Specific absorption rate

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Specific absorption rate (SAR) is a measure of the rate at which energy is absorbed by the human body when exposed to a radio frequency (RF) electromagnetic field; although, it can also refer to absorption of other forms of energy by tissue, including ultrasound.[1] It is defined as the power absorbed per mass of tissue and has units of watts per kilogram (W/kg).[2]

SAR is usually averaged either over the whole body, or over a small sample volume (typically 1 g or 10 g of tissue). The value cited is then the maximum level measured in the body part studied over the stated volume or mass.

Calculation[edit]

SAR for electromagnetic energy can be calculated from the electric field within the tissue as:

\text{SAR} = \int_\textrm{sample} \frac{\sigma(\mathbf{r}) |\mathbf{E}(\mathbf{r})|^2}{\rho(\mathbf{r})} d\mathbf{r}

where

\sigma is the sample electrical conductivity
E is the RMS electric field
\rho is the sample density

SAR measures exposure to fields between 100 kHz and 10 GHz (generally known as radio waves).[3] It is commonly used to measure power absorbed from mobile phones and during MRI scans. The value will depend heavily on the geometry of the part of the body that is exposed to the RF energy, and on the exact location and geometry of the RF source. Thus tests must be made with each specific source, such as a mobile phone model, and at the intended position of use.

Mobile phone SAR testing[edit]

When measuring the SAR due to a mobile phone the phone is placed at the head in a talk position. The SAR value is then measured at the location that has the highest absorption rate in the entire head, which in the case of a mobile phone is often as close to the phone's antenna as possible. Various governments have defined maximum SAR levels for RF energy emitted by mobile devices:

  • United States: the FCC requires that phones sold have a SAR level at or below 1.6 watts per kilogram (W/kg) taken over the volume containing a mass of 1 gram of tissue that is absorbing the most signal.
  • European Union: CENELEC specify SAR limits within the EU, following IEC standards. For mobile phones, and other such hand-held devices, the SAR limit is 2 W/kg averaged over the 10 g of tissue absorbing the most signal (IEC 62209-1).
  • India: switched from the EU limits to the US limits for mobile handsets in 2012. Unlike the US, India will not rely solely on SAR measurements provided by manufacturers; random compliance tests are done by a government-run Telecommunication Engineering Center (TEC) SAR Laboratory on handsets and 10% of towers. All handsets must have a hands free mode.[4]

SAR values are heavily dependent on the size of the averaging volume. Without information about the averaging volume used, comparisons between different measurements cannot be made. Thus, the European 10-gram ratings should be compared among themselves, and the American 1-gram ratings should only be compared among themselves.

MRI scanner SAR testing[edit]

For Magnetic Resonance Imaging the limits (described in IEC 60601-2-33) are slightly more complicated:

Whole body SAR Partial body SAR Head SAR Local SAR (a)
Body Region → whole body exposed body part head head trunk extremities
Operating Mode ↓ (W/kg) (W/kg) (W/kg) (W/kg) (W/kg) (W/kg)
Normal 2 2 - 10 (b) 3.2 10 (c) 10 20
1st Level Controlled 4 4 - 10 (b) 3.2 20 (c) 20 40
2nd Level Controlled >4 >(4 - 10) (b) >3.2 >20 (c) >20 >40
Short duration SAR The SAR limit over any 10 s period shall not exceed two times the stated values
Note: Averaging time of 6 minutes.

(a) Local SAR is determined over the mass of 10 g.

(b) The limit scales dynamically with the ratio "exposed patient mass / patient mass":

NORMAL OPERATING MODE: Partial body SAR = 10 W/kg – (8 W/kg * exposed patient mass / patient mass)
FIRST LEVEL CONTROLLED OPERATING MODE: Partial body SAR = 10 W/kg – (6 W/kg * exposed patient mass / patient mass)

(c) In cases where the orbit is in the field of a small local RF transmit coil, care should be taken to ensure that the temperature rise is limited to 1 °C.

Criticism[edit]

SAR limits set by law don't consider that the human body is particularly sensitive to the power peaks or frequencies responsible for the microwave hearing effect.[5][6] Frey reports that the microwave hearing effect occurs with average power density exposures of 400 μw/cm2, well below SAR limits (as set by government regulations).[5]

Notes:

In comparison to the short term, relatively intensive exposures described above, for long term environmental exposure of the general public there is a limit of 0.08 W/kg averaged over the whole body.[3] A whole-body average SAR of 0.4 W/kg has therefore been chosen as the restriction that provides adequate protection for occupational exposure. An additional safety factor of 5 is introduced for exposure of the public, giving an average whole-body SAR limit of 0.08 W/kg.

FCC advice[edit]

The FCC Guide, "Specific Absorption Rate (SAR) For Cell Phones: What It Means For You," after detailing the limitations of SAR values, offers the following "bottom line" editorial:

"ALL cell phones must meet the FCC’s RF exposure standard, which is set at a level well below that at which laboratory testing indicates, and medical and biological experts generally agree, adverse health effects could occur. For users who are concerned with the adequacy of this standard or who otherwise wish to further reduce their exposure, the most effective means to reduce exposure are to hold the cell phone away from the head or body and to use a speakerphone or hands-free accessory. These measures will generally have much more impact on RF energy absorption than the small difference in SAR between individual cell phones, which, in any event, is an unreliable comparison of RF exposure to consumers, given the variables of individual use." [7]

See also[edit]

References[edit]

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