Seasonal Decline in Lung Function Among Residents of Northern Thailand: A Longitudinal Study Before and After the...

Background: Seasonal haze from biomass burning in northern Thailand frequently elevates PM_2.5 above national and WHO standards, while evidence on short-term lung function changes in exposed populations remains limited. Method: This longitudinal study evaluated changes in lung function in 130 adults before and after the haze season from December 2023 to May 2024 in San Pa Thong District, Chiang Mai Province. Spirometry was conducted following ATS/ERS standards. Demographic and health-risk data were gathered using validated Thai questionnaires addressing diabetes, cardiovascular, and chronic obstructive pulmonary disease risk categories. Results: A significant reduction in the FEV₁/FVC ratio was observed, from 85.3% to 82.4% (p = 0.001) after the haze period, while the proportion of participants with FEV₁/FVC < 70% increased from 2.3% to 6.9% (p = 0.031). No statistically significant independent predictors were identified in multivariable analysis. Greater reductions were observed in older adults, low-income individuals, and those at moderate to high diabetes risk. Conclusions: These findings suggest modest changes in spirometry parameters during the seasonal haze period. The observed reduction in FEV₁/FVC should be interpreted cautiously and may reflect early functional variation rather than confirmed airway obstruction. Although the magnitude of change was relatively small, the findings highlight the potential value of ongoing respiratory monitoring and early detection strategies in haze-affected regions.