CLASSE Safety Handbook

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Magnetic Field Safety

The primary potential hazard from magnetic fields is the physical pull on objects in the workplace (tools, jewelry, etc.); this hazard can be minimized by not carrying unnecessary objects into proximity of such fields and adequately securing those that are carried. Secondarily, even relatively weak magnetic fields (just a few gauss) can affect devices implanted in the body such as pacemakers; in this case, maintaining a safe distance from magnetic objects (usually just a few feet) is necessary. Third, biological effects could be induced by long-term exposure to very high strength and/or high-frequency fields (although such damage has not been unambiguously proven); very few places at CLASSE exist where such fields exist except inside an exclusion area while radiation producing equipment is on and hence where personnel cannot be present anyway. The concepts of distance and time apply to magnetic fields: a field drops off quickly with distance from the object, and time you spend near known locations of magnetic fields should be minimized.

Anywhere electric current flows, magnetic fields also exist; conversely, a time-varying magnetic field can induce electric current in a nearby conducting loop. Magnetic materials like iron (and many types of steel) can become magnetized and have magnetic fields without current flow. HallProbe.jpg Most magnets used at CLASSE to steer, focus, or defocus the electron and positron beams contain both conducting coils as well as magnetic material; hence a residual magnetic field can remain after current is off. Ion pumps have magnetic fields supplied by permanent magnets, so the fields are present at all times. Contact the Safety Director to get a magnetic field measured with a Hall Probe (at right).

You may find useful related information in the EHS Magnet Safety Manual.

There are no federal regulatory limits on exposure to magnetic fields, so Cornell EHS has adopted prudent limits. Common units for magnetic fields are Tesla (T) and Gauss (G); 1 T = 10,000 G. For reference, the earth's magnetic field on its surface is about 0.6 G, some consumer electronic devices can generate magnetic fields up to 0.2 G up to an inch away, and medical MRI imaging typically involves a static magnetic field of up to 5 T (50,000 G). There is no evidence that short-term exposure to the static fields in MRI scans causes biological damage.

Frequency (Hz) Exposure Applies to Cornell Limit (T) Cornell Limit (G)
Static (<1) Instantaneous Extremities 5 50,000
Static (<1) Instantaneous Whole Body 2 20,000
Static (<1) 8-hour average Extremities 0.6 6000
Static (<1) 8-hour average Whole Body 0.06 600
Static (<1) Instantaneous Magnetic Implant (e.g. Pacemaker) 0.0005 5
Static (<1) Instantaneous Any public area 0.0005 5
f=1-300 Instantaneous Hands and Feet 0.6 / f 6000 / f
f=1-300 Instantaneous Arms and Legs 0.3 / f 3000 / f
f=1-300 Instantaneous Whole Body 0.06 / f 600 / f
f=300-30,000 Instantaneous Whole Body 0.0002 2
f=50-60 8-hour average Whole Body 0.0005 5
f=50-60 8-hour average Magnetic Implant (e.g. Pacemaker) 0.0001 1
f > 30,000 Instantaneous Whole Body Contact Safety Director  

Topic revision: r13 - 27 Aug 2021, RigelLochner
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