Current legislative control of particulate matter (PM) based on ambient mass concentrations does not encompass source dependent variation in PM toxicity or the impact of high numbers of nanoparticles (ultrafine particles; UFPs) which contribute little to PM mass. UFPs may pose high risks due to their small size and high number/surface area concentrations and higher propensity to penetrate tissue barriers and reach the circulation and secondary target organs. Non-regulated nanoparticle emissions from transport sources belong to this category of high concern, and the effects of some specific emissions (e.g. particles from wear components or natural gas and jet engines) are either not sufficiently understood or remain undetected by current air quality or certification procedures. ULTRHAS will specifically address (i) the impact of different transport modes, fuel technologies and wear components, including atmospheric ageing processes, on the physicochemical characteristics of particulate and gaseous emissions; and how these processes and characteristics affects (ii) the biological responses leading to harmful effects in the lung and beyond by applying state-of-the-art emission measurement, exposure and toxicity-testing approaches under highly controlled laboratory conditions. ULTRHAS will (iii) rank the health hazards of different transport mode emissions and (iv) apply an advanced health impact assessment framework incorporating burden of disease methods to quantify baseline and policy scenario impacts for development and prioritization of mitigation measures; and (v) evaluate the future impact and acceptance of new policies on public health, taking into account social aspects. ULTRHAS will provide enhanced understanding of health threats posed by nanoparticles from different transport modes; optimize a framework for toxicity and hazard assessment of aerosols; provide guidance for prioritization of mitigation measures for future legislation on air pollutants.