What Is 1 Watt Per Kg?

1 watt per kilogram (W/kg) is a measurement used to determine the Specific Absorption Rate (SAR) of wireless devices like cell phones, tablets, and laptops. The SAR measures how much radio frequency (RF) energy is absorbed by the body when using a device. It quantifies the rate of energy absorption in watts per kilogram of body tissue.

Specifically, 1 W/kg refers to the SAR limit set by regulatory bodies for safe exposure to RF energy. It represents the maximum whole-body SAR deemed safe for the general population when using wireless devices.

SAR Explained

SAR stands for “specific absorption rate” and is a measurement of how much radio frequency (RF) radiation energy is absorbed by the body when exposed to a radio frequency (RF) transmitting device like a cell phone.

SAR measures the rate at which RF energy is absorbed by the body, specifically by 1 gram of tissue. It is expressed in watts per kilogram (W/kg).

To measure SAR, researchers use sophisticated instruments and techniques to evaluate the levels of RF radiation absorbed. This involves testing cell phones in positions next to a model of a human head and body while they are transmitting at the maximum power levels.

The SAR measurement allows comparisons of the amount of radiation different devices expose a user to. A higher SAR value means more radiation is being absorbed.


The 1 W/kg SAR limit originated in the late 1970s and early 1980s as scientists began researching the biological effects of radiofrequency (RF) radiation from newly emerging wireless technologies. Groups like the United States Naval Medical Research Institute and the World Health Organization convened panels of experts to review the scientific literature and make recommendations for safe exposure limits.

In the US, the American National Standards Institute (ANSI) C95 committee and the Institute of Electrical and Electronics Engineers (IEEE) helped develop the first guidelines in 1982, recommending a SAR limit of 0.4 W/kg averaged over the whole body. Meanwhile, in Europe, the International Commission on Non-Ionizing Radiation Protection (ICNIRP) proposed a SAR limit of 0.08 W/kg in 1984.

Over the next decade, various national and international bodies proposed revised SAR limits as more research became available. This led to the current international standard of 2 W/kg averaged over 10 grams of tissue, which the FCC adopted in 1996 for cell phones sold in the US. Most other countries have since harmonized with the same limit.

current sar limits aim to balance device function and user health

Current Standards

Most countries and regulatory bodies have set limits on the amount of RF exposure and SAR allowed from wireless devices like smartphones. Here are some of the key standards:

  • The FCC limits SAR exposure to 1.6 watts per kilogram (W/kg) averaged over 1 gram of tissue for the head and body, and 4 W/kg averaged over 10 grams of tissue for the hands, wrists, feet and ankles.
  • The European Union limits the SAR level to 2 W/kg averaged over 10 grams of tissue, although each member state can pass its own standards.
  • China has set a SAR limit of 2 W/kg averaged over 10 grams of tissue.
  • Other countries like Canada, Australia and Japan have similar limits to those set by the FCC and EU.

These SAR limits provide a substantial safety margin, assuming reasonable usage of wireless devices. Standards differ slightly between countries but most keep RF exposure thousands of times below danger levels.

Measuring SAR

SAR levels are measured using standardized tests defined by regulatory bodies like the FCC. These tests aim to replicate how people use devices in real-world scenarios.

For mobile phones, SAR testing involves placing a phone model next to a plastic head and body model filled with a liquid that simulates human tissue. The phone operates at its highest power level and transmits at multiple frequencies and positions near the head and body.

SAR measurements are taken based on how much radiofrequency energy is absorbed into the simulated body tissues. Probes within the head and body models detect temperature changes to calculate the absorption rate, measured in watts per kilogram (W/kg).

For larger devices like laptops and tablets, SAR testing uses specialized robots that simulate human usage. The device is operated while robots record SAR across multiple areas to determine peak spatial SAR.

All SAR tests are carried out in FCC-accredited laboratories under highly controlled conditions to ensure consistent and repeatable results.

Effects on the Body

There is ongoing research into the biological effects of exposure to radiofrequency radiation at levels around 1 W/kg in humans and animals. Some studies have reported biological changes associated with these exposure levels, while others have not observed significant effects.

Changes that have been reported in some studies include increased permeability of the blood-brain barrier, oxidative stress, changes in gene and protein expression, increased frequency of DNA damage, altered brain development and activity, reproductive effects, and increased cancer risk. However, many of these findings remain inconclusive due to study limitations and inconsistencies across different investigations.

One of the challenges is that biological effects do not always imply health hazards. More research is needed to determine if the changes seen in some studies at around 1 W/kg have negative impacts on human health and well-being.

Overall, the scientific community agrees more data is needed to make definitive conclusions about biological and health effects. Ongoing research aims to better characterize effects and identify mechanisms of interaction between radiofrequency fields and biological tissues.


The 1 W/kg SAR limit has been the subject of debate and controversy since its introduction. Some critics argue that the limit does not adequately protect the public from potential health risks of cell phone radiation exposure.

One major criticism is that the 1 W/kg limit only measures the amount of heating that occurs from RF energy absorption in the head and body in the short term. However, there are concerns that other biological effects, like oxidative stress or DNA damage, could occur at lower levels over time. These potential long-term or non-thermal effects are not considered in the SAR limit.

Additionally, the testing used to determine a phone’s SAR level has been controversial. SAR values are based on tests conducted by manufacturers under very controlled conditions. Consumer watchdog groups have argued that the actual SAR experienced by users in real-world conditions may be higher than reported.

There is also debate around whether certain groups, like children and pregnant women, require extra protection beyond the standard SAR limit. Childrens’ developing brains and bodies are thought to be more vulnerable to RF energy. Yet, SAR limits do not differentiate between adults and child cell phone users.

While organizations like the FDA and WHO continue to deem the 1 W/kg limit as safe based on current research, some experts argue that the limit may need revising as more studies emerge. There are ongoing calls from some scientists and advocacy groups for stronger regulations and lower SAR limits as a precaution.

Reducing Exposure

There are several tips for minimizing radiation exposure from cell phones, laptops, tablets, and other devices:

  • Use speaker mode or a headset when talking on your cell phone. Keeping the device away from your head reduces exposure to RF energy.

  • Text more instead of talking on the phone. This limits the device’s close proximity to your body.

  • Carry your phone in a bag instead of in a pocket. The greater distance from your body reduces radiation absorption.

  • Opt for a device with a lower SAR value. Choose a phone with an SAR level well below the maximum permitted.

  • Limit children’s use of cell phones. Young people’s developing brains and skulls are more vulnerable to radiation.

  • Don’t sleep next to your phone. Keep devices away from your bed to reduce exposure during sleep.

  • Use a wired headset or speaker for laptops and tablets. Wireless Bluetooth devices also emit radiation.

  • Keep laptops off your lap. Use a table or stand to create some distance from the device.

Following basic precautions like these can help minimize unnecessary radiation exposure from everyday devices.

Latest Research

In recent years, there has been growing interest in understanding the potential biological effects of low-level radiofrequency radiation exposure from wireless devices and networks. This has been fueled in part by the massive growth in cell phone use globally, as well as the rollout of new wireless technologies like 5G.

While high levels of RF radiation are known to cause tissue heating and other adverse health effects, there is uncertainty about what impacts, if any, chronic exposure to low-level radiation may have. Some studies have pointed to possible links to certain types of cancer, reproductive issues, and neurological effects, but the research remains inconclusive.

Several new studies published in the last few years have attempted to further characterize biological and health impacts:

  • In 2018, the National Toxicology Program released results from a multi-year study on radiofrequency radiation exposure in rodents. The study found some evidence of tumors in the brains and hearts of male rats, but no clear associations in female rats or mice.
  • A 2019 study exposed fruit flies to daily RF radiation across their lifespan and found reduced reproductive success and indications of possible DNA damage.
  • Researchers in 2020 examined brain scans from over 450 children and identified some correlations between RF radiation exposure and potential effects on brain structure and function.

However, many experts caution about drawing firm conclusions from these early findings, given the limitations of extrapolating animal research to humans. More longitudinal studies tracking large populations over time are needed to truly untangle any potential risks.

Ongoing research continues to explore aspects like oxidative stress, neurotransmitter disruption, and blood-brain barrier permeability. While a definitive answer remains elusive, the accumulation of evidence will help guide future policies and recommendations on safe levels of exposure.


In summary, SAR or Specific Absorption Rate is a measure of the rate at which radio frequency energy is absorbed by body tissues when exposed to electromagnetic fields from mobile devices. The current standard for cell phones in most countries is around 1 to 2 W/kg over a small sample volume of tissue. While there is no firm evidence that these levels have negative effects on health, some scientists argue that developing brain tissues in children may be more sensitive and current standards should be lowered as a precaution.

Overall, more long-term research is still needed to fully understand the impacts of electromagnetic radiation on the body, especially for chronic exposures over many years. Given the widespread use of mobile devices today, ensuring safety standards are sufficiently protective remains an important area for study. Consumers interested in reducing potential risks can use speaker mode, text messaging, and limit duration of calls. The industry continues working to design phones that absorb less power. Ultimately the 1 W/kg standard seeks to find a reasonable balance between device functionality and precaution for user health.

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