Background: Exposure to ambient PM 2.5 is known to affect lipid metabolism through systemic inflammation and oxidative stress. Evidence from developing countries, such as India with high levels of ambient PM 2.5 and distinct lipid profiles, is sparse. Methods: Longitudinal nonlinear mixed-effects analysis was conducted on >10,000 participants of Centre for cArdiometabolic Risk Reduction in South Asia (CARRS) cohort in Chennai and Delhi, India. We examined associations between 1-month and 1-year average ambient PM 2.5 exposure derived from the spatiotemporal model and lipid levels (total cholesterol TC, triglycerides TRIG, high-density lipoprotein cholesterol HDL-C, and low-density lipoprotein cholesterol LDL-C) measured longitudinally, adjusting for residential and neighborhood-level confounders. Results: The mean annual exposure in Chennai and Delhi was 40 and 102 μg/m 3 respectively. Elevated ambient PM 2.5 levels were associated with an increase in LDL-C and TC at levels up to 100 µg/m 3 in both cities and beyond 125 µg/m 3 in Delhi. TRIG levels in Chennai increased until 40 µg/m 3 for both short- and long-term exposures, then stabilized or declined, while in Delhi, there was a consistent rise with increasing annual exposures. HDL-C showed an increase in both cities against monthly average exposure. HDL-C decreased slightly in Chennai with an increase in long-term exposure, whereas it decreased beyond 130 µg/m 3 in Delhi. Conclusion: These findings demonstrate diverse associations between a wide range of ambient PM 2.5 and lipid levels in an understudied South Asian population. Further research is needed to establish causality and develop targeted interventions to mitigate the impact of air pollution on lipid metabolism and cardiovascular health.