Most clinicians believe that any device that is marketed as a "bacterial/viral filter" must necessarily be capable of capturing any individual bacteria or viruses that might be suspended within inhaled or exhaled gases. We were surprised to discover that this is, by no means, a justifiable assumption. This article describes testing methods that manufacturers employ to generate the often-misleading efficiency specifications that are claimed for some of these devices. We discuss articles that have documented the presence of airborne pathogens in the effluent of a ventilator circuit, and characterize the attributes that a competent filter must exhibit if it is to succeed in protecting patients and caregivers from incidental exposure to bacteria, viruses, aerosolized drugs, and endotoxins. This article continues with a discussion of the numbers of particles that are commonly produced with commercially available pneumatic nebulizers, the comparative performance characteristics of filters and heat/moisture exchanging filters (HMEFs), and the success or failure of various brands of HMEFs to comply with the guidelines recently developed by the Centers for Disease Control and Prevention for the management of patients who are harboring active tuberculosis. The presentation concludes with a description of the standards that apply to any filter that classifies as a high-efficiency particulate aerosol (HEPA) device, and demonstrates that the performance of filters/HMEFs in common clinical use can range from approximately 1/50th to > 30-fold the efficiency of a HEPA-grade device. Those who frequent the bedside of patients receiving ventilation might unwittingly be placing themselves at considerable risk of exposure to infectious microaerosols, but methods are available to dramatically decrease those risks.