Association of co-exposure to extreme cold and nitrogen dioxide with psoriasis outpatient visits and systemic inflammation.

BACKGROUND: Psoriasis, a chronic inflammatory skin disorder, is closely linked to environmental triggers. However, the combined effects of extreme cold and nitrogen dioxide (NO2), two common environmental factors, on psoriasis patients remain unclear. This study aims to explore the associations of co-exposures to extreme cold and NO2 with psoriasis outpatient visits and systemic inflammatory markers among patients with complete blood count (CBC). METHODS: Data from patients with psoriasis were collected in a subtropical megacity of China between January 2021 and December 2025. A distributed lag non-linear model (DLNM) was used to evaluate the outpatient visit risk associated with extreme cold, extreme NO2, and their co-exposure, identifying susceptible subgroups. Additive interaction was assessed using an interaction term and the relative excess risk due to interaction (RERI). Furthermore, to evaluate the effect of co-exposure on systemic inflammatory markers, a sub-cohort of patients with CBC records was extracted for exploratory analysis. RESULTS: A total of 63,411 psoriasis outpatient visits and 3,048 patients who underwent CBC tests were recorded during the cold seasons of 2021-2025. Co-exposure to extreme cold and NO2 was associated with an increased risk of psoriasis outpatient visits at Lag14 (RR = 1.017, 95% CI: 1.004-1.030), with a higher point estimate compared to single-factor exposure; however, the interaction test was not significant (RERI = 0.026, 95% CI: -0.055 to 0.108). Stratified analysis revealed that the association between co-exposure and outpatient visits varied by age, with a stronger effect observed in younger age groups (Q-test for interaction, p = 0.004). In exploratory analyses, SII showed a significant global association (p = 0.016) at Lag7 (OR = 1.109, 95% CI: 1.031-1.194), whereas NLR and PLR were not significant. CONCLUSION: Co-exposure to extreme cold and NO2 is associated with an increased risk of psoriasis outpatient visits, particularly in susceptible age groups, although no synergistic effect was detected between the two exposures. Exploratory analyses also suggest a potential association with elevated systemic inflammation, warranting further investigation. These findings provide preliminary evidence for the role of combined environmental exposures in psoriasis and highlight the importance of considering age-specific vulnerability when developing targeted public health interventions for at-risk populations.