In an electromagnetic measurement report, two quantities come up often — and are, just as often, confused with one another even though they don't measure the same thing.
Specific Absorption Rate (SAR)
SAR, expressed in watts per kilogram (W/kg), measures the energy absorbed by biological tissue during close contact with an emitting device — typically a mobile phone held against the head or body. It's a near-field measurement.
Ambient power density
Power density, expressed in microwatts per square metre (µW/m²), measures the radiation present in the air at a given location — independent of any contact with a device. It's a far-field measurement, used to characterise the ambient exposure of a room or living space.
Why you can't convert one into the other
These two quantities describe different physical situations: one is direct contact with a source, the other is exposure at a distance in free space. There is no universal formula for converting one into the other. A report that mixes them up, or translates one into an equivalent of the other, is misleading.
What this means for reading your report
An EMesura report always specifies which quantity is used for each measurement, and which reference it's compared against — ICNIRP regulatory limits for SAR, precautionary scales such as SBM-2024 for ambient power density. This distinction, simple once explained, avoids misleading comparisons.