Subclinical impairment of colour vision among workers exposed to styrene.

Sir,-Does chronic exposure to styrene impair colour vision? Fallas et al (1992;49:679-82) found subclinical impairment of colour vision among workers exposed to styrene applying the Farnsworth 100 hue test during working hours in daylight. In daylight, however, neither colour temperature nor illumination are constant and good results depend on the use of standard lighting conditions,2 for instance standard illuminant C or D65.3 The authors do not state if those wearing glasses used their own. In our experience most glasses are coloured at least slightly and it is imperative that colour testing should not be performed on subjects wearing coloured glasses or coloured contact lenses.4 In such cases we use clear glasses with different refraction taking into account the fact that we cannot correct astigmatism. The Farnsworth 100 hue test was designed to test hue discrimination among subjects with normal colour vision and to evaluate chromatic discrimination loss in those with congenital defects of colour vision.3 Subsequently it was applied to test acquired defects. The prevalence of congenital dyschromatopsia is about 8% among men.5 Fallas et al apparently did not distinguish between those with congenital and acquired colour vision defects when calculating the error scores and the ranges. We guess that the results are influenced by congenital defects in colour vision. Furthermore the term "range" was not defined by the authors. Acquired dyschromatopsias can be caused by many systemic and ocular diseases. Therefore a complete ophthalmological examination is desirable, but probably not feasible in many epidemiological studies. For screening at least the visus should be examined, however. The mean of the error score of the controls given by the authors is high compared with data published by others5 6; we think that this discrepancy could be caused by extraprofessional and by congenital dyschromatopsias. The subjects were examined during the shift so that they were actually exposed to styrene before testing. Ethanol, another organic solvent, is known to cause an acute and transient impairment of colour vision.78 To our knowledge, comparable studies on the effect of styrene have not been published. It is an obvious supposition that styrene can cause an acute and transient impairment of colour vision, too, if there are effects caused by a chronic exposure.9 If the colour vision is examined during a shift, it is impossible to differentiate between acute and chronic effects. In conclusion we think that the paper does not give any evidence of an impairment of colour vision caused by styrene. AXEL MUTTRAY DETLEV JUNG JOHANNES KONIETZKO InstitutfiirArbeitsund Sozialmedizin der _Johannes Gutenberg-Uniersitdt Mainz, Obere Zahlbacher Stra,Be 67, 6500 Mainz, Germany