One limitation of this study is that we could not link the death data to the previous medical records of the decedents, so that we were unable to abstract the pre-existing comorbidities for each subject. Although the subjects died of cardiorespiratory diseases may probably have the pre-existing chronic cardiorespiratory diseases, modification of the specific primary causes of death identified in this study may still not exactly represent the modification of the pre-existing comorbidities.