A critical review of the neurotoxicity of styrene in humans.
Styrene monomer is an aromatic industrial solvent. It is used to make polystyrene, resins, rubber, reinforced material and boats. Humans are exposed to styrene in occupational situations mostly during spraying processes at work, most of which is inhaled. The major neurotoxic effects of styrene as reviewed are prenarcotic effects, electroencephalographic abnormalities, slowing of motor, sensory and distribution nerve conduction velocities that reveal the possibility of polyneuropathy, dysfunction of the autonomic nervous system, slowing of reaction times, and centrally-controlled otoneurotoxicity. In acute exposure situations reversal of adverse effects has been observed; however, the impact of long-term exposure needs further studies. Dopamine depletion has been reported as a neurochemical basis of the neurotoxicity of styrene. Styrene epoxide, a toxic intermediate metabolite, has also been reported to deplete glutathione and cause lipid peroxidation, possibly leading to neuronal membrane damage. Raised concentrations of glial fibrillary acidic protein has been reported as an indicator of neurotoxicity in rats, which may damage the brain through astrogliosis phenomenon. This damage is also correlated to the toxic effects of styrene epoxide. Recently decreased monoamine oxidase type B activity in peripheral blood cells has been investigated as biochemical indicator of neurotoxicity of styrene in workers. There is a need for long-term studies, consideration of confounding factors (smoking, alcohol, diet, drugs, working environment and exposure to other solvents), and the effects on different ethnic and racial groups. More electroencephalographic studies and computer tomographic investigations are desired. Further reduction of the exposure limit to below 50 ppm is recommended.