Answer to Question #189 Submitted to "Ask the Experts"

Q

What level of radiation and what type of radiation should one expect from a transformer? What type of instrument would be used to detect any radiation from the transformer? Are there any limits to this type of radiation? What government agency has responsibility? Thanks.

A

The primary source of electromagnetic fields, not radiation, from transformers used in the distribution of electricity is either 50 or 60 hertz electric and magnetic fields and their associated harmonics. I would not expect that transformers would be significant sources of other forms of electromagnetic radiation. 60 hertz fields are found primarily in North America, while 50 hertz is predominant throughout the rest of the world. The electric fields, which have units of volts per meter (V/m), are driven by the voltage on the line and are essentially independent of time. Magnetic fields, on the other hand, at these frequencies are usually described in term of magnetic flux density in tesla or gauss. Magnetic fields are a function of the current flowing in the line and will vary throughout the day. A number of companies sell equipment that can measure these fields. Electric field sensors usually consist of two parallel plates. A displacement current must flow between the plates when they are shorted and the magnitude of this current is related to the electric field in which the plates are immersed. Magnetic field sensors are based on the induction of a current in a loop, and this current is proportional to the magnetic field present. In general, electric field measurements are more difficult to make than magnetic field ones, primarily due to the perturbation of the fields when humans and other conducting or lossy dielectric objects are in the fields. Humans, on the other hand, are relatively transparent to the external magnetic field present, so that such measurements are easily performed. Presently, both the American Conference of Governmental Industrial Hygienists (ACGIH) and the International Commission on Non-ionizing Radiation Protection (ICNIRP) have standards of 50/60 hertz electric and magnetic fields. The ACGIH limits are for the protection of the worker, while ICNIRP has limits for both occupational and public exposures. The occupational limits of ACGIH and ICNIRP are very similar and based on the gross effects of electric and magnetic fields. For electric fields, this would be the prevention of spark discharges, and for magnetic fields the electrostimulation of tissue and the disruption of aortic blood flow. Neither ACGIH nor ICNIRP explicitly addresses the potential carcinogenic effects of chronic, low-level exposure to these fields, which has been the subject of much study in recent years. The epidemiological data to date is contradictory, open to much interpretation and discussion. There are also other studies which associate exposure of these fields to changes in the immune, melatonin, and calcium systems of the body. They do, however, take into account that people with implanted medical devices are potentially susceptible to electromagnetic interference from power frequency fields and have adjusted the limits for these individuals to much lower levels than the general working population. At present, Standards Coordinating Committee 28 of the Institute of Electrical and Electronic Engineers (IEEE) has a subcommittee looking a sub-radiofrequency fields below 3 kHz, but has yet to produce a standard. There is also no federal or OSHA standard covering these fields. Many states, however, do have limits for electric fields at the edges of power company right of ways. It is instructive to note that the Swiss government adopted, on February 1, 2000, stringent exposure limits for public exposures to power lines. Magnetic fields from new power lines, substations, or electric railway lines must not exceed 10 milligauss in places where people spend time. This is a level that is a factor of 100 times lower than ICNIRP. The Swiss limits are very similar to the controversial draft recommendations of an NCRP committee on EMF health risks issued in 1994, which are still under review. Our society is facing an impressive increase in the number and diversity of sources of electric and magnetic fields, and transformers are just one contributor to our daily exposure to these fields. In addition to the already well-understood continuous exposure to sinusoidal or periodic fields, recent developments and emerging technologies increase the probability of transient exposures or exposures to transient electromagnetic fields. Transients are understood to be exposures to electromagnetic events the duration of which is short compared with the time constants of possible biological responses. This includes single exposure events such as pass-by exposures when crossing article surveillance or personal identification gates as well as periodically pulsed fields with long enough pulse intervals and aperiodic field transients such as those produced by temperature-regulated heating blankets or electric trains. These types of exposures are currently the focus of further research and could form the basis for the revision of present standards. References

• 2000 TLVs and BEIs Threshold Limit Values for Chemical Substances and Physical Agents. ACGIH; Cincinnati, OH.
   
• ICNIRP Guidelines. Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields (up to 300 GHz). Health Phys. 74:494-522; 1998.

John Leonowich
Pacific Northwest National Laboratory