Hidden hazards and dangers associated with the use of HME/filters in breathing circuits. Their effect on toxic metabolite production, pulse oximetry and airway resistance.

Current airway dust ®lter technology emanates from the USA mining industry during the 1930s. It was further developed and improved by the USA military during World War II to prevent workers from inhaling very ®ne radioactive particles in the nuclear industry. These later ®lters we now recognize as high ef®ciency particulate air ®lters (HEPA). The ef®ciency of ®lters varies according to the number and size of particles they capture. Filters may be classi®ed commonly as 95, 99.95, and 99.97% ef®cient (95, 99 and 100, respectively). The higher the ef®ciency the higher the classi®cation number used. The ef®ciency of ®lters may also be affected by volatile chemicals which may be inhaled with the particles. Industrial ®lters which are categorized by the above system may also have a pre®x of N, R, or P; N for Not resistant to oil, R for Resistant to oil, and P for oil Proof; thus, a ®lter would be classi®ed as for example a P100. Requirements for military and environmental biohazard ®lters are based on these classi®cations and surprisingly may only achieve 95% ef®ciency. A generally perceived increased threat from biological hazards that may be inhaled, including bioterrorism threats, especially in hospital environments, has increased interest in respiratory ®lters. Protecting personnel from inhaled biological hazards requires a ®lter ef®ciency of several orders of magnitude greater than the industrial dust ®lters used in respirators alluded to above. As few as 10 inhaled smallpox viruses may be suf®cient to infect patients with the disease. This has resulted in the sub-classi®cation of HEPA ®lters into true HEPA ®lters and HEPA type ®lters. A further sub group has also developed. ULPA ®lters (Ultra Low Penetration Air) and `absolute' ®lters are designed for industrial applications such as microelectronic clean rooms, but have too high a resistance for medical breathing apparatus. All HEPA ®lters are manufactured from glass ®bre materials supported on a rigid frame. In order to reduce resistance to air ̄ow and increase ef®ciency, the surface area is increased by pleating. Filtration is achieved for larger particles (>0.3 m) by inertial impaction and interception; smaller particles are captured by Brownian diffusion. The size of particles used to test ®lters is measured in microns. Microns are units used for particles that can be seen with a light microscope. A micron is a thousandth of a millimeter, which is in turn a thousandth of a meter (1 m=1000 nm or 0.001 mm). The variation in ®ltration ef®ciency is tested by the British BS3928 Sodium Flame method and the USA Hot DOP method. The most dif®cult particle size to capture by ®ltration is a particle of 0.3 m as at this size the effects of inertial impaction, interception and Brownian motion are least effective. Particles of 0.3 m are also most likely to be deposited in the lungs if inhaled. The di-octyl-phthalate (DOP) test, used for testing the ef®ciency of ®lters, takes advantage of the properties of DOP. In particulate form, DOP has a constant mean diameter of 0.3 m. By passing DOP through the ®lter, the capturing ef®ciency can be classi®ed. Bacterial size generally equates to 0.3 m; virus sizes are considerably smaller (Table 1). Wilkes has recently used the sodium chloride test for measuring ®lter ef®ciency on 33 breathing system ®lters (nine pleated hydrophobic and 24 electrostatic ®lters). The particles in this test are considerably smaller than those of the DOP test and have a size distribution with a median diameter of 0.07 m and a geometric standard deviation not exceeding 1.83. REVIEW ARTICLES British Journal of Anaesthesia 91 (2): 249±64 (2003) DOI: 10.1093/bja/aeg154