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Air quality is a growing concern worldwide because of its impacts on both the environment and the human health. The road transport sector is a major contributor to this poor air quality. To reduce the emission of particulate matter, all diesel passenger cars were equipped with diesel particulate filters since the EURO5b emission standard. Unfortunately, these filters can be damaged or intentionally removed during the lifetime of a vehicle. This work presents the particle number emission factors for EURO5 and EURO6 diesel passenger cars, based on the measurements of 757 vehicles. These measurements were performed at low idle, which shows a high correlation to particle number emission factors obtained during homologation cycles or real-driving emission measurements. The results show that the average Particle Number (PN) emission factors are highly impacted by high emitters present in the fleet and that the mileage has a significant impact on the PN emission factors. Finally, the estimated PN emission factors based on low idle measurements were higher by a factor 5.6 for EURO5a, 2.5 for EURO5b and 5.5 for EURO6, compared to their respective HBEFA (Handbook Emission Factors for Road, Transport) emission factors.