SAR stands for Specific Absorption Rate, and it measures how much radiofrequency energy your body absorbs from a wireless device like a smartphone. It’s expressed in watts per kilogram (W/kg) and represents the rate at which your tissue heats up when exposed to radio waves. Every phone sold in the U.S. must have a SAR value below 1.6 W/kg, and most countries enforce similar limits to prevent tissue heating that could cause harm.
How SAR Works
When your phone transmits a signal, it emits radiofrequency electromagnetic fields. Some of that energy gets absorbed by your body, particularly by whichever part is closest to the antenna. The primary biological effect of this absorbed energy is heating. It’s the same basic principle as a microwave oven, just at far lower power levels. Your body can easily dissipate the small amount of heat a phone generates, but regulators set SAR limits to ensure that even under worst-case conditions, localized heating stays well within safe territory.
SAR is measured over a specific mass of tissue and averaged over a six-minute window. In the U.S., the measurement covers 1 gram of tissue. In Europe and most other regions following international guidelines, it covers 10 grams of contiguous tissue. This difference in averaging mass is why the numerical limits differ between the two systems, even though both aim for a comparable level of protection.
U.S. and International Safety Limits
The FCC caps SAR for cell phones at 1.6 W/kg, averaged over 1 gram of tissue. This is the number you’ll see listed in phone specifications for devices sold in the United States.
Most of the rest of the world follows guidelines from the International Commission on Non-Ionizing Radiation Protection (ICNIRP), which sets the limit at 2 W/kg averaged over 10 grams of tissue for the head and trunk. Because 10 grams of tissue smooths out “hot spots” more than 1 gram does, the ICNIRP number is higher without actually allowing more heating in any one spot. Both standards trace back to the same science: whole-body exposure at about 4 W/kg for 30 minutes raises core body temperature by roughly one degree Celsius. The public limits build in large safety margins below that threshold, with ICNIRP using a 50-fold reduction factor for the general public.
ICNIRP also sets a whole-body average SAR limit of 0.08 W/kg for the general public and 0.4 W/kg for occupational exposure, along with separate, higher limits for limbs (4 W/kg public, 20 W/kg occupational), since arms and legs tolerate more heating than the head and torso.
How SAR Is Tested
Manufacturers don’t test SAR on real people. Instead, they use a human-shaped model called a SAM phantom (Specific Anthropomorphic Mannequin) filled with a liquid that mimics the electrical properties of human tissue. For head measurements, the phone is placed against the phantom’s ear. For body measurements, a flat phantom is used. The liquid must be at least 10 centimeters deep so the measurement probe can scan properly along the phantom’s curved surfaces.
The tissue-simulating liquid is a carefully calibrated mixture, typically combining water with non-polar liquids like mineral oil or glycerol, plus emulsifiers. Its electrical properties must fall within tight tolerances of the target values for human tissue. A robotic probe then scans through the liquid in two passes. First, an area scan at a resolution of 1 centimeter or finer maps the general absorption pattern. Then a finer “zoom scan” at 5-millimeter resolution or better zeroes in on the peak absorption area. The highest SAR value recorded becomes the device’s official rating.
This testing represents a worst-case scenario. The phone transmits at maximum power, pressed directly against the phantom. In everyday use, your phone almost never operates at full power because it adjusts its output based on signal strength. A strong signal means the phone can dial back its transmission power significantly, resulting in SAR levels well below the tested maximum.
What Changes With 5G
Traditional SAR measurement works for frequencies up to 6 GHz, which covers 4G and lower-band 5G. But higher-frequency 5G (sometimes called millimeter wave) operates above 6 GHz, and at those frequencies, radiofrequency energy barely penetrates the skin. It gets absorbed almost entirely in the outermost layers of tissue rather than deeper inside the body.
Because of this shallow penetration, regulators switched the measurement approach for devices transmitting above 6 GHz. Instead of SAR, these devices are evaluated using power density, measured in milliwatts per square centimeter at a minimum distance of 5 centimeters from the device. The underlying goal is the same: preventing excessive tissue heating. The metric just fits the physics better at higher frequencies.
How to Find Your Phone’s SAR Value
Every phone manufacturer is required to disclose SAR values, and there are several easy ways to find yours. On most Android phones, you can navigate to Settings, then About Phone, then Legal Information, where you’ll find a section on RF exposure and SAR. On iPhones, the same information lives under Settings, then General, then Legal and Regulatory.
You can also look up any phone’s SAR value on the FCC’s website by searching the device’s FCC ID, which is printed on the phone’s label or listed in its settings. The manufacturer’s support website will also have SAR data listed alongside the device manual.
Keep in mind that a phone with a SAR of 1.5 W/kg isn’t meaningfully more dangerous than one rated at 0.5 W/kg. Both are tested under maximum-power conditions that rarely occur in practice, and both fall within the safety limit that already includes a wide margin below levels known to cause any biological effect. The SAR value is a regulatory compliance number, not a real-world exposure reading.
Does SAR Radiation Pose Health Risks?
The World Health Organization states that the primary biological effect of radiofrequency fields is tissue heating, and that existing exposure guidelines are specifically designed to prevent health effects from both localized and whole-body heating. At the power levels phones actually operate at, the temperature increase in tissue is negligible, typically fractions of a degree that your body’s blood flow handles effortlessly.
The question of whether long-term, low-level exposure causes non-thermal effects (impacts unrelated to heating) has been studied extensively for decades. Major health agencies have not found consistent evidence that RF exposure below current limits causes cancer or other diseases, though some agencies classify radiofrequency fields as “possibly carcinogenic” as a precaution, a category that also includes things like pickled vegetables and talcum powder. If you want to reduce your exposure anyway, using speakerphone or a wired headset keeps the antenna farther from your head, which drops absorption dramatically since RF energy weakens rapidly with distance.