## 4 Conclusions  
At present, studies on ozone deposition on indoor materials from a non-Western perspective are still limited. In this work, experiments were carried out to study ozone deposition on seven common indoor materials from Chinese markets. We obtained the ozone deposition velocity and VOC emissions from each material when exposed to various ozone concentrations. A simplified model was used to discuss ventilation requirements based on a standard room by considering the indoor concentrations of both ozone and VOCs. The major conclusions are outlined below.  
First, ozone deposition velocities varied between materials, ranging from 0.005 to 0.062 cm-s\({}^{-1}\). In general, engineering wood (i.e., green-core OSB and lauan plywood) and some synthetic fibers, such as flannel, are prone to ozone deposition.  
Second, 15 out of the 24 targeted VOCs were detected after exposing the materials to ozone, and the corresponding area-averaged emission rates were obtained. Overall, the higher the ozone concentration, the more plentiful the VOCs. Among others, formaldehyde, toluene, and xylene were frequently observed, often in high concentrations. The materials that emit and/or produce the most formaldehyde were also the ones most prone to ozone deposition, suggesting that although some of the gas-phase formaldehyde is directly linked to the ingredients of the materials, such as the adhesives used during manufacturing the engineering wood, some could result from ozone-initiated reactions.  
Third, both the deposition velocity and the VOC emissions play a role in affecting the indoor ozone concentration, although the former is more crucial than the latter. The simulations indicate that when the ozone concentration in the outdoor air is not severely high, it is possible to use ventilation to maintain acceptable indoor air quality, considering the concentrations of both ozone and major VOCs.  
The major limitations of this work are summarized as follows. First, although the selected materials were freely-stored in an office/lab environment for enough time before the experimental procedure, only one specimen of each material was subjected to testing. A follow-up study should be done to examine at the changes in ozone deposition and the corresponding VOC emissions, as well as the changes in the physical and chemical properties of free-running materials responding to ozone deposition over the long term. Second, the proposed model to investigate ventilation requirements is based on many assumptions that do not represent applications in practice. For example, the product yield on material surfaces was not measured, and the emissions of VOCs were thus grouped together. In addition, the concept of a standard room is not rigorous, so interpretations should be tentative.  
The following are available online at [[http://www.mdpi.com/2076-3417/10/12/4146/s1](http://www.mdpi.com/2076-3417/10/12/4146/s1)]([http://www.mdpi.com/2076-3417/10/12/4146/s1](http://www.mdpi.com/2076-3417/10/12/4146/s1)), Table S1. Emission factors of detected VOCs from wood-based panels. Table S2. Emission factors of detected VOCs from synthetic fibers.  
Conceptualization: W.Y. Data Curation: W.Y. Formal Analysis: H.W., Z.C. and W.Y. Funding Acquisition: W.Y. Methodology: H.W. and W.Y. Investigation: W.Y., H.W. and Z.C. Resources: W.Y. and X.Z. Supervision: X.Z. Writing--Original Draft: W.Y., H.W. Writing--Review and Editing: W.Y. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the China National Key R&D Program during the 13 th Five-year Plan Period, grant number 2017 YFC0702700.  
The material emissions database by the National Research Council Canada (NRC) is the outcome of two consortium projects: CMEIAQ I and II. The authors would like to thank the members of the consortiums and the technical advisory committees. In addition, the authors would like to express their gratitude toward all the lab staff who assisted in the measurements in this study.  
The authors declare no conflict of interest.