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Electromagnetic Fields

Electromagnetic radiation covers a wide range of frequencies (Fig 1.0), which are generally divided in to 2 groups, ionizing and non-ionizing.

Figure 1.0 The Spectrum of Electromagnetic Radiation

Ionizing radiation when absorbed by materials can change the atomic structure of the material by knocking electrons out of atoms. The ionizing spectrum includes x-ray, gamma rays and short wave ultraviolet radiation.

A Federal standard exists which requires all VDT distributors to control for X-ray emission such that no VDT can be sold in Canada unless the VDT meets or exceeds the standard (0.05 mR/hr).

Non-ionizing radiation does not change the atomic structure of the human body and includes frequencies from ultraviolet radiation to ELF (extremely low frequency). Levels of non-ionizing radiation are emitted from VDTs such as ultraviolet, visible and infra-red, but these are found to be well below levels which are considered dangerous ( Table 1).

Also included in the non-ionizing electromagnetic radiation group are very low frequency and extremely low frequency electric and magnetic fields. Before explaining the origin of these two fields, some basic concepts of the VDT operation will be discussed.

The cathode ray tube (CRT), which forms the screen with its face, releases electrons which are accelerated towards the screen. Images on a VDT screen are produced by this projected electron beam which moves horizontally and vertically across the screen. This horizontal and vertical 'scanning' is controlled by horizontal and vertical deflection coils, such that each time the beam travels across the screen in response to the horizontal deflection coil, it also moves slightly downward in response to the vertical deflection coil.

Very low frequency (VLF) fields originate from the current in the vertical deflection coil and arise from the output lead of the fly back transformer (see Fig. 2.0). VLF electric and magnetic (E and H respectively) fields from a VDT are really no different from sitting in front of a colour television screen. Extremely low frequency (ELF) fields fall below the 300 Hz range. Both ELF electrical and magnetic fields are produced by the electric circuits responsible for the vertical motion of the electron beam on the VDT. More specifically, the magnetic field originates from the current in the vertical deflection coil, and the electric field arises from a modulation of the static charge on the screen (Fig. 2.0).

These fields will be measured in the survey. Very low frequency (VLF) fields will be measured for both magnetic and electric, and ELF (extremely low frequency field) will be measured for magnetic and electric.

Figure 2.0 Cross Section of a Typical Video Display Terminal

 

Reproductive Disorders

There have been reports of miscarriages from VDT operators. However, research in this area has been unable to provide any scientific evidence that VDT work was related to miscarriages or birth defects.

If you wish to investigate this topic further, the following references will be able to provide more detailed information :

 

Non-Ionizing Electromagnetic Radiation Exposure Guidelines

As mentioned earlier, VLF will be measured for both electric (E-Field) and magnetic fields (H-Field), and ELF will be measured for both electric (E-Field) and magnetic field (H-Field).

Table 1.0 lists exposure guidelines used by the Ministry of Labour (derived from Federal Safety Code #6), and from the American Conference of Governmental Industrial Hygienists (ACGIH).

Table 1.0 Exposure Guidelines for Electromagnetic Radiation

Spectrum Maximum Levels Recommending Agencies
X-ray
> 10 keV


0.5 mR/h

Radiation Emitting Devices Regulations
Very Low Frequency (VLF)
0.01 - 1 MHz
1 - 10 MHz

Electric (V/m)
280  
280/f

Magnetic (mA/m)
2190
2.19/f

 Ontario Ministry of Labour
Federal Safety
Code # 6
Extremely Low Frequency (ELF)
(60 Hz)

Electric (V/m)
25000

Magnetic (mG)
        10,000

 ACGIH

 

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