Editing Specific absorption rate (section)

== Calculation ==
SAR for electromagnetic energy can be calculated from the [[electric field]] within the tissue as

:<math>\text{SAR} = \frac{1}{V}\int_\text{sample} \frac{\sigma(\mathbf{r}) |\mathbf{E}(\mathbf{r})|^2}{\rho(\mathbf{r})} \,d\mathbf{r},</math>

where
:<math>\sigma</math> is the sample [[electrical conductivity]],
:<math>E</math> is the [[Root mean square|RMS]] electric field,
:<math>\rho</math> is the sample [[density]],
:<math>V</math> is the volume of the sample.
SAR measures exposure to fields between 100&nbsp;kHz and 10&nbsp;GHz (known as radio waves).<ref name=icnirp>{{cite web|url=http://www.icnirp.org/documents/emfgdl.pdf|title=ICNIRP Guidelines For Limiting Exposure To The Time-Varying Electric, Magnetic And Electromagnetic Fields (Up To 300 GHz)|publisher=[[International Commission on Non-Ionizing Radiation Protection]]|year=1998|access-date=2013-12-12|url-status=dead|archive-url=https://web.archive.org/web/20140606044606/http://www.icnirp.org/documents/emfgdl.pdf|archive-date=2014-06-06}}</ref> It is commonly used to measure power absorbed from [[mobile phones]] and during [[MRI]] scans. The value depends 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 ===
{{See also|Wireless device radiation and health}}
When measuring the SAR due to a mobile phone the phone is placed against a representation of a human head (a "SAR Phantom") 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. Measurements are made for different positions on both sides of the head and at different frequencies representing the frequency bands at which the device can transmit. Depending on the size and capabilities of the phone, additional testing may also be required to represent usage of the device while placed close to the user's body and/or extremities. Various governments have defined maximum SAR levels for RF energy emitted by mobile devices:
* [[United States]]: the [[Federal Communications Commission|FCC]] requires that phones sold have a SAR level at or below 1.6&nbsp;[[watt]]s per [[kilogram]] (W/kg) taken over the volume containing a mass of 1&nbsp;gram of tissue that is absorbing the most signal.
* [[European Union]]: [[CENELEC]] specify SAR limits within the EU, following [[International Electrotechnical Commission|IEC]] standards. For mobile phones, and other such hand-held devices, the SAR limit is 2&nbsp;W/kg averaged over the 10&nbsp;g of tissue absorbing the most signal ([http://webstore.iec.ch/webstore/webstore.nsf/artnum/033746 IEC 62209-1]).

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.
To check SAR on your mobile phone, review the documentation provided with the phone, dial *#07# (only works on some models) or visit the manufacturer's website.

=== MRI scanner SAR testing ===
For [[magnetic resonance imaging]] the limits (described in [http://webstore.iec.ch/Webstore/webstore.nsf/artnum/043851?opendocument IEC 60601-2-33]) are slightly more complicated:

{| class="wikitable" style="text-align: center;"
|-
| 
! Whole-body SAR
! Partial-body SAR
! Head SAR
! colspan="3" | 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
| &gt;4
| &gt;(4–10) (b)
| &gt;3.2
| &gt;20 (c)
| &gt;20
| &gt;40
|-
! Short-duration SAR
| colspan="6" | 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&nbsp;g.
: (b) The limit scales dynamically with the ratio "exposed patient mass / patient mass":
:: Normal operating mode: Partial body SAR = 10&nbsp;W/kg − (8&nbsp;W/kg × exposed patient mass / patient mass).
:: 1st level controlled: Partial body SAR = 10&nbsp;W/kg − (6&nbsp;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&nbsp;°C.