Association between humidity and respiratory health: the 2016-2018 Korea National Health and Nutrition Examination Survey.

Background: Ambient humidity has a significant impact on respiratory health and influences disease and symptoms. However, large-scale studies are required to clarify its specific effects on lung function and respiratory symptoms. We examined the relationship between relative humidity (RH), lung function, and respiratory symptoms using data from the Korea National Health and Nutrition Examination Survey (KNHANES).

Methods: In this cross-sectional study, we analyzed data from KNHANES participants aged ≥ 40 years, collected between 2016 and 2018. Pulmonary function tests (PFTs) and health questionnaires were used to assess lung function and respiratory symptoms. Individual environmental data, including RH, were obtained from the Community Multiscale Air Quality model and linked to the participants' addresses. Short-term (0-14 days), mid-term (30-180 days), and long-term (1-5 years) RH exposures were examined. Linear regression models were used to evaluate the associations between RH and PFTs. Univariate and multivariable logistic regression models were applied to assess the risk of lung function abnormalities and respiratory symptoms.

Results: In total, 10,396 participants were included (mean age: 58.3 years, male: 43.6%). In multiple regression analysis, higher RH was negatively associated with the forced expiratory volume in one second (FEV)/forced vital capacity (FVC) ratio across various time lags, while FVC was positively correlated with long-term RH exposure. In multiple logistic analysis adjusted for clinical and environmental covariates, long-term higher RH exposure was associated with a lower risk of restrictive lung disease (odds ratio [OR] at 4-year moving average [MA]: 0.978, 95% confidence interval [CI]: 0.959-0.997), while mid-term RH exposure decreased the risk of chronic cough (OR at 90-day MA: 0.968, 95% CI: 0.948-0.987) and sputum production (OR at 90-day MA: 0.985, 95% CI: 0.969-1.001).

Conclusions: Higher RH was negatively associated with lung function and increased the risk of obstructive lung disease, whereas mid-term RH exposure reduced the risk of chronic cough and sputum production.