Hello Earl, 

I am sending you the information as it is in the Respiratory therapy book (EGAN). This information will support the theory I have given about how the nose hairs filters out the bigger particles from reaching the lung tissue and being more therapeutic using a nebulizer.  I am going to type it directly from the EGAN (the respiratory therapist bible) so that you can understand better. I hope this helps. 

EGAN’S Fundamentals of Respiratory Care—Seventh edition—copyright 1999 by Mosby, INC. Page 158 (nasal cavity) and pages 684-685 (Particle Size)

 The Nose

The defense function of the nose involves several mechanisms. Hairs in the vestibules provide gross filtration. Filtration is augmented by the flow pattern through the nasal cavity. Inspired gas is accelerated to a high velocity through the anterior nares. It then changes direction sharply as it enters the internal nasal cavity. The pattern causes particle larger than a few um to impact on the nasal mucosa. These particles are cleared by ciliary action or nose blowing. Beyond the external nares, the cross-sectional area increases. This results in a decrease in gas velocity. Low velocity and turbulence combine to remove any remaining particles. Filtration is based on impaction, sedimentation, and diffusion. 

Particle Size

Aerosol particle size depends on the substance being nebulized, the nebulizer chosen, and the method used to generate the aerosol. Because clinical aerosols always contain many different size particles, we express the average size using a measure of central tendency called the mass median aerodynamic diameter (MMAD). For given aerosol distribution, the MMAD describes the particle diameter (in micrometers {um}), which corresponds to the most typical settling behavior. For example, in an aerosol distribution with an MMAD of 5um, 50% of the particles will be smaller, have less mass, and settle more slowly, while 50% will be larger, have greater mass and settle more quickly.

            The unaided human eye cannot see particles less then 50 to 100um in diameter (equivalent to a median sized grain of sand). Thus, you cannot visually determine whether a nebulizer is producing an optimal particle size. The only reliable way to determine the characteristics of an aerosol suspension is by laboratory measurement. The two most common laboratory methods used to measure MMAD are staged impaction and laser refraction. 

Deposition

Aerosol deposition occurs whenever aerosol particles make contact with the respiratory tract mucosa and are retained. Whether aerosol particles can enter the respiratory tract and where they deposit depends on their size, shape, and motion, as well as the physical characteristics of the airways. Key mechanisms causing aerosol deposition include sedimentation and diffusion. 

Sedimentation

Sedimentation occurs when aerosol particles settle out of suspension due to gravity. The greater the mass of the particle, the faster it settles. Sedimentation is the primary mechanism for deposition of particles in the 1 to 5 um range, occurring mostly in the central airways. Sedimentation increases with time, affecting particles down to 1 um in diameter.  

Diffusion

Brownian diffusion is the primary mechanism for deposition of small particles (<3um), mainly in the alveolar region (deep part of the lung). In this area, bulk gas flow ceases and aerosol particle inertia is very low. Here, most remaining aerosol particles have very low mass and are easily bounced around by collision with carrier gas molecules. These random molecular collisions cause some particles to contact and deposit on surrounding surfaces. Particles smaller than 1 um are so stable that most remain in suspension and are cleared with the exhaled gas. 

So what I am getting at Earl, is the RULE OF THUMB, 

A particle’s depth of penetration into the respiratory tract varies inversely with its size. Therefore, it is possible to target aerosols/oils for delivery to the respiratory tract by using nebulizers that produce particles in the size range where most deposition will occur: 

DESIRED LOCATION                                                           RECOMMENDED MMAD

Upper airway: nose, larynx, trachea                                                5 to 20 um

Lower airways                                                                                   2 to 5 um         

Parenchyma: alveolar region      (deep part of the lung)               1 to 3 um

I hope this helps. I tried to make it as simple as possible.

Jennifer Stephens LRRT        (Licensed Registered Respiratory Therapist)