# Specific absorption rate

SAR is the abbreviation for the specific absorption rate and a measure of the absorption of electromagnetic fields in a material. It always leads to its heating . The specific absorption rate is expressed as power per mass in units of W / kg .

## determination

The specific absorption rate can be calculated from

1. the field strength in the tissue: ${\ displaystyle \ mathrm {SAR} = {\ frac {1} {2}} {\ frac {\ sigma | {\ vec {E}} | ^ {2}} {\ rho}}}$ 2. the current density in the tissue: ${\ displaystyle \ mathrm {SAR} = {\ frac {1} {2}} {\ frac {J ^ {2}} {\ rho \ sigma}}}$ 3. the temperature increase in the tissue: ${\ displaystyle \ mathrm {SAR} = c_ {i} {\ frac {dT} {dt}}}$ there is:

E = electric field strength ([ E ] = V / m ) at a point inside a body to be measured (e.g. measuring phantom).
J = current density ([ J ] = A / ), caused by the magnetic and electric fields. (The limit value of the current density for skilled workers is, for example, 10 mA / m². For normal people, 2 mA / m².)
ρ = density of the fabric ([ ρ ] = kg / )
σ = electrical conductivity of the tissue ([σ] = S / m )
c i = specific heat capacity of the tissue ([ c i ] = J / kg K )
dT / dt = time derivative of tissue temperature ([ dT / dt ] = K / s )

## Cellular area

A SAR value is often given for cell phones . With modern devices, this is approximately between 0.10 and 1.99 W / kg. The lower the SAR value, the less the tissue is heated by the radiation. The recommended upper limit of the World Health Organization is 2.0 W / kg. In the past, the SAR value for mobile phones was determined by manufacturers under inconsistent conditions, which made it unreliable. A European standard ( EN 50361) has existed since autumn 2001 , which precisely defines the measurement conditions. In March 2007, EN 50361 was replaced by EN 62209-1.

A maximum SAR value of 0.5 W / kg when used on the ear and 1.0 W / kg when used on the body is permitted for labeling a mobile phone with the blue angel . A maximum SAR value of 0.8 W / kg is permitted for labeling with the TCO '01 Mobile Phone logo.

The SAR value for mobile phones is given for the maximum transmission power . Due to the power regulation , however, a smaller SAR value usually occurs during operation, which depends on the respective cellular network . In well-developed networks with a high density of transmission masts , the mobile phone usually transmits with significantly lower transmission power than in poorly developed networks. As a result, exposure when making calls is also lower in areas with a high density of transmission towers than in areas with a low density of transmission towers. Staying in vehicles and buildings (especially those made of reinforced concrete ), on the other hand, increases the required transmission power. The power levels of standard GSM mobile phones are between 1 and 2000 mW depending on the reception situation and generate permissible SAR values ​​within this range.

## Other technically relevant areas

Other areas where SAR values ​​are relevant: