Short-term exposure to outdoor nitrogen dioxide and respiratory mortality, with high-risk populations: a nationwide time-stratified case-crossover study

Numerous existing studies reported the negative impacts of outdoor nitrogen dioxide (NO₂) on respiratory mortality. However, the evidence of related high-risk populations was considerably limited, especially associated with ages, causes of death, and district-level characteristics. In addition, most earlier studies were based on monitored areas, thus previous risk estimates of NO₂ could be biased to provide nationwide risk estimates and high-risk populations. Therefore, this study performed a nationwide time-stratified case-crossover study to evaluate the association between short-term ambient NO₂ and respiratory mortality in South Korea (2015-2019). A machine learning-ensemble daily NO₂ prediction model was used to cover unmonitored areas. To examine high-risk populations, we assessed NO₂ risk estimates by age group, sex, cause of mortality, and district-level characteristics. In the total population, NO₂ was weakly associated with increased mortality risk due to respiratory disease (OR [odds ratio]: 1.011, 95% CI [confidence interval]: 0.995-1.027), and the association became evident only in individuals aged 80 y or older (1.022, 1.000-1.044), especially related to pneumonia. Further, in people aged 60-69 years, NO₂ was marginally associated with mortality for chronic lower respiratory diseases. Lower district-level socioeconomic status and medical services were marginally related to higher respiratory mortality risks related to NO₂. The excess respiratory mortality fractions and YLL (year of life lost) attributable to NO₂ were 4.13% and 93,851.63 years, and around 70% of the excess deaths were due to noncompliance with the World Health Organization air quality guidelines (daily average NO₂ > 25 µg/m³). This study provides evidence for high-risk populations and the appropriateness of target-specific action plans against NO₂. In addition, based on the excess death estimates, we suggest stricter NO₂ standards are required.