The paper describes methodologies for the particle-size-dependent performance measurement of air cleaners. It discusses the use of laser optical particle counters and condensation nucleus counters with preseparators in evaluating air cleaners. The selection of a challenge aerosol is particularly important; materials include nebulized dissolved solids in water or low-vapor-pressure organics. Another area of increased attention is the generation of ozone by electronic air cleaners. Ozone production rates from a wire-plate-electrode configuration are summarized. Activated carbon adsorbents have been used in commercial air cleaners to remove organic materials. The absorption of low concentrations (below 200 ppb) of materials such as benzene and acetaldehyde obtained in laboratory isotherm experiments is reviewed.