Background <p>Hypertension, dyslipidemia, and liver diseases often coexist as multimorbidity, yet its assocaition with environmental drivers like nighttime light (NL) and air pollution remain underexplored. This study investigates the independent and joint effects of NL, air pollutants, and PM_2.5 components on multimorbidity risk.</p> Methods <p>Based on data from the China Health and Retirement Longitudinal Study (2015–2020), we analyzed 11,216 adults aged 45 years and older who did not have multiple chronic conditions at baseline. We adjusted the covariates, including age, gender, education level, marital status, smoking status, drinking status, and sleep state. We employed Cox models that incorporated various combinations of these covariates to estimate the hazard ratios (HR). The exposure variables included NL, sulfur dioxide (SO₂), carbon monoxide (CO), ozone (O₃), nitrogen dioxide (NO₂), and particulate matter (PM₁, PM_2.5, PM₁₀), along with components of PM_2.5 (black carbon [BC], organic matter [OM], nitrate [NO₃⁻], sulfate [SO₄^2-], and ammonium [NH₄⁺]). Dose-response relationships and synergistic effects were quantified using restricted cubic spline functions and interaction analyses.</p> Results <p>During a median follow-up period of five years, 1,504 participants (13.4%) developed multiple conditions. A significant dose-response relationship (<i>P</i> &lt; 0.05) was observed between the risks of multiple diseases and NL, as well as several air pollutants, including PM₁, PM_2.5, PM₁₀, NO₂, SO₂, CO and components of PM_2.5 (BC, OM, NO₃⁻, SO₄²⁻, and NH₄⁺), with the exception of O₃. The relative excess risk in interaction (RERI) for NL and PM₁ was 0.24 (95% CI: 0.01–0.46), for PM_2.5 it was 0.26 (95% CI: 0.03–0.48), for O₃ it was 0.35 (95% CI: 0.11–0.58), and for ammonium (NH₄⁺) it was 0.25 (95% CI: 0.03–0.47), indicating synergistic risk. The most pronounced interactions were observed between NL and BC (HR= 1.85, 95% CI: 1.57–2.18) as well as NO₂ (HR = 1.63, 95% CI: 1.39–1.93). Concurrent exposure to high levels of NL and moderate levels of PM₂.₅ increased the risk by 61% (HR = 1.61, 95% CI: 1.29–1.99), while high levels of NL combined with black carbon resulted in an 85% increase in risk (HR = 1.85, 95% CI: 1.57–2.18).</p> Conclusions <p>NL and air pollutants independently and synergistically elevate multimorbidity risk, with PM_2.5 components (e.g., BC, NO₃^-) and O₃ exhibiting pronounced interactions. Policy interventions targeting light pollution reduction and air quality improvement are urgently needed to mitigate environmental health risks.</p>