Background Styrene is an organic solvent and a well-documented neurotoxin with effects on reaction time, colour vision, and hearing thresholds. Mixed solvent exposure has been associated with encephalopathy. We investigated the long-term risk of encephalopathy and other non-vascular dementia in styrene exposed reinforced plastics workers.

Methods We followed 77,271 workers of 456 Danish reinforced plastics companies 1968–2011 and investigated the exposure response relation between cumulated styrene exposure and hospitalisation-defined cases of encephalopathy (n = 258), unspecified dementia (n = 689) and Alzheimer’s disease (n = 236) as defined by ICD-8 and ICD-10 codes. Individual styrene exposure levels were modelled from information on occupation, measurements of work place styrene levels, product, work tasks, and year of employment. Data were analysed with a discrete survival function. Adjusted models included age, sex, calendar year, and level of education.

Results We observed positive exposure response relations for Alzheimer’s disease and unspecified dementia in the crude but not in the adjusted analyses. For encephalopathy, however, a statistically significant exposure response relation (p < 0.01) was observed in the crude as well as the adjusted analyses, the latter showing an adjusted rate ratio of 2.18 (95% CI: 1.38–3.46) for the highest exposure quintile (>170 mg/m3-years). A combined analysis of encephalopathy and unspecified dementia showed no exposure response relation.

Conclusion The strong association observed for encephalopathy is supportive of a causal association as indicated for other solvents. However, patients with cognitive deficits may be more likely to have a diagnosis of toxic encephalopathy – instead of unspecified dementia that they qualify for - if they have a longer and thus higher cumulated styrene exposure. The findings may thus reflect the physicians’ diagnostic practices as well as causal effects. The no effect seen in the combined analysis of encephalopathy and unspecified dementia support the former interpretation.