Extremely Low Frequency Electromagnetic Radiation - (ELF)
I am going to arbitrarily split this large topic into just the 2 ranges of electromagnetic fields which are of most interest to me: 0 to 300 Hz and microwaves from 2 to 3 GHz.
Is electromagnetic radiation dangerous to humans? There is no question that high electromagnetic fields are dangerous to humans. People can die from high voltage shocks such as lightning and high current such as an electric chair and is pretty obvious the interior of a microwave oven would be an unhealthy environment, so the question becomes at what level are voltages, currents and electromagnetic fields safe for people?
This topic is far too complex to cover in a short article, but I want to take a quick look at it from the unusual perspective of electrical engineering.
Human bodies involve incredibly complex electro-chemical processes which are beyond the scope of my discussion, but these processes generate voltages in the range of 2 mV to 40 mV and sometimes as high as 100 mV. This implies that fairly small external voltages might interfere with our physiology. But do they?
The human body model for capacitance is a 100 pf capacitor in series with a 1.5K resistor. Internal body impedance may be 300 – 1000 ohms while dry skin resistance may be well over 100K ohms. A person can fairly easily be charged up with “static electricity” to over 20,000 volts, which can result in a painful spark if discharged suddenly, but we seem to survive just fine.
So if our normal human body control voltages are less than 0.1 volts, how come 20,000 volts doesn't completely disrupt our system? I'm not sure why we evolved with this protection because I grew up in the jungle, running around barefoot and only wearing a pair of cotton shorts, and I don't believe I ever experienced a “static electricity” jolt in that environment.
Lets try to put the effect of static electricity in perspective with some extremely hypothetical numbers and an extremely simplified model to look at how it might be superficially analyzed:
- Suppose we have a human body 50 cm across charged up to 5,000 volts.
- Assume the skin impedance is 100 times greater than the internal impedance so 99% of the voltage is across the skin barrier and only 1% or 50 volts is across the internal body, which might be an internal voltage gradient during discharge of 50 volts across 50 cm or 1 volt per cm.
- Say we have a muscle 1 cm across so there is 1 volt across the muscle during the discharge.
- Suppose it takes 10 mV across a synapse to fire this muscle.
- A synapse gap is only 30 nm but there could be thousands of synaptic junctions in a motor neuron. So assume the synapse cluster feeding the axon is 0.2 mm wide, this implies there might only be 0.2 mV across the synapse, which is less than the trigger voltage.
Some of these assumptions could easily be out by an order of magnitude, but you can see how a high voltage discharge from static electricity might not necessarily cause convulsions.
It could just as easily be that the discharge happens much faster than a muscle can react. I haven't seen any studies of adverse health effects or mechanics of static electricity, perhaps because the total discharge time in a year is probably only a few milliseconds. The main concern seems to be that body charge could attract particles like allergens, dust or pollutants to the facial area.
If static discharge is a concern, there are many measures that can be taken to minimize charge buildup in your body such as:
- Wear cotton – other fabrics, especially synthetics can contribute to charge buildup
- Keep yourself discharged by touching grounded surfaces whenever possible.
- Increase the humidity in your area
- The electronics industry has also developed many methods of controlling human ESD if you want to take it a step further – such as conductive shoe straps, wrist straps and ESD garments.
Clearly we can stimulate heart muscles with pacemakers that only output up to 5 volts and we can externally stimulate muscles with physiotherapy devices generating anywhere from 60 to 500V.
Defibrillators generate between 300 and 1000V. All of these devices are designed to provide more benefits than drawbacks, so there don't seem to be a lot of studies on their health risks. However these devices illustrate it doesn't require extremely high voltage to cause reactions in our bodies if the voltage is in the right location.
Moving up to mains frequencies, 50-60 Hz. At these frequencies the electric and magnetic fields will penetrate buildings and people unless they are shielded. Typical urban transmission lines can be up to 69 KV. Power lines in a house may only be 220 V but this can still result in higher fields than those from high voltage lines that are far away. Usually people know better than to touch either type of power line.
This picture defies sanity - fill in your own caption….
Typical fields in households can range from 5 to 100 V/m. In my house it is between 2 and 5 V/m. The field close to household appliances can be from 200 to 10,000 V/m. Exposure to these fields do not cause our muscles to twitch, although some people can feel a tingle from high fields. A cursory calculation of a 50 cm body in a 200 V/m field might conclude that because these fields penetrate bodies there would be 100 volts across the body or 10 volts/cm. (Which would be more than a pacemaker directly on all muscles). However this is not what happens, the body is a dramatically better conductor than air, so you could think of it as internally shorting out the 100 volts, because not enough current can flow through air to sustain more than a few microvolts across a 1000 ohm body. Of course if you touch 2 electrodes that can supply enough current to sustain 100 volts across your body, you can receive a very dangerous shock.
Clearly, fields of hundreds of volts/m that we are all exposed to do not cause our muscles to twitch, but they do cause small voltages and currents in our bodies. The question is, are these small voltages harmful to people?
There have been many studies to try and answer this question. Some statistical studies find adverse effects such as these:
While other studies find no conclusive causal evidence. For example the most definitive statistical studies indicate the incidence of leukemia in children living within 600 m of high voltage lines is double that of children living further away. But there is no known causal mechanism that explains how these fields might cause leukemia. Maybe the high fields alter something else in the environment that promotes leukemia. It would seem that real adverse effects from power lines are at most a low probability – even in the leukemia studies, of the 1 in 5,000 children that contract leukemia, less than 1% of these would be living near power lines. This is over 500 times rarer than the incidence of prostate cancer in men.
However, if you want to minimize exposure it will probably cost you money or comfort.
Perhaps you could rig a copper mesh anechoic chamber “mosquito net” over and under your bed.
One of my neighbors is going to the trouble of grounding all the aluminum siding on his house to see if it makes a difference. I will go over and take some measurements at some point during this project to see how it compares to my house.
The power lines on my street are buried underground.
Here are the fields outside near my house.
Here are the fields a couple of streets away where the power is on telephone poles.
Here are the fields a few blocks away under power lines.
2000 volts per meter and I couldn't feel a thing.
The meter was in its red zone and beeping away to indicate high fields.
These power lines run pretty close to houses.
I don't know how safe or dangerous it is under these power lines, but the fields are up to1000 times greater than at my house - so lets go inside.
This is a normal baseline reading in my house, although the RF strength can be less if WiFi is not active.
Lets compare these readings to some household appliances:
computer and LCD
My meter has green, yellow and red bar graphs to indicate when the manufacturer thinks the fields are safe, not safe for long exposure and unsafe for even short exposure. Most household appliances are green in the ELF bands and many of them are only on for short periods. This does not mean they are all safe – for example electric razors and vacuum cleaners can and will damage your hearing if you don't take precautions, but this blog is about things you cannot sense yourself.
There are lots of studies that show correlation between high electric fields and adverse health effects, but the symptoms are generally minor and it is difficult to causally link electric fields as the true cause.
As with most things, the higher the dose the more likely it is to have an adverse effect. The standards that are in place are sufficient to protect people from immediate damage, but are generally not based on long term risks, so it is up to the individual to decide if they want to draw the line at some more conservative level.
If you are concerned, it is useful to have a good meter to know what you are being exposed to, because you probably cannot feel exposure to even very high fields.
Even though we are just skimming the surface of the subject, this blog is getting too long, so I will deal with microwaves in a different blog.
Still waiting for some parts and working on other blogs.
All links to blogs related to this project can be found in the first blog here: