Microsoft Lumia 640 LTE Quick Manual page 13

The exposure guidelines are based on the Specific Absorption Rate (SAR),
which is an expression of the amount of radio frequency (RF) power
deposited in the head or body when the device is transmitting. The
ICNIRP SAR limit for mobile devices is 2.0 W/kg averaged over 10 grams
of tissue.
SAR tests are carried out with the device in standard operating positions,
transmitting at its highest certified power level, in all its frequency bands.
The maximum recorded SAR values for this device are:
When held against
the head
When operated at a
separation distance
of 1.5 centimetres
(5/8 inch) from the
body
This device meets RF exposure guidelines when used against the head
or when positioned at least 1.5 centimetres (5/8 inch) away from the
body. When a carry case, belt clip or other form of device holder is used
for body-worn operation, it should not contain metal and should provide
at least the above stated separation distance from the body.
To send data or messages, a good connection to the network is needed.
Sending may be delayed until such a connection is available. Follow the
separation distance instructions until the sending is finished.
During general use, the SAR values are usually well below the values
stated above. This is because, for purposes of system efficiency and to
minimise interference on the network, the operating power of your
mobile is automatically decreased when full power is not needed for the
call. The lower the power output, the lower the SAR value.
Device models may have different versions and more than one value.
Component and design changes may occur over time and some changes
could affect SAR values. The latest available SAR information for this and
other Microsoft Mobile device models can be found at
sar.microsoft.com.
For more info, go to www.sar-tick.com. Note that mobile devices may
be transmitting even if you are not making a voice call.
The World Health Organization (WHO) has stated that current scientific
information does not indicate the need for any special precautions when
using mobile devices. If you are interested in reducing your exposure,
they recommend you limit your usage or use a hands-free kit to keep
Maximum SAR value
0.65 W/kg over 10g
0.53 W/kg over 10g
Frequency bands
used when Maximum
SAR value was
recorded
3-slot GPRS1800 +
WLAN2450
2-slot GPRS850 +
WLAN2450