The current national ambient air quality standards for PM_2.5 are based on mass. Although, recent studies indicate that bulk PM_2.5 mass is not the best surrogate of aerosol health effects, yet it continues to remain the regulatory parameter. In recent years, certain aerosol properties such as carbonaceous and metallic content, oxidative potential, and surface area have emerged promising better linkages with the health effects. New instruments and methods have also been developed to measure these properties. However, the epidemiological studies exploring these linkages are sparse and scattered across multiple disciplines. In this interdisciplinary session, we invite the researchers working in atmospheric measurement technology development, atmospheric sciences, aerosol toxicology and environmental epidemiologists to contribute to the discussion on what, why and how to measure the most biologically relevant parameters of ambient air pollutants (i.e. gases and particles) in order to better address the health impact of air pollution.