Specific Absorption Rate

The Specific Absorption Rate (SAR) is a scientific measure used to quantify the rate at which the human body absorbs electromagnetic radiation energy from a radiofrequency (RF) source, such as mobile phones, Wi-Fi devices, or other wireless transmitters. It is an important parameter for evaluating exposure safety and ensuring that electronic devices operate within internationally prescribed radiation limits to prevent adverse health effects.

Definition and Concept

The Specific Absorption Rate (SAR) is defined as the power absorbed per unit mass of biological tissue, expressed in watts per kilogram (W/kg). It represents the amount of RF energy that is converted into heat inside body tissues when exposed to electromagnetic fields.
Mathematically,
SAR=σE2ρ\text{SAR} = \frac{\sigma E^2}{\rho}SAR=ρσE2​
where:

• σ = electrical conductivity of the tissue (S/m)
• E = root mean square (r.m.s.) electric field strength inside the tissue (V/m)
• ρ = density of the tissue (kg/m³)

This equation shows that SAR depends on both the strength of the electromagnetic field and the physical properties of the tissue.

Importance of SAR Measurement

The SAR value serves as a biophysical index of human exposure to electromagnetic radiation. Since excessive RF energy absorption can raise tissue temperature and potentially cause biological effects, SAR limits are established to ensure that electronic devices remain safe for human use.
Regulatory agencies require manufacturers to test and certify that mobile phones and other RF-emitting devices conform to these limits before they can be marketed.

Units and Measurement

SAR is measured in watts per kilogram (W/kg), indicating the rate of energy absorption by 1 kilogram of tissue.
There are two main types of SAR measurement:

• Localized SAR: Measures the energy absorbed in specific parts of the body, typically the head or torso, during mobile phone use.
• Whole-body SAR: Represents the average absorption throughout the entire body, relevant for full-body exposure to RF fields such as those from base stations or MRI scanners.

The measurement is typically conducted using a phantom model (a human-shaped liquid-filled structure that simulates tissue conductivity) and a robotic probe to record energy absorption levels across different positions.

International Standards and Regulatory Limits

Different regulatory agencies around the world prescribe specific SAR limits based on scientific studies and safety margins:

• International Commission on Non-Ionizing Radiation Protection (ICNIRP):
  • Localised SAR limit for head and trunk: 2.0 W/kg averaged over 10 grams of tissue.
  • Whole-body SAR limit: 0.08 W/kg.
• Federal Communications Commission (FCC), USA:
  • Localised SAR limit: 1.6 W/kg averaged over 1 gram of tissue.
• India (Department of Telecommunications):
  • Adopted the same limit as the FCC, i.e., 1.6 W/kg averaged over 1 gram of tissue, effective from 2012.

These limits ensure that devices emit radiofrequency energy at levels far below those that could cause thermal or biological harm.

SAR and Mobile Phones

SAR values are commonly associated with mobile phones, which are the primary source of personal RF exposure. When a mobile phone is held close to the head or body, some of the transmitted energy is absorbed by the tissues.
Key aspects include:

• The SAR value of a phone is measured under controlled laboratory conditions at maximum transmission power.
• Actual exposure during normal use is usually much lower, as modern phones adjust their transmission power based on signal strength and proximity to the network tower.
• Manufacturers are required to disclose the SAR rating of each phone model, which can typically be found in the user manual or on the official website.

Factors Influencing SAR Values

The SAR value for any given device or exposure situation depends on several factors:

• Frequency and Power Output: Higher frequencies and power levels increase the rate of energy absorption.
• Distance from the Source: Energy absorption decreases rapidly with distance; keeping the device slightly away from the body significantly reduces exposure.
• Duration of Use: Longer exposure leads to higher cumulative absorption.
• Tissue Composition: Water-rich tissues (like the brain and muscles) absorb more energy than fatty tissues.
• Device Design and Antenna Position: The placement of antennas and shielding affects radiation distribution.

Biological and Health Implications

Research on the health effects of RF exposure primarily focuses on two aspects:

1. Thermal Effects:
   • When tissues absorb RF energy, the primary effect is a rise in temperature.
   • SAR limits are designed to prevent any significant heating (greater than 1°C) that could damage tissue.
2. Non-Thermal Effects:
   • Some studies have suggested possible biological changes unrelated to heating, such as effects on DNA, sleep patterns, or cell function.
   • However, scientific consensus from bodies such as the World Health Organization (WHO) and ICNIRP indicates that there is no conclusive evidence linking mobile phone radiation within permissible SAR limits to adverse health effects like cancer.

Methods to Minimise SAR Exposure

While mobile devices are designed to comply with safety standards, users can further reduce exposure through simple precautions:

• Use Hands-Free Devices: Earphones, Bluetooth, or speaker mode keep the phone away from the head.
• Text Instead of Calling: Messaging reduces direct contact time with the body.
• Avoid Weak Signal Areas: Phones emit higher power when searching for signals.
• Carry Phones Away from the Body: Avoid keeping phones in pockets for prolonged periods.
• Limit Long Calls: Frequent short calls reduce cumulative exposure.

SAR Testing and Compliance

Before being approved for sale, electronic communication devices undergo rigorous testing for SAR compliance. These tests are conducted using standardised procedures defined by regulatory bodies and international organisations such as IEEE, IEC, and CENELEC.
The results are verified and certified by accredited testing laboratories to ensure public safety.

Criticism and Misconceptions

Despite scientific safeguards, public concern about electromagnetic radiation persists. Some misconceptions include:

• Belief that lower SAR means a “safer” phone — in reality, all certified phones operate well below harmful levels.
• Misunderstanding that SAR directly correlates with health effects — SAR is only a regulatory safety indicator, not a direct measure of biological risk.
• Variability in SAR across countries leads to confusion, although all limits are set with substantial safety margins.