Testing EMF and Magnetic Fields on BART

by / Thursday, 23 January 2014 / Published in Healthy Building Inspections & Testing

I have seen blogs and articles on the increase in RF (Radio Frequency) radiation in public places, especially Public Transportation, due to the ubiquity of Wi-fi and personal devices containing transmitters such as smart phones & tablets.  However, I have not seen any information on the low frequency magnetic fields that may be emitted on electric trains.  As a resident of the San Francisco Bay Area, the obvious candidate would be testing EMF on the BART (Bay Area Rapid Transit) Electric Commuter Rail system.

The BART system began in 1972 and currently has 44 Stations in 4 Counties, covering 104 Miles and carries over 390,000 passengers on an average weekday.

Testing EMF is something I do at least once a week. I was curious to see, anecdotally, what type of magnetic field radiation exposure was being experienced by this largely unknowing commuter population.

DISLAIMER:  This is far from a scientific study. My observations and measurements are a single sample and this information is anecdotal at best. My goal is to increase awareness, stimulate discussion, and perhaps motivate a properly designed study of the actual EMF exposure experienced by passengers on the BART and other electric commuter rail systems.

On October 30, 2013 I took my Tenmars TM-192D Triaxial Magnetic Field Meter to the Lafayette, California BART Station to measure low frequency magnetic fields at the station and on the train. Testing EMF this way is fairly common.

Testing EMF

EMF and MF (magnetic field) reading on BART platform

On the station platform, with no train near the station, the overall background MF (Magnetic Field) measured quite low, similar to general background levels, at 0.30 mG (MilliGauss).

Testing EMF

Testing EMF and MF (magnetic field) – on platform as train arrived at the station

As the train arrived at the station, the MF measurement increased to 2.55 mG.

Testing EMF

EMF and MF (magnetic field) reading – seated on BART with train stopped

I entered the train and took a seat in the middle of the car.  With the train stopped at the station, with me seated in the train, the MF measured 1.02 mG.

Testing EMF

EMF and MF (magnetic field) reading – on train as BART accelerates

As the train started accelerating the MF measurement increased almost instantly to 17.72 mG.

Testing EMF

Testing EMF and MF (magnetic field) reading – as BART reaches steady speed

Testing EMF as the train reached a steady speed, I realized the magnetic field measurements settled down to 12.8 mG.

As I passed through several stops, the measurements were quite consistent, highest during accelerating and braking (due to dynamic braking, I presume), dropping to around 12 mG when at steady speed, and less than 2 mG with the train stopped.

Testing EMF

EMF and MF (magnetic field) reading – on train as BART runs along route

As I went along I measured the MF on the floor (nearer the electric motors) and the reading was much higher at 92.90 mG.

Testing EMF

EMF and MF (magnetic field) reading – on ceiling as BART runs along route

As magnetic fields typically reduce quickly with distance from the source, I also measured the MF near the ceiling of the car and found the reading to be 1.62 mG.

After taking these measurements I did some research on the BART system.  BART operates on a third rail electrical system that provides the rolling stock with 1,000 volt DC power.  Typically, we are not as concerned with DC, or static, magnetic fields.  After all, the earth itself is a giant DC electromagnet, with the magnetic field protecting us from harmful cosmic rays and also giving us the Northern Lights! Humans have had millions of years of evolution to get used to DC magnetic fields. Alternating Current magnetic fields, on the other hand, have only been around for 150 years or so, and we are still very much in the dark as to the actual effects of this type of radiation.

So why are the AC magnetic fields measured so high on the BART trains?

BART trains originally used DC traction motors, but the newer Rohr cars were rebuilt with ADtranz model 1507C 3-phase Alternating Current (AC) traction motors with insulated-gate bipolar transistor (IGBT) inverters, both of which emit AC magnetic fields proportionate with the amount of current running through the motors and inverters.

So although the supply current is DC, the use of inverters and AC traction motors emits significant AC magnetic fields, especially when accelerating and braking, as the trains use dynamic breaking where the motors generate electricity while braking and emit high MF during that process, due to the large amount of current (amperage) needed to power these trains.

EMF Testing Summary of Results

To reiterate all of my measurements:

Empty Platform

0.30 mG

Platform with train arriving

2.55 mG

Inside train, train stopped

1.02 mG

Train accelerating

17.72 mG

Train at steady, full speed, on seat

12.80 mG

Train at steady, full speed, on floor

92.90 mG

Train at steady, full speed, at ceiling

1.62 mG


So what do these levels mean?  As with all radiation, exposure is based on exposure time and field strength.  I would estimate the average BART commuter is on a train for between 1-2 hours per day, with the longest commutes being up to 4 hours per day.  Train operators obviously have the highest exposure.

IBE (Institute of Biological Engineering) Sleeping Area Concern Levels in milliGauss, mG.

None <0.2 mG
Slight 0.2 – 1.0 mG
Severe 1.0 – 5.0 mG
Extreme >5.0 mG


For non-sleeping areas, levels of less than 2.0 mG are usually considered acceptable by most building biologists, although many prefer to keep long term exposure under 1.0 mG.

For a complete overview of published health-related effects from exposure to all types of electromagnetic radiation, I suggest the BioIniative Report, which is available for $2 online.   www.bioinitiative.org

There are thousands of peer-reviewed, published studies demonstrating physiological responses to low-frequency EMF (electric and magnetic) and higher-frequency radiation (RF EMR). I suggest the following resources if you are interested in learning more about the health effects from artificial electro-smog. If you have health related questions speak to your physician.




Let us know if your home or workplace could benefit from Testing EMF.


6 Responses to “Testing EMF and Magnetic Fields on BART”

  1. Sherrie says :

    I found this article after searching ‘San Francisco’s BART trains magnetic force’. I was curious about it because while reading the archive of Mother Earth News magazine, I came across a short note about the health concerns in a 1977 issue of M.E.N. I was curious to see if anything was ever done about the problem since being brought up so long ago.

    Here’s the excerpt: “THE MAGNETIC FIELDS CREATED BY SAN FRANCISCO’S BART TRAINS ARE SO STRONG -according to Antony Fraser-Smith of the Stanford Radioscience Laboratory-that they set up measurable electric currents in trees. Fraser-Smith worries that exposure of the human body (which is a better “conductor” than any tree) to such fields may pose a future health hazard. “No one monitors our total exposure to electromagnetic fields,” FraserSmith is quoted as saying in New Scientist, “and it is conceivable that the BART signals-although probably harmless themselves-may increase the possibility of harm from other electromagnetic signals.””

    Here’s a link to the page: http://www.motherearthnews.com/nature-and-environment/usda-pest-management-program-zmaz77ndzgoe?pageid=2#PageContent2

    Mr. Fraser-Smith is still an active Stanford educator and I thought it might be interesting to get his opinion on his theories today compared to 1977. Someone knowledgeable in the area should contact him for an update – despite SF getting new trains – it might be nice to know what damage has been done.

  2. David Sasse says :

    From https://en.wikipedia.org/wiki/Bay_Area_Rapid_Transit#Traction_motors

    “Prior to rebuilding,[102] the Direct Current (DC) traction motors used on the 439 Rohr BART cars were model 1463 with chopper controls from Westinghouse, who also built the automatic train control system for BART. The Rohr cars were rebuilt with ADtranz model 1507C 3-phase alternating current (AC) traction motors with insulated-gate bipolar transistor (IGBT) inverters. The Westinghouse motors are still in use on the Alstom C (C1) and Morrison-Knudsen C2 cars and the motors that were removed from the Rohr cars were retained as spare motors for use on them.”

    The DC powered motors should emit far less MF than the Rohr cars with the AC traction motors and inverters. The inverters especially are likely to be a strong source for magnetic fields.

    I am not sure how the new cars that will go into service are powered, but I imagine they are AC with inverters.

    David Sasse

  3. Bill McClinton says :

    Since the different model BART cars have different traction motor systems (ie: chopped dc, inverted created variable frequency AC) I would think the EMF fields might be different from one type to the other.

  4. Mitigating and Eliminating Magnetic EMFs – A Mom’s Guide (part 4) - Organic Housewife says :

    […] interesting link to magnetic fields on Bay Area Rapid Transit.  If you ride the train, this is a good quick […]

  5. Lilly says :

    I strongly agree with Candice’s comments. The public transit should put the public’s health as the most important concern. If the EMF might pose adverse health impact to the public, then something should be done to change that. A better design of the BART system would benefit millions of daily BART riders.

  6. Thanks to the folks at Healthy Building Science for this informative article.

    BART is currently in the process of designing “The Fleet of the Future.” Are EMFs even being considered? The first prototype is scheduled to come to the Bay Area this year and the first trains will be put into use in 2017. BART hopes to order 1,000 of these new models.

    You inspired me to post an inquiry on BART’s website.The new BART trains being built by Bombardier Transit. The New Train Car design team can be contacted through this webform: http://www.bart.gov/about/projects/cars/contact

    I am a daily BART rider. Even if you are not, If enough people inquire to see if this is a design consideration early enough in the design phase, I would like to think it could result in a healthier ride for millions of future BART riders.

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