface of the body is between $10^{-14}$ and $10^{-11}$T. I guess heart has a higher magnetic field than brain. So, surprisingly, the higher value is close (about 3 times higher) to @count_to_10's estimate, although (s)he did not take into account the decrease of magnetic field with distance.