Background As a group of environmental pollutants, polycyclic aromatic hydrocarbons (PAHs) are neurotoxic and may cause mild cognitive impairment (MCI) by inducing inflammation. Whether neutrophil-lymphocyte ratio (NLR), an inflammatory indicator, plays a mediating role in the relationship between PAHs exposure and MCI is unclear yet.

Objective To investigate a potential mediating role of NLR in the association between exposure to PAHs and MCI in coke oven plant workers.

Methods Eleven urine hydroxylated PAHs (OH-PAHs) of 530 coke oven plant workers were determined by high performance liquid chromatography tandem mass spectrometry. NLR was derived from participants' routine blood examination results using a fully automated haematology analyser. The associations between urinary OH-PAHs and MCI were analyzed by binary logistic regression, the associations between urinary OH-PAHs and NLR were analyzed by multiple linear regression, and the role of NLR in the relationship between urinary OH-PAHs and MCI was evaluated by mediating effect analysis.

Results After controlling for confounding factors and other OH-PAHs, the results of binary logistic regression showed that for every e-fold (e is the base of the natural logarithm) increase in the concentration of 2-hydroxynaphthalene (2-OHNap) and 1-hydroxyphenanthrene (1-OHPhe), the OR (95%CI) values of reporting MCI positive were 1.21 (1.02, 1.43) and 1.25 (1.04, 1.51) respectively. For each unit increase of NLR, the OR (95%CI) of reporting MCI positive was 1.56 (1.12, 2.18). The results of multiple linear regression showed that each unit increase in natural log-transformed levels of 1-OHPhe was associated with 0.05 (95%CI: 0.01, 0.10) increase of NLR. The results of mediating effect analysis showed that the association between urinary 1-OHPhe and MCI was partially mediated by peripheral blood NLR, with a mediation ratio of 9.8%.

Conclusion Exposure to PAHs in coke oven plant workers may increase the risk of reporting MCI positive partially through increased NLR in peripheral blood.