Human exposure to vehicle exhaust during engine start-up can be encountered on a daily basis in parking lots, home garages, and vehicle stop/star traffic environment. This work is the first pilot study to characterize solid particle number and size distribution during engine start-up using various light-duty vehicles with different technology engines. A total of 84 vehicles were tested in this pilot study, consisting of post-2007 diesel engines equipped with high efficiency diesel particulate filters (DPFs) as well as modern gasoline port fuel injected (PFI) and gasoline direct injected (GDI) engines equipped with three-way-catalysts (TWCs). Particle concentration from DPF equipped diesel engines were found to be the lowest, while GDI and 8-cylinder PFI engines had the highest particle emissions. The average solid exhaust particle concentration observed with GDI engines during engine start-up was 12 × 106 part/cm3, this is a factor of more than 4000 higher than the ambient background concentration. A PN emissions index concept was developed to rank post 2010 model-year vehicles tested relative to a 4-cylinder diesel with DPF, used as the best available technology for low solid particle emissions. GDI engines had the highest PN indices, with up to a factor of 8000 higher for particles larger than 25 nm (Dp > 25 nm) and up to 900 times higher for particles smaller than 25 nm (DP < 25 nm).This study presents the differences seen in solid particle number emissions at engine start-up among the modern vehicle fleet. While all vehicles tested are expected to meet the respective emissions standard in the engine laboratory, there seems to be a drastic difference in their particle emissions performance in the real world under engine start-up. Lowering the PN emissions index from gasoline engines during engine start-up in future vehicles could be beneficial to the environment and the public as a whole. The priority should be on GDI engines followed by 8-cylinder PFI engines. High efficiency particle filters in engine exhaust such as diesel with DPF provides the best available technology for solid particle reduction, as shown in this work.