Aerosol droplet size within

A draft European Nebulizer Standard (30) has been submitted for formal approval to CEN (Comite Europeen de Normalisation European Committee for Standardization). CEN is responsible for European standardization in all fields except Electrotechnical (CENELEC) and Telecommunications (ETSl). Publication of the European Nebulizer Standard is expected during 2001. Included within the European standard are detailed descriptions of two test methods for (1) assessing nebulizer aerosol output inhaled using breath simulation similar to that described previously and (2) assessing nebulized aerosol droplet size using low-flow cascade impaction. Because this standard is expected to be adopted throughout Europe (and possibly more widely), a summary of its contents may be useful to readers. 

Infectious disease can be produced by any pathogen able to be aerosolized and subsequently transported into the respiratory tract at the appropriate concentration. Some common diseases which may result from airborne transmission in indoor environments are listed in Table 5. The rate of infection within any environment is a function of the viability and virulence of the pathogen, its concentration in the inhaled air, and characteristics, such as droplet size, of the carrier aerosol within which it is contained. Some biological agents produce disease at low concentrations, while others must accumulate to a higher level. Furthermore, individual susceptibility to infection depends on a number of factors, such as age and health, as well as concomitant exposure to chemical pollutants. 

The reservoir incorporated within the Circulaire nebulizer system may help increase the inhaled proportion of aerosol from a given fill volume compared to the same driving nebulizer without incorporation of the valved reservoir bag system. Like the Halolite, this implies reconsideration of prescribed doses and volume fills of drug solution to avoid overdosing. The extent to which the droplet size from conventional nebulizers is affected by adaptations in Halolite and Circulaire nebulizer system is not clear. Meaning in vitro measurements (e.g., European standard) characterizing droplet size from either system is not available. 

Since the 1970s laser sizers have offered convenient and rapid estimation of the optical size distributions of nebulized aerosol size distributions. However, just as weight loss as a measure of aerosol output is confounded by evaporation, droplet size distributions are now clearly understood also to be affected by evaporation once the aerosol cloud is mixed with drier ambient air. The methodology adopted within the European standard was inevitably a compromise but may arguably present the most representative clinical compromise. 

Typical mechanisms for aerosol removal from gas streams by filters are diffusion to surfaces, interception and impaction. Very large particles can be removed by gravitational settling. These mechanisms are quite dependent on the particle size and it is usually found that conventional filters have a minimum in filter efficiency for particles in a narrow size range less than 1 im. When the gas is hot relative to the filter, thermophoresis can enhance particle removal. When the aerosol laden gas stream contains elevated concentrations of steam that condenses within the filter, difflisiophoresis will enhance particle removal. These phoretic enhancements of filtration are attractive because filtration efficiencies by these mechanisms are not especially dependent on the aerosol particle size. Washed Venturi scmbbers involve the injection of water droplets into the aerosol laden gas and these water droplets act much like spray water droplets to remove aerosol particles. Electrostatic precipitation is, in principle, a very attractive decontamination process, but it is difficult to assure that the necessary power will be available to operate the precipitators under accident conditions. 

Figure 27.8 shows the relative difference of droplet vaporization times predicted by the aerosol model, compared to the HE limit, for ammonia-water interaction cases with 85% and 30% liquid initially. The vaporization time has been defined here as the time by which 80% of the liquid ammonia within the droplet has been vaporized. In accordance with the earlier results, the differences of the model predictions and the infiuence of droplet ventilation increase with the droplet size. 

Currently, suspensions prepared from micronised active substances are the only marketed dehvery system for nebulisation of poorly water soluble substances such as steroids and cyclosporine [53]. Several problems are inherent in nebulising micro-suspensions and they vary from non-optimised lung deposition for the active substance to heterodispersity of the active substance concentration in the aerosol droplets and poor compatibility with different types of nebulisers, particularly ultrasonic devices. Suspensions may also have poor stability and the two components (solid and liquid) tend to separate with time within the formulation by sedimentation or flocculation, depending on the particle density relative to that of the liquid. Several jet nebulisers can deliver suspensions quite effectively, even independently of the primary particle size [54], but ultrasonic devices may convert primarily the continuous phase into aerosol whereas vibrating mesh inhalers can be blocked by particles being larger than the pore diameter of the membrane. 

---