Mikyoung Jun's research was supported by NSF grant DMS-0906532. Eun Sug Park's research was supported by contract with the Health Effects Institute (HEI), an organization jointly funded by the United States Environmental Protection Agency (EPA) (Assistance Award No. R-82811201) and certain motor vehicle and engine manufacturers. The contents of this article do not necessarily reflect the views of HEI, or its sponsors, nor do they necessarily reflect the views and policies of the EPA or motor vehicle and engine manufacturers. This publication is based in part on work supported by Award No. KUS-C1-016-04, made by King Abdullah University of Science and Technology (KAUST). The authors gratefully acknowledge the help of Ms. Melanie Hotchkiss, Dr. Jim Price, and Dr. Clifford Spiegelman with the acquisition of the 24-hr canister VOC data. The authors also thank the editor, associate editor, and two referees for valuable comments and suggestions that led to substantial improvements in the article.