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The overall aim of this thesis is to increase our knowledge of the physical environment of office workers with special focus on electromagnetic fields and to address the question of whether electromagnetic fields can directly or indirectly contribute to symptoms or discomfort among video display terminal ( VDT) workers. Furthermore, we have measured light modulation from various commonly used light sources in laboratory conditions and, as a second step, used modulated light as stimulus for provocation of neurophysiological responses in persons with perceived “electrical hypersensitivity” (EHS).During the last 20 years work-related illness among office workers has received increased attention. Changes in the physical environment, the introduction of VDTs and other electrical equipment and changes in light conditions have been discussed in this context. The basis for this thesis is the interdisciplinary Office Illness Project in Northern Sweden. Using a questionnaire completed by 4,943 office workers, 150 VDT workers with or without facial skin symptoms were selected for a case referent study of the electromagnetic fields in offices.When the measurements in the offices were performed in 1989, the general level of the 50 Hz magnetic fields in the offices was rather low, but in 5% of the offices the flux density exceeded 0.5 pT. At this level VDT monitors were shown to display detectable instability (jitter). Furthermore, the ability of test subjects (healthy volunteers) to detect jitter was shown to depend on both the amplitude and frequency characteristics of this instability. The study indicates that the instability of computer monitors and thereby the instability of the visual image of the VDT screen might be an increasing problem since it is known that the harmonic content of the general magnetic field in offices is on the rise.VDT monitors contributed to the magnetic field level at VDT workplaces in both extremely low and very low frequency ranges. However, the dominant source of electric fields in rooms was ungrounded electrical equipment, not VDT screens.High electric fields in the extremely low frequency range in the offices were associated with skin symptoms among VDT workers. The causal nature of this association cannot be determined since it may depend on undetected factors related to exposure. No associations were found, however, for any of the VDT-related electromagnetic fields and skin symptoms.Commonly used fluorescent tubes in our office environment have a degree of modulation of the light (flicker) that varies widely from less than 1% (fluorescent tubes with high frequency gear) up to nearly 100%. When persons with perceived EHS were exposed to flickering light, a higher amplitude of brain cortex responses were found at all tested frequencies compared with control subjects. These findings are of considerable interest, but further studies are required in order to establish a possible relationship between flickering light and discomfort/symptoms in persons with perceived EHS.