The authors compared four methods of aerosol measurement. Samples from a pilot iron grinding operation and a production foundry operation were analyzed by a scattering aerosol spectrometer (OPC), an aerodynamic particle sizer (APS), a quartz crystal microbalance (QCM) cascade impactor, and a scanning electron microscope. Spherical aerosol particles were of 0.1 to 15 micrometers in diameter. Plots of the number concentration or mass concentration versus aerodynamic diameter, calculated from an empirical equation, showed large differences among the instruments. Samples from a production foundry were similar in particle size to those from the pilot operation. APS method was chosen as the reference. For particle size less than 0.3 micrometers, laser OPC spectrum indicated an increase in the number of particles with decreasing particle size, while SEM showed the opposite trend. The QCM spectrum was of questionable accuracy below 0.5 micrometers. The counting efficiency of APS dropped off below about 1.0 micrometer. The slopes of particle size distribution between 1.0 and 7.0 micrometers obtained on APS, OPC, and QCM were similar, while the SEM slope was almost linear on mass weighted plot. On the same plot QCM gave lower particle mass concentration than APS. At 8.0 micrometers APS showed a dip caused by circuitry. QCM underestimated the aerosol concentration by a factor of about two. The authors conclude that a combination of instruments is to be used to get an estimate of true size distribution.