Answer to Question #11053 Submitted to "Ask the Experts"

Category: Micro/Radio Waves, Radar & Powerlines — Microwaves and Radiofrequency

The following question was answered by an expert in the appropriate field:

Q

Question 1: Is it unsafe to place a laptop on your body instead of a desk?

Question 2: The instruction manual for my laptop says that "human proximity to the antennas should not be less than 20 cm, including when the computer display is closed." Does this mean that the laptop is unsafe if I use it on my lap?

A

Answer to Question 1: The short answer is no, but let me explain further. I assume this question refers to possible health risks from the electromagnetic fields produced by laptops. Nearly all laptops have Wi-Fi modules, and many have Bluetooth (Bluetooth SIG, Inc.) modules as well. A few laptops also have transmitters that enable them to communicate with mobile-telephone networks as well. In addition, laptops (as do all computers) produce a variety of low-frequency electric and magnetic fields associated with the operation of the internal circuits. All of these are potential sources of exposure to the user to electromagnetic fields of varying characteristics.

Under any circumstances that I can imagine, your exposure to radiofrequency (RF) energy from a laptop would be far below national (United States) and international exposure limits. This is because:

  • Of the low operating power of the RF devices in the laptop.
  • Of their low duty cycle of transmission (i.e., the small fraction of time in which they are actually transmitting RF energy).
  • Most laptops have the antennas mounted in the back cover beneath the screen, and they face away from the body even when the laptop is on the lap of a user.
  • Laptops, together with tablets and smartphones, are tested to ensure that they comply with government exposure limits, in the United States those of the Federal Communications Commission (FCC). Also, the low-frequency fields produced by laptops are most certainly far below accepted safety limits.

Tests for compliance with FCC exposure limits are done under conditions that approximate highest exposure scenarios, i.e., setting the device to transmit at the highest level that it is physically capable of producing, as opposed to levels at which the device would normally operate. To receive an FCC authorization number (which is usually on the bottom of the case), the manufacturer must provide evidence that the laptop meets relevant exposure limits. But the regulatory situation is rather complicated. See the second Q/A below.

Question 2: The instruction manual for my laptop says that "human proximity to the antennas should not be less than 20 cm, including when the computer display is closed." Does this mean that the laptop is unsafe if I use it on my lap?

Answer to Question 2: Again, the short answer is no. In brief, the "20 cm" advisory in the computer's manual may be a consequence of regulatory issues as opposed to indicating an actual likelihood of overexposure. I will explain what I mean by that. Conceivably, it may mean that the RF exposure very close to the back of the case exceeds government limits. But I think that it is much more likely that the manufacturer of the device simply had not tested it under such circumstances.

To explain, the Wi-Fi antennas in laptops are usually mounted inside the top cover of the device behind the screen. Such antennas would typically be more than 20 cm from the user's body even when the laptop is on the user's lap.  

A manufacturer of a laptop can often choose to apply for FCC authorization under one of two different rules that specify different ways to establish compliance with RF exposure limits. One, an older set of rules, had been developed initially for "mobile" devices such as car phones and applies for devices that are operated more than 20 cm from the user. For such devices, the appropriate measure of compliance is the RF signal strength in air near the antenna. This is quite easy to determine.

By contrast, cell phones are typically used with their antennas close to the body. For these kinds of devices, a second and different set of rules applies, requiring the manufacturer to determine the power deposition inside the user's body, the so-called Specific Absorption Rate (SAR). This is a much more difficult and expensive quantity to measure. If the laptop would be used with its antenna closer than 20 cm from the body, the FCC would require SAR tests similar to those performed on cell phones. (Tablet computers, as well as laptops whose antennas are beneath the keypad, are all tested for SAR.) Otherwise, the manufacturer can apply for FCC authorization under the earlier set of rules, which only requires it to determine the RF fields in air near the antenna.

So why does the caution appear in the user's manual? The most likely reason is that the manufacturer has chosen not to have the comparatively expensive SAR tests done, since users would not operate the laptop with its cover (which contains the antenna) against the body and the "worst case" conditions of testing (cover against the body) are not likely to represent real-world exposure conditions. But since the manufacturer has not tested the device with the cover against the body, it adds a caution in fine print to the instruction manual.

As a practical matter, Wi-Fi transmitters in laptops operate at low power levels and at a very low duty cycle (low fraction of time transmitting energy). Under any realistic usage scenario, the SAR produced in the user's body by the Wi-Fi transmitter would almost certainly be far below FCC limits, even if the case were pressed directly against the body.

Kenneth Foster, PhD
University of Pennsylvania
 

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