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Ionized Air

July 9, 2015

Biochemist and renowned science fiction author, Isaac Asimov, had a simple test of whether or not someone was a scientist. He would ask them to pronounce the word, "unionized." Non-scientists would invariably say, "union-ized;" that is, represented by a trade union. Scientists would say, "un-ionized;" that is, neutrally charged.

We live in a sea of electricity. The friction between molecules in the air generates static charges such that the air is positively charged with respect to the Earth. This results in a atmospheric potential gradient of about a hundred volts per meter. This works out to nearly 200 volts from my head to my toes, and the only reason that I'm not perpetually in a "bad hair day" is that the current is low, since the number of ions is small.

As I wrote in an earlier article (Kelvin and Atmospheric Electricity, June 19, 2013), William Thomson, better known as Lord Kelvin, made the first accurate measurement of the atmospheric potential gradient. Attesting to Kelvin's ability as an experimentalist, such a measurement was extremely difficult in his age devoid of electronic amplifiers. For his experiment, Kelvin built a novel water spray electrometer, and he used the primitive photography of the time as a means for data logging.[1]

Using
zinc-copper cells as a voltage reference, Kelvin recorded a voltage gradient of 137 volts/meter.[1] Since this gradient changes with atmospheric state, Kelvin thought that such measurements would help in weather forecasting. A modern atmospheric potential gradient measuring apparatus called a field mill can be used to give warning of impending thunderstorms.[2] These are inexpensively constructed using today's technology (see figure).

A simple field mill.

Schematic diagram of a simple field mill.

If the motor shaft is not
grounded, as would be the case when it spins on a nylon bearing, a conducting brush is needed to ground the shaft.

(Illustration by the author, using
Inkscape.)


As Bob Dylan sang in Subterranean Homesick Blues,[3] "You don't need a weatherman to know which way the wind blows." Likewise, your nose can function like a field mill by detecting ozone, ionized oxygen, in the air ahead of a thunderstorm. Our word, ozone, derives from the classical Greek word, οζω, to have a smell.

Starting many decades ago, our society's demand for increased electric power has been satisfied by an increased number of electric power transmission lines. I used to pass beneath one of these on my way to work, and there's one within a mile of my home. The voltage on these lines is typically about 100 kilovolts, although newer technology high-voltage direct current lines can run to around half a megavolt.

High voltages are used in transmission lines as a way to reduce power loss. Since power P is the product of voltage E and current I, P = EI, and loss in a resistance R is a large function of current, only, P = I2R, we get lower loss for the same transmitted power when the voltage is high and the current is low.

One consequence of these high voltages is a large field gradient beneath the transmission lines. A middle-sized transmission tower is about 100 meters high; so, with 100 kilovolts on the lines, the electric field gradient is a thousand volts per meter, or ten times the typical atmospheric value. This makes you wonder about the safety of being under these transmission lines for a long time.

The World Health Organization has a website devoted to public health affects of electromagnetic fields.[4] Although it would not be prudent to build a school under a high voltage power line, for at least one reason aside from possible electric field affects, there doesn't seem to be any conclusive evidence for any biological effect of an electric field gradient of a thousand volts per meter.[5] However, a few people complain of symptoms of electromagnetic hypersensitivity, sometimes so severe as being affected by low-power Wi-Fi signals. If I had such problems, I would relocate to Green Bank, West Virginia.

Although people are fixated on electric transmission towers, presumably because they are megaliths of our modern age, There are other sources of large electric fields in the environment. A thunderstorm can produce a field gradient of about 3 kV/m. Walking across a polyester carpet on a dry winter's day can charge your body to about 6 kV, which is enough to generate a 2 millimeter spark to a light switch plate. The dielectric strength of dry air is about 3 MV/m, 30 kV/cm, or 3 kV/mm.

Corona rings on a 400 kV power line

High voltages can produce a corona, an electric discharge in the air caused by ionization.

Corona can be suppressed by corona rings, examples of which are shown on this 400 kV power line.

These rings spread the electric field over a larger volume.

(Detail of a photo by Jean-Pierre Daniel, via Wikimedia Commons.)


A team of scientists from the International Laboratory for Air Quality and Health of the Queensland University of Technology (Brisbane, Australia) have discovered that other common features of our modern technological infrastructure, highways, generate many more charged particles than electric transmission lines.[6-7]

Previous studies by this same group found that ion concentration in parks in a typical urban environment had a background level of about 270 ions per cc.[8-9] They found that power lines and electrical substations were strong unipolar sources of ions, while motor vehicle exhaust was a strong bipolar source.[8] Positive and negative ion concentrations near a highways were found to be linearly related to traffic density, with heavy diesel vehicles being the main source of ions near busy roads.[9]

Their current work showed that charged particle concentrations within ten meters of busy roads exceeded those under power lines by a factor of fifteen.[6] The concentration is still twice as much at forty meters distance.[6] For power lines, these ions originate from corona ions that attach to aerosols to form charged particles.[6] For motor vehicles, the culprits are nanoparticles present in exhaust, and this effect is most pronounced for heavy diesel vehicles. Such vehicles have high particle and charge emissions.[6]

Figure caption

Charged particle concentration near power lines and roadways.

(Inkscape illustration by the author, using data from ref. 6.)[6]


Rohan Jayaratne, a Research Fellow at QUT's International Laboratory for Air Quality and Health who participated in the study, readily states that no firm connection between power lines and health has been found, but the influence of motor vehicles hasn't been realized.

"Although the effects of ions and charged particles generated by high-voltage power lines on human health is still open to conjecture, there has been a lot of attention on increased exposure due to expanding power networks in urban residential areas... However what people do not realize is that a large number of charged particles in urban environments come from motor vehicle emissions.[7]
There is no evidence that breathing in air ions is a health risk, but half of fine particles inhaled through normal breathing are deposited in the lungs. Since, as Jayaratne says, "Diesel emissions contain a range of toxic chemicals and have recently been classified as 'probably carcinogenic to humans'," ionized particles are a problem.

"We do not believe that ions are dangerous - the danger comes from the pollutants. The ions merely assist the particles to stick to the lungs. If there are no dangerous particles in the air to attach to the ions, there is no risk of ill health."[7]

References:

  1. K. L. Aplin and R. G. Harrison, "Lord Kelvin's atmospheric electricity measurements," arXiv Preprint Server, May 23, 2013.
  2. Chapter 6, "Instrumentation And Measurements," from Lars Wåhlin, "Atmospheric Electricity," Research Studies Press, John Wiley & Sons (New York, 1989), ISBN 0-471-91202-6.
  3. This song is part of Dylan's Bringing It All Back Home. studio album, a vinyl copy of which I have on my record shelf.
  4. Electromagnetic hypersensitivity, Electromagnetic fields and public health, World Health Organization, December, 2005.
  5. Chapter 9: Health Risk Assessment, Static Fields, in Environmental Health Criteria Monograph No.232, World Health Organization, 2006.
  6. E.R. Jayaratne, X. Ling, and L. Morawska, "Comparison of charged nanoparticle concentrations near busy roads and overhead high-voltage power lines," Science of The Total Environment, vol. 526 (September 1, 2015), pp 14-18.
  7. Roadside air can be more charged than under a high-voltage power line, Queensland University of Technology Press Release, May 28, 2015.
  8. Xuan Ling, Rohan Jayaratne, and Lidia Morawska, "Air ion concentrations in various urban outdoor environments," Atmospheric Environment, vol. 44, no. 18, (June 2010), pp. 2186-2193. Available, also, at the journal web site.
  9. Rohan Jayaratne, Xuan Ling, and Lidia Morawska, "Ions in motor vehicle exhaust and their dispersion near busy roads," Atmospheric Environment, vol. 44, no. 30 (September 2010), pp. 3644-3650. Available, also, at the journal web site.

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