The size range of airborne particles that is closely related to specific deposition regions in the human respiratory tract and excess lung burden of these deposited particles is associated with disease. Size-selective sampling, therefore, needs to be performed to assess the related health risks. Performance criteria applied to these samplers must be well characterized in order to provide accurate and reliable results. The PM 10 samplers that have been used in place of the total suspended particulate samplers for the collection of ambient air particles are more relevant to potential inhalation hazards. In order to be certified, a PM 10 sampler must meet reliable performance specifications, primarily the aerosol penetration test with liquid and solid particles in a wind tunnel (wind speeds of 2, 8, and 24 km/hr). This testing is intended to assure reasonable accuracy in aerosol measurements. However, the sampler performance under calm air conditions has not been well studied.In the present study, the sampling heads of three devices-the Harvard impactor, the Personal Environmental Monitor (PEM), and the Sierra Andersen model 241 dichotomous sampler PM 10 inlet head-were tested for aerosol separation efficiency. With the consideration of bias and imprecision of the measurements, five specimens of each type of sampler were chosen for performance testing, repeating the tests 5 times for each specimen. An ultrasonic atomizing nozzle was used to nebulize potassium sodium IMPLICATIONS This work demonstrates that the mean square error (MSE) is a good indicator to show the overall performance of size-selective samplers. The MSE approach provides a clear way to differentiate the inaccuracy from imprecision so that a better sampler can be made. The sampling head of the dichotomous sampler, which is the only PM 10 sizeselective sampling inlet head certified by the U.S. Environmental Protection Agency, has a good fit to the PM 10 criterion, while other samplers have sharper separationcurve slopes and exhibit significant particle bounce when challenged with solid particles.