Deposition of NaCl Microaerosols in the Respiratory Tract

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Abstract
The lung deposition characteristics of airborne NaCl micro-crystals were studied on 2 normal subjects breathing 32, 10, or 3 times per minute, the tidal volume being passively adapted to the corresponding frequency, i.e., respectively 350, 850, and 2,200 ml. It is again confirmed that the deposition of NaCl crystals in the lungs taken as a whole and in the parts of the lungs corresponding to the last 50 ml. of the tidal volume is inversely proportional to the breathing frequency. In the deep alveolar air, however, the deposition (as shown in previous papers dealing with water-insoluble particles) appears to always exceed 90% of the number of inhaled particles, whatever the breathing frequency. By comparing the deposition characteristics of particles of different mean sizes, from 0.1[mu] to 0.7 [mu], either soluble in water or insoluble, it may be stated that, unexpectedly, the percent of deposition in the lungs according to the breathing frequency is practically the same for all these particles: ca. 22% - 23% for 32 breaths per minute; 30% - 32% for 10 breaths per minute; and 50% - 60% for 3 breaths per minute. This holds true for the deposition rate in the lung regions corresponding to the last 50 ml of the tidal volume; respectively 37% 40%, 54% - 64% and 81% - 90%. It is also shown that for every breathing frequency, any voluntary increase in the normal tidal volume leads to an increase in the deposition rate in the lungs taken as a whole, while it decreases considerably the percentage of particles deposited in the deep alveolar spaces. Confirming studies dealing with insoluble dust microparticles, it was found that a "minimum size deposition" for NaCl particles exists in the lungs taken as a whole and in the end of the tidal air. For NaCl crystals, it is about O.1 [mu]. For determining the size of the NaCl droplets as found in the lungs (99.6% relative humidity), the size of the NaCl crystals was multiplied 6 by 7, and therefore the "minimum retention size" of the NaCl droplets is ca. 0.[mu], a figure close to that found previously for coal dust but quite different from that found for insoluble submicronic particles (0.2[mu] - 0.3[mu]). As a matter of fact, when multiplying the diameter of every airborne NaCl crystal by 7, it was found that quite fortuitously the size distribution curve of the NaCl particles (now drop-lets) was superposed to that of a coal dust sample previously studied and that the deposition characteristics of the NaCl droplets were similar to those of these coal dust particles. As previously found for coal dust particles (mean size-0.7[mu]) and for water-insoluble submicronic dust particles (mean size-0.2[mu]), the deeper the lung regions where the NaCl droplets are deposited, the smaller their size. However, when the alveolar spaces are overventilated by a very high tidal volume, there is no significant difference in the size of the particles collected in alveolar air and in the air corresponding to the last part of the tidal volume. This is attributed to the fact that in these extreme conditions, the alveolar spaces can be assimilated to simple airways. Because of their hygro-scopicity, NaCl particles measured as crystals after they have been exhaled as droplets from the deep parts of the lungs are smaller than the insoluble dust submicronic particles expired from identical lung regions.