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\bibitem{deHartog_etal2009} {\textquotedblleft}\textit{Associations between PM$_{2.5}$ and heart rate variability are modified by particle composition and beta-blocker use in patients with coronary heart disease}{\textquotedblright}. de Hartog JJ, Lanki T, Timonen KL, Hoek G, Janssen NAH, Ibald-Mulli A, Peters A, Heinrich J, Tarkainen TH, Van Grieken R, van Wijnen JH, Brunekreef B, Pekkanen J, Environmental health perspectives \textbf{117}, 105 (2009).   http://doi.org/10.1289/EHP.11062

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