Human Exposure to Radiofrequency Energy From Wireless Local Area Networks
Kenneth R. Foster
Professor of Bioengineering
University of Pennsylvania
Wireless computer networks have become commonplace in our environment. Wireless hot spots are found in many public areas and, increasingly, in homes and schools. Wireless networks use low-powered radiofrequency (RF) transmitters called access points to communicate with other low-powered transmitters called client cards that are located in users' laptop computers or other portable equipment. Nearly all of these wireless networks use Wi-Fi technology, although other wireless technologies are coming into use as well.
Despite the very low power at which wireless networks operate, some citizens have questioned the possibility that the RF signals associated with the networks might pose a health threat. This column addresses those concerns.
The question of possible health effects of RF signals from Wi-Fi networks has two parts: What levels of exposure do people experience from the networks? What are the possible adverse effects of the RF energy from the networks on the human body?
Wireless networks operate at low power levels and, consequently, the levels of exposure to users of Wi-Fi-equipped computers are low. Other people, who are not using Wi-Fi-enabled equipment, experience still lower exposures to RF energy. The maximum power output of client cards (located in computers) or access points (typically located in the ceiling of public areas with hot spots) is typically lower than the maximum power output of most mobile telephones. Moreover, this signal characteristically falls off as the square of the distance of the user to the antenna of the transmitter.
Another factor serves to limit public exposure to Wi-Fi fields: the very small fraction of time that the client cards or access points are actually transmitting signals. A number of factors limit the fraction of time that a particular client card or access point is transmitting energy. This includes the requirement that only one transmitter (client card or access point) is operating at a particular time, limitations in the capacity of the wired network to which the wireless network is connected, and error correction schemes used by the network.
Consequently, a laptop containing a wireless client card invariably produces far smaller exposures to the user than does a mobile-phone handset operated at the same distance from the body. Because of the greater distance of an access point to the user, the exposures produced by access points are far lower still. In fact, surveys show that RF fields from Wi-Fi networks in ordinary environments are nearly always smaller than fields in the same area from nearby cellular base stations, broadcast transmitters, and other commonplace sources of RF energy.
In 2006 I conducted an industry-supported survey of RF field levels in urban and suburban areas in four countries—the United States, France, Germany, and Sweden (Foster 2007). The survey made 356 measurements of background RF signals at 55 sites: private residences, commercial spaces, health care and educational institutions, and other public spaces. Measurements were conducted in public spaces as close as practical to access points.
The results, which are detailed in "Radiofrequency Exposure From Wireless LANs" (Foster 2007), show that in all cases the measured Wi-Fi signal levels were very far below international safety limits, specifically, those of the Institute of Electrical and Electronics Engineers and the International Commission on Nonionizing Radiation Protection (ICNIRP 2002). These limits were designed to protect against all known hazards of RF energy. In nearly all cases, these signals were also considerably lower than those from other nearby sources of RF energy, including cellular telephone base stations.
Concerns about possible health risks from exposure to low levels of RF fields in ordinary environments have been expressed by a number of individuals over the years in connection with many technologies that use RF energy. To address such concerns, health agencies around the world have repeatedly reviewed the scientific literature and found no convincing evidence of any health hazards from RF fields below international safety limits. For example, the World Health Organization (WHO) stated recently in a fact sheet that "no health effects are expected from exposure to RF fields from [cellular] base stations and wireless networks" (WHO 2006).
A few individuals have reported that RF signals from Wi-Fi and other low-level sources of RF fields can trigger allergy-like reactions—a phenomenon called electrical hypersensitivity. This is a complex issue that scientists have studied with respect to low-level RF fields from various sources for a number of years.
While the distress of electrically hypersensitive individuals is very real, controlled studies have failed to connect their symptoms to the exposure to fields. These studies show that the symptoms appear to be associated with whether the individual believes that he or she is being exposed, rather than the actual exposure. The WHO fact sheet quoted above states that electromagnetic fields "have not been shown to cause such symptoms. Nonetheless, it is important to recognize the plight of people suffering from these symptoms" (WHO 2006).
Thus, electrical hypersensitivity is a complex psychosocial phenomenon, not a straightforward toxicity response to RF fields. Indeed, given the presence of RF fields from many sources in the environment, many stronger than fields from wireless networks, it is difficult to imagine that wireless networks by themselves could be a cause of significant health problems or that an electrically hypersensitive individual could reliably identify wireless networks as the cause of his or her problems.
I conclude that levels of exposure of citizens to RF fields from wireless networks is far below international safety limits. Moreover, in nearly all of the places that I surveyed, the Wi-Fi signals were far below other RF signals that were present from other sources. Given the low level of exposure to people from RF fields from wireless networks in comparison to that from other sources of RF energy that are ubiquitous in the modern environment, any health concerns about wireless networks would seem to be moot.
- Foster KR. Radiofrequency exposure from wireless LANs. Hlth Phys 92:280–289; 2007.
- International Commission on Non-Ionizing Radiation Protection. General approach to protection against non-ionizing radiation. ICNIRP Statement in Hlth Phys 82:540–548; 2002.
- World Health Organization. Electromagnetic fields and public health: Base stations and wireless technologies; May 2006. WHO Fact Sheet. Available at: who.int/mediacentre/factsheets/fs304/en/index.html. Accessed 2 April 2007.