Monday, 21 March 2011
Electromagnetic Radiation (EMR) and Its Effects
In recent years there has been considerable discussion and concern about the possible hazards of electromagnetic radiation (EMR), including both RF (Radio Frequency) energy and power frequency (50-60 Hz) electromagnetic fields.
All life on Earth has adapted to survive in an environment of weak, natural low-frequency electromagnetic fields (in addition to the Earth's static geomagnetic field). Natural low-frequency EM fields come from two main sources: the sun and thunderstorm activity. But in the last 100 years, man-made fields at much higher intensities and with a very different spectral distribution have altered this natural EM background in ways that are not yet fully understood. It has been known since the early days of radio that RF energy can cause interference in most of the wireless operated systems and in devices which process extremely low power signals associated with control systems and in medical equipments. Very high density of Electromagnetic Fields have been known to have resulted in injuries by heating body tissue. These heat-related health hazards are called thermal effects. In addition, there is evidence that magnetic fields may produce biological effects at energy levels too low to cause body heating. The proposition that these thermal effects may produce harmful health consequences has produced a great deal of research. Human body nervous system is also known to work on extremely low intensities of electrical signals. Very little has been done to investigate the effect of Electromagnetic Fields on the human nervous and control system which as such leaves a lot of scope for future research.
Potential Sources of EMR
The list could be endless starting with seemingly harmless AC (alternating current) operating devices to all the intentional and non- intentional transmitters. Appreciable radiation occurs as the size of the components and connecting wires approaches one tenth of the wavelength of the operating frequency. The intensity of the field so radiated also depends on the power handled by the device. To name a few tube lights, spark plugs, washing machines, music systems, power amplifiers, computers, lifts, air conditioners, cable TV, computer screen etc all fall in the category of non-intentional transmitters but can be potential source of Electromagnetic Radiations. All radio and TV broadcasting systems, telecommunication systems and equipments, RADARS, mobile phones etc are intentional transmitters of Electromagnetic Radiations and may result in typical effects on various systems in addition to causing serious interference problems and affecting human body.
With ever increasing use of modern digital technology in almost all the modern processing and communication equipments the threat of unintentional Electromagnetic Radiation is further enhanced. As all digital signals positively have a very large bandwidth, its increasing use will result in Electromagnetic Radiations in almost all the frequency ranges there by leaving a possibility of affecting most of electronic devices and also human beings.
Some Typical Effects of EMR
Electromagnetic Interference: Interference is the energy levels introduced by electronic or communications systems that have a detrimental effect on other systems. Any electronic system is capable of receiving Electromagnetic Radiations if the size of the components or connecting wires approaches one tenth of frequency that may be present in the surrounding environment due to any intentional or unintentional transmitter. Depending on the intensity of this unwanted received radiation there could be instances of malfunctioning of the device receiving this energy. There have been number of instances where such a condition had resulted in catastrophic failure of the equipments. The most famous example is the failure of the Electronic surveillance system installed at one of the ships of United Kingdom Naval forces during Falkland war due to operation of its own Radar system installed on the same warship. As a result of this only one system could be operated at one time resulting in the loss of the ship. Intentional Jamming of the radio receivers by the security and police forces is the utilization of this effect to make the enemies receivers inoperative, however it can also happen due to unwanted and unintentional Radiation of Electromagnetic fields.
Effects on Human Body: Body tissues that are subjected to very high levels of RF energy may suffer serious heat damage. These effects depend upon the frequency of the energy, the power density of the RF field that strikes the body, and even on factors such as the polarization of the wave.
However, additional longitudinal resonances occur at about 1 GHz near the body surface.
Health Care Engineering: Is electromagnetic Interference (EMI) becoming a problem in the healthcare environment? Although electrical interference in hospitals is often regarded as no more than a minor nuisance, there are documented cases in which equipment failures due to Electromagnetic Interference (EMI) have lead to injury or death. Some examples fare as follows: -
o In 1992, a patient attached to a monitor-defibrillator in an ambulance died because of interference from the ambulance radio prevented the machine from working.
o In 1987, patient monitoring systems failed to sound alarms because of interference; two patients died as a result.
o In 1993, a patient fitted with a pacemaker went into ventricular fibrillation shortly after being scanned with a metal detector outside a courtroom.
As in many other fields, the amount and complexity of electronics in hospitals and other medical environments is increasing year by year. Despite this, the number of reported incidents of EMC (Electromagnetic Compatibility) problems fortunately does not appear to be growing. This is probably because most manufacturers and designers of medical electronic products have developed a good awareness of EMC. Pacemakers are typical examples, where their design with respect to compactness and immunity to radio frequency interference has greatly improved over the years. Nowadays, pacemakers are very reliable, but can still fail under extreme conditions. Pacemakers have failed in patients undergoing electro surgery (2) and in other cases where patients kept mobile phones in their chest pockets, a few centimeters from the pacemaker leads (3). The powered Wheelchair is another typical example: there are many stories of radio frequency interference. (RFI) from mobile phones or police 'walkie-talkies' causing the wheelchair to drive itself and its occupant into traffic. These stories are based on real occurrences; reports of incidents in the USA in the early 1990s prompted the Food and Drug Administration (FDA) to investigate the problem and recommend that the manufacturer change the design to give an immunity of at least 20V/m to RFI.
The use of many items of electronics equipment in close proximity in the hospital environment means that the same sort of EMC problems are encountered as with other types of electronic products. However there are some special features of the hospital EM environment:
o Failure of medical devices can lead to injury or death.
o Some equipment found in hospitals is intentionally designed to emit electromagnetic energy, often for therapy. Other equipment, which may be located nearby, is designed to detect very small physiological signals. This combination has the potential to create EMC Problems.
o Also in hospitals, there is the question of whether to ban or restrict the use of mobile phones. They improve communications but can interfere with critical equipment.
o Many medical devices are connected directly to patients. For mains-powered devices, the designer must prevent electric shock as well as ensure EMC. Electronic equipment is constantly evolving, so there is always the possibility of new problems arising, e.g. interference from new types of mobile communications devices.
Some Remedial Measures
Ensuring EMC of the devices/ components: Electromagnetic compatibility describes a state in which the electromagnetic environments produced by natural phenomena and by other electrical and electronic devices do not cause interference in electronic equipment and systems of interest. In order to reach this state, it is necessary to reduce the emissions from sources that are controllable, or to increase the immunity of equipment that may be affected, or to do both.
To try to eliminate all possibility of interference by decreasing emissions and increasing immunity further could incur a high cost to industry and could prevent new technologies from emerging. For example, a restriction lowering the transmitting power of cellular telephones so that consumers could place their cell phones on top of any electronic equipment might compromise the performance and economic viability of such communication systems. On the other hand, a requirement that all commercial electronic equipment perform without malfunction at ambient levels of 50 V/m would place a financial burden on manufacturers of a large range of equipment.
The following are some of the techniques used to counter the effects of EMI:
Source Elimination: An effective technique to eradicate interference is through identification and elimination of the source. In theory, this represents arguably the most effective of any measures, but is not practical in most situations, whereas this would require the source to be periodically or permanently disabled from operation. Grounding a grounding point represents a common reference point for a device or multiple devices that functions to ensure the safety or the equipment and operator, and its effects provide some immunity to noise and interference. Certain transmission and other Electronic equipment require adequate grounding to ensure proper operation. The Conductor used to ground the equipment should be the shortest necessary length to avoid a ground loop condition. This could result in energy transfer through conduction to connected devices.
Filters The use of filters allows selected frequencies to pass through to the connected device, while rejecting or attenuating any frequencies that are outside the filter specifications. Examples of filters include low-pass, band-pass, and high-pass.
Shielding. An effective manner used to minimize, and in some instances eliminate, EMI is to effectively shield components from interaction with electromagnetic energy. This technique is often expensive and causes major design engineering challenges, especially to fully shield a device, which requires that conductive material completely enclose the equipment or circuitry. Any separation in the shielding material reduces the effectiveness of the shielding technique.
With ever increasing use of the spectrum due to modern communication equipments and also use of latest digital processing techniques for most of the devices there is a need for laying down the guidelines for Emission standards. The extent of the spurious radiation by any device should be limited to the extent possible without increasing the cost of the product. Various governmental and nongovernmental agencies must come forward at national and international level to coordinate and standardize the permissible radiation limits