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Size-selective aerosol samplers are used to assess the health effect, because the retention of deposited particles in the respiratory tract is strongly size dependent. The main objective of this work is to examine the particle loading effects on PM2.5 cyclone samplers.In this work, five PM2.5 cyclones of different body diameters (9.3–35.6 mm), derived from the BGI VSCC, were designed and fabricated to investigate the effects of particle loading. An ultrasonic atomizing nozzle was used to generate micro-meter-sized potassium sodium tartrate (PST) particles and Sodium chloride (NaCl) particles as solid challenge aerosols, and di-ethyl-hexyl-sebacate (DEHS) particles as liquid challenge particles. Aerosol number size distributions and concentrations, both upstream and downstream of the cyclones, were measured using an aerodynamic particle sizer. In addition to the cyclone body diameter, other parameters investigated in this work included: challenge aerosol size distribution, chamber humidity, and the material of the cyclone.The PM2.5 cyclones could be used to sample liquid particles without any bias, because the deposited liquid aerosols dripped down and did not accumulate on the inner wall of the cyclone. However, when challenged with solid particles, the deposited and accumulated aerosols on the wall reduced the aerosol penetration, and changed the curve to be less sharp. The extent of underestimation was affected by many parameters, such as challenge aerosol size distribution, humidity, test agent, and the elastic properties of the cyclone and the test agent. On average, there was an underestimation of 20% of 2.5 µm aerosol penetration when challenged with PST particles, regardless of cyclone body size. This suggested that cyclones might not be ideal for sampling solid particles.Cyclone samplers currently used for size-selective sampling are likely subject to aerosol loading effect, and resulted in underestimation of the PM2.5 measurements. The use of virtual cyclone or wet cyclone might solve parts of the problem.