Absence of mutagenic response to radiation from a video display terminal.

K. Absenceof mutagenic responseto radiation from a video display terminal. Scand J Work Environ Health 14(1988)49-51. The standard AmesSalmonella test (TA 100) was used to detect the mutagenicity of radiation from a video display terminal. The Ames test is a sensitive assay that detects the ability of a chemical to damage deoxyribonucleic acid. It has also been employed to detectthe mutagenicityof electromagnetic radiation. An extremely short distance (62mm) from a video display terminal and an extremely high electrostatic field strength (250 kv/ m) was employed. No mutagenic response was found in this test system.

Sk in problems among people wo rking at a video display terminal (VDT) were first reported from Norwa y in 1981 (10) . Sin ce then cases and such problems ha ve been published in Great Britain (14) , th e United States (4,5, II), and Sweden (2,7,8,15). Most them ha ve been Ro sace a-like.
Several attempts ha ve been made to establish a reasonable etiology for rashes possibly related to exposure to video displ a y terminals . Studies ha ve sho wn that , besid es the desired visible radiation from the pattern generated on the screen , the radiation at shorter optical wavelengths is negligible and no ionizing radiation occurs (13). The ph ysical effects generated by the terminals con sist mainly of alternating ma gnetic field strengths that leak from the magnetic de fection of the electron ra y gen erating the screen pattern and an electrostatic field strength from a positively charged translucent film inside the front glass of the display. The field streng th acc elerates th e negatively cha rged electrons to wards the screen . No biological effects of the magnetic field strengths hav e been proved . Th e electrostatic field strength , on the other hand , seems to be a rea sonable cause o f skin rashe s (2,3,8,10).
However, in the popular pre ss in Sweden there ha s been debate on whether work with video display terminals co uld ca use severe side effects such as ter atogenic effects and aging of th e skin. Many pregnant wo me n do not dare wo rk at a terminal, a nd man y operators a re an xiou s about the po ssibility of skin deterioration . Fo r this rea son it was considered desirable to st ud y th e po ssible mutagen ic effect of video display terminals in the Ames test using the Salmonella tester strain T A 100. This is a sensitive as say for detecting the abilit y of a chemical to damage de oxyribonucleic acid (I). This test system has also previously been shown to detect mutagenic effects of electromagnetic radiation (9) , such as ultraviolet radi ation type A (9), ultraviolet radiation type B (12) , 250-kV X ray s, and Cobalt-Sf) gamma rays (6).

Materials and methods
Salmonella typhimurium strain T A 100 was obtained from Dr Bruce Ames (Uni versity of California, Berkeley, Cali forn ia , United States). This ba cterial test er strain mainly detects mutagens causing base-pair substitutio ns . It lack s the uvrB excision repair system. Thi s bac terial strain is regularly used in the laboratory and was checked for proper genetic characteristics. Rat liver homogenate for metabolic activation was not used in thi s experiment. The mutagenic compound 2-nitrofluorene was used as a positive control. Two milliliters of soft agar containing traces of biotin and histidine was mixed with 0.1 ml of an overnight (14 h) bacterial culture and poured onto minimal glucose agar plates (20 ml aga r in petri dishes with a diameter of 90 mm) (I). Thus the thickness of the agar layer containing the ba cterial strain became a pproxi ma tely 0.3 mm, which co rr espo nds well to the th ickness of the epidermis. A co mmo n monochromatic video display terminal , (DM -14 , manu factured by th e Ad vanced Datum Info rmat ion Corporation in Taiwan) was used in the experiments. In thi s type o f terminal no attempts to eliminate the emiss io n of altern a ting ma gnetic or electr ic field s streng ths have been made . The terminal was used for a ll kinds of emission except that of the electrostat ic field strength, which wa s elim inated by a met al screen in front of the terminal. This shi elding net also decreased the alternating magnetic field strengt hs a t the test system . The agar plates were situated horizontally without lids 62 mm from the front glass of the terminal.

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An approximatel y homogeneou s electros tatic field was generated between a pair of metal plate s, 0.5 x 0.7 01, at a distance of 62 0101 . The upp er plate was connected to the high voltage generator in a black and white television set, and the lower one was grounded. Since the agar was molded into insulating petri dishes, the potential of the exposed agar must be defined by a ground path to the lower plat e. The gro unding was don e through a short , conveniently bent metal wire. The agar plates were situated horizontally without lids above the lower metal plate. In this par allel plat e system, the electrost atic field strengths at the sur faces of the exposed agar plates were about 250 kV1m .
Twent y-one TA 100 plate s were used; 13 between the metal plates in the electros tatic field, three und er the term inal without electrosta tic field str ength, and five as exposed controls at different sites in the room . The experiment was run twice and the agar plates were counted blind. To permit blind counting, grounding wire imp ressions were made on all the agar plat es. Because of the possibility of a bacterial contamination from the use of metal wires (though they were handled sterile), we had thr ee sterile control plat es with nutrition agar ; two between the metal plate s and one under the term inal. During the experiment (6 h) the air temperature in the room was 14-18°C, and the indoo r air humidity was 16-20070.
The number of revertants per plate were cou nted after the plates had been incub ated at 37°C for 48 h. The mean number of revertants for the exposed and control plates were estimated separately for each of the two experiments. A doubling of the numb er of spontaneous revertants was con sidered to show a mutagenic effect, according to Ames et al (I).

Results
The result s are shown in ta ble I . Non e of the th ree sterile plat es showed more than 10 revertant s in the two experiments, and thu s we excluded them from the counting. The exposed plates showed fewer revertants than the controls. Thu s, we did not find any significant mutagenic effect in this test system, either from the electrostatic field strength or fro m the video display terminal without an electrostatic field strength. , The ratio exposed : cont rols was less tha n 1 fo r all the experimental s itua tion s.

Discussion
In this study it has been shown tha t no mut agenic effect s, as detect ed by the Ames test, occur in front of a video displa y terminal or in exposur e to electrostatic field strengths. The distance from the terminal was very sho rt (62 0101), and the field strengths very high (250 kV1m ), as compare d to tho se of a common work situation at a terminal. The normal work distance at a term inal is in the range of 0.5-0.8 01 from the screen. On a very cold and dry winter day the electro static field str ength observed at this distance is no more than 20 kV1m (2).
The absence o f any mut agenic effects, as detected by the Ames test system, does not support the claims that work with video display terminals is associat ed with risks of skin cancer. Ho wever. this study does not pro vide information abou t othe r kinds of side effects, ie, eye and skin symptoms and possible teratogenic effect s. The Ames test also covers only one type of genotoxic effect.
Th e Ames test is a biological test which has been constru cted for the rapid detection of the possibility of mutagenic effects and is used, fo r example, for testing chemicals, foodstuffs, and ph ysical effects (6,9,12). Thus the Ames test would reasonabl y respond to exposure in a time span (here 6 h) which is very short in comp ar ison to VDT oper ators' accumulated worktime at a terminal. We believe that th is mutagenicity study supports the view that there is no increased risk of skin cancer associated with work at video display terminal s.