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Abstract Introduction The association between ambient air pollution and length of hospital stay (LOS) for children with lower-respiratory infection is unclear. Materials and methods The association was examined using admission data for 75,432 children in Hanoi, aged 0–5 years, hospitalized with acute lower-respiratory infection (ALRI) and daily air pollutant levels in Hanoi between 2007 and 2016. We estimated the odds ratio of being discharged for an interquartile range (IQR) increment (IQR) of ambient pollutants during one to four days prior to admission date. We used pooled logistic regression models including a B-spline function of calendar time and terms describing the baseline odds, adjusting for meteorological factors, age, gender, residential location, week-day of admission, discharge on weekends or holidays, and number of respiratory hospitalizations one day prior to the discharge date. Results An IQR increase in O 3 concentrations (85.8 μg/m3) was associated with a 5% (95%CI: 2%–8%) decrease in the odds of discharge from hospital among children with ALRI. Results were similar for PM 10 , but only for children aged 2–5 years. For the other pollutants, corresponding associations were close to null or in the opposite direction. For example, increase in CO was associated with shorter LOS. Conclusions Our study provides the first investigation of the associations between lengths of hospital stays and air pollutant levels prior to hospitalization in Asian children. Increased levels of O 3 prior to admission predicted prolonged hospitalizations. The inconsistencies in the findings for other pollutants call for further investigations. Graphical abstract Image 1 Highlights • Acute effects of ambient air pollutants on prolonged hospitalization were examined. • Data from 75,432 hospital admissions were analyzed using pooled logistic regressions. • O 3 was associated with prolonged hospitalizations amongst children age 0–5 years. • PM 10 were associated with prolonged hospitalizations in children aged 2–5 years. • Further studies are needed to scrutinize the inconsistent findings of SO 2 and NO 2. [ABSTRACT FROM AUTHOR]

Background Lower respiratory diseases are the most frequent causes of hospital admission in children worldwide, particularly in developing countries. Daily levels of air pollution are associated with lower respiratory diseases, as documented in many time–series studies. However, investigations in low-and-middle-income countries, such as Vietnam, remain sparse. Objective This study investigated the short-term association of ambient air pollution with daily counts of hospital admissions due to pneumonia, bronchitis and asthma among children aged 0–17 in Hanoi, Vietnam. We explored the impact of age, gender and season on these associations. Methods Daily ambient air pollution concentrations and hospital admission counts were extracted from electronic databases received from authorities in Hanoi for the years 2007–2014. The associations between outdoor air pollution levels and hospital admissions were estimated for time lags of zero up to seven days using Quasi-Poisson regression models, adjusted for seasonal variations, meteorological variables, holidays, influenza epidemics and day of week. Results All ambient air pollutants were positively associated with pneumonia hospitalizations. Significant associations were found for most pollutants except for ozone and sulfur dioxide in children aged 0–17. Increments of an interquartile range (21.9 μg/m 3 ) in the 7-day-average level of NO 2 were associated with a 6.1% (95%CI 2.5% to 9.8%) increase in pneumonia hospitalizations. These associations remained stable in two-pollutant models. All pollutants other than CO were positively associated with hospitalizations for bronchitis and asthma. Associations were stronger in infants than in children aged 1–5. Conclusion Strong associations between hospital admissions for lower respiratory infections and daily levels of air pollution confirm the need to adopt sustainable clean air policies in Vietnam to protect children's health. [ABSTRACT FROM AUTHOR]