#specific absorption rate

Why Benzoate, Benzoic Acid in Soft Drinks May Cause Cancer

The story of soft drinks and cancer has been on the news for several weeks now; and I decided to do a little research on it . As you read on, you will find out what it is and what it is not. There are countless of information (facts) that those who control these systems will not want you to know, because it will have negative effect on their businesses / companies, so they will do anything within…

So I think this safety issue came up before when I discussed Food Ham vs Ham Radio and what we concluded (if I remember correctly), was that exposure to low-level electromagnetic radiation (EMR) has not been linked to any health risks. However, if the combination of frequency and power is such that the body absorbs excessive amounts of energy, there are thermal effects (heating).  This only occurs in very strong fields or fields that originate very close to the body. 

Biological effects have not been shown at amateur frequencies and power levels.  This is largely due to the fact that RF radiation is not high enough to cause ionization like radioactivity.

Thermal effects occur because RF energy causes the molecules to vibrate at the same frequency.  The energy of the vibrations is dissipated within the body as heat.  The stronger the field, the more the molecules vibrate and the more heating of the body's tissues results.

Absorption also varies with frequency because the body absorbs more RF energy at some frequencies than others.  The total amount of heating depends on the RF field's intensity and frequency and is called the specific absorption rate or SAR.

Despite the lack of health risks (besides thermal effects), it's common sense to not be exposed to too much of one thing and therefore the FCC has set limits on the Maximum Permissible Exposure (MPE) from radio transmitters of any sort. 

The full body is resonant at about 35 MHz if the body is grounded and 70mHz if they are not grounded.  Body parts are resonant at higher frequencies.  An adult's head is resonant at the much higher frequency of 400MHz

Above and below the ranges of highest absorption, the body responds less and less to the RF energy.  The frequencies at which the body has the highest SAR (absorption rate) are from 30-1500MHz and these are the regions on the MPE graph where the limits for exposure are the lowest.

So with this when comparing MPE for amateur bands at 3.5, 50, 440, and 1296MHz, you can see that MPE is lowest at 50MHz and highest at 3.5MHz.  Controlled environments are those where people are aware of their exposure and can take the necessary steps to minimize it (ham shack).  Uncontrolled environments are where people aren't aware of their exposure (public park).

Some additional safety tips:

Prevent access to locations where strong fields are present

Make sure strong fields are not created in or directed to areas where people might be present.

RF burns caused by touching or coming close to conducting surfaces with a high RF voltage are an effect of heating.  And these can be eliminated by proper grounding techniques or by preventing access to an antenna.

So when we talk about the intensity of an RF field, we're talking its power density - most commonly measured in milliwatts per square centimeter (mW/cm2).  The power density of an RF field is highest near antennas and in the directions where antennas have the most gain.  It can also be very high inside transmitting equipment.

Increasing transmitter power increases power density everywhere around an antenna to the same degree that transmitter power increased. 

Increasing distance from an antenna lowers power density in proportion to to the square of the distance from the antenna.  (Ex) At twice the distance from an antenna, power density is divided by four.