What is SAR in mobile devices? And why is it different for head and body?

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Mobile devices have a Specific Absorption Rate value, which in most cases can be checked by dialing *#07#
What are those values?

In: Technology

SAR is a measure of how much the radio waves cause tissue to heat up.

Radio waves are a form of electromagnetic energy. When they hit something which conducts electricity (like a metal antenna) they are converted into electricity, and when an electrical current flows in a conductor, the conductor heats up.

Your body can conduct electricity. So, when radio waves pass through your body, they cause electricity to flow in your body and this causes your body to heat up. This is effect is normally ridiculously tiny, and completely irrelevant.

However, a mobile device contains several radio transmitters – a cellular radio, bluetooth, wifi, etc. The cellular radio transmitter has to be powerful enough to be able to send a signal for several miles. So when you are making a call, you are holding a powerful radio transmitter right next to your head.

When you do this, the radio waves when they hit your head are strong enough that they can cause detectable heating. The SAR is a measure of how much heating there is. To make sure that people don’t get burned by this heating, there are laws about the maximum SAR allowed.

For a cell phone, the SAR is calculated for when you hold the phone next to your ear, and also for when you have the phone in a pocket (body worn). Because of the different shape and different tissue in your body at these different places, the SAR is different. For example, bone doesn’t conduct electricity much at all, so there is virtually no electrical flow in the skull bone, so no heating. However, next to your body, it is next to fat and muscle, which heat up more.