Correct sampling of airborne biological particles is the most critical step in aerobiological studies. It is common not to take account of the difference between the true atmospheric concentration, and the apparent concentration, as measured with rotating‐arm impactors. The difference can be quantified by the sampling efficiency (E5). I demonstrate that such differences can lead to substantial errors in many fields of aerobiology. Because of the continuing mis‐use of such aerobiological samplers I present a review of concepts and an examination of previous experimental and theoretical attempts to ascertainEs.I take the example of the widely‐used rotorod sampler.