Understanding how ozone behaves indoors is critical for assessing human health risks, as people spend the majority of their time inside. A new study published in Eco-Environment & Health on July 9, 2025, presents the first large-scale machine learning model capable of predicting hourly indoor ozone (O₃) concentrations using easily accessible predictors, including outdoor O₃, meteorological conditions, and window-opening behavior. The research, conducted by scientists from Fudan University and the Chinese Academy of Sciences, addresses a significant gap in exposure assessment, as most epidemiological studies rely solely on outdoor data despite people spending 70%–90% of their time indoors.
Ozone is a key air pollutant formed by chemical reactions between nitrogen oxides and volatile organic compounds under sunlight. In 2021, long-term O₃ exposure contributed to nearly 490,000 deaths worldwide. Traditional mechanistic models require detailed indoor parameters that are difficult to obtain in large-scale studies, while linear regression models struggle with nonlinear environmental relationships. To overcome these limitations, the team built a random forest model trained on over 8,200 hours of indoor O₃ data collected using portable electrochemical sensors in 23 households across 18 Chinese cities. Predictor variables included outdoor O₃ levels from high-resolution random-forest and MERRA-2 datasets, meteorological parameters (temperature, humidity, wind, solar radiation, boundary-layer height, and surface pressure), and window-opening status recorded manually by volunteers.
The researchers compared two models: one excluding and one including window status. Incorporating window behavior raised cross-validation R² from 0.80 to 0.83 and lowered RMSE from 7.89 to 7.21 ppb. The model accurately captured hourly O₃ fluctuations and regional differences, performing better in southern than northern China and in the cold than warm season. Predictor-importance analysis showed surface pressure, temperature, and ambient O₃ as dominant factors, with ventilation emerging as a crucial behavioral determinant. Diurnal comparisons revealed that indoor O₃ concentrations were 40% lower than outdoor levels during the day, underscoring the buffering effect of indoor environments.
"Most exposure studies still rely on outdoor O₃ data, but that's only half the story," said Prof. Xia Meng, senior author of the study. "Our findings show that ventilation behavior—something as simple as whether a window is open or closed—can change exposure dramatically. By integrating such behavioral data with meteorological information through machine learning, we can finally estimate indoor O₃ more precisely at large scales." This research introduces a practical, low-cost strategy for predicting indoor O₃ exposure in real time across large geographic areas. The model can be integrated into health-risk assessments, smart-home monitoring systems, and public-health surveillance platforms, enabling policymakers and scientists to better understand indoor–outdoor exposure differences.
Future work could extend the framework to other pollutants such as fine particulate matter or nitrogen dioxide, incorporate smart sensors for automated window tracking, and expand monitoring to diverse climatic zones. The study, published with DOI: 10.1016/j.eehl.2025.100170, was funded by the National Natural Science Foundation of China. Ultimately, this machine-learning approach bridges environmental modeling with daily life, promoting healthier indoor environments in rapidly urbanizing regions.
