In its most-recent document summarizing the state of the science related to PM, the National Research Council (NRC) stated that "a finding that the effect of particles depends on the concentration of another pollutant--that is, 'effect modification'--would have implications for setting NAAQS independently for the various criteria pollutants". This dissertation contains a series of manuscripts describing new methods and approaches to evaluate the dynamic entanglements of the composition and toxic effects of gases and particles described by the NRC. The first study develops a new in vitro exposure method that is able to expose cultured human lung cells to the particulate matter (PM) present in air pollution mixtures while they are still in equilibrium with the gases surrounding them. When coupled with a gas-only in vitro exposure method already in use, and the analytical equipment in our laboratory, we have a uniquely suited set of tools to investigate 'entanglement' phenomena. The remaining three manuscripts describe a three-part study designed to demonstrate dynamic entanglements among gaseous organic compounds (VOCs), PM, and their subsequent biological effects. We studied these entanglements in increasingly complex VOC and PM mixtures in urban-like conditions in a large outdoor chamber. First, we demonstrate the existence of 'effect modification' for the case of single gas-phase toxicants and an inherently non-toxic PM (mineral oil aerosol, MOA). In the presence of the single gas-phase toxicant in the dark, the initially non-toxic PM became toxic to lung cells in the PM-only in vitro exposure system. Next, we used sunlight to create secondary gas-phase toxicants from a complex mixture of 54 VOCs similar to those in urban air, with the MOA used in part one. Finally, testing was further applied to systems with primary diesel exhaust (gas and PM) from a single vehicle operated with different types of fuel. In almost every case, we observed increases in cellular toxicity when chamber contents were photochemically aged. These studies prove unambiguously that the toxicity inherent to one phase of an air pollution mixture can affect the toxicity of another - precisely the 'effect modification' described by the NRC.