Electrostatic charge and lung deposition

The mechanical model is based on Weibel’s (1963) symmetric adult lung model. The first four branches of the bronchial tree are made from metal piping at the correct size and branching angle. After the fourth generation, which has 128 tubes, this technique is no longer a viable engineering option and the lower generations are represented by wire meshes. A wire cloth made with 0.03 mm diameter wire and an aperture mesh size of 0.036 mm represents the alveolar region. These are the same dimensions as those used by Hopke et al. (1990) in their experiments for radon daughter deposition. Three other wire cloths with different mesh sizes are used to represent the small bronchioles. Sample air is drawn either through the lung or through a control tube into a Grimm 5.400 SMPS-C (Sequential Mobility Particle Sizer – Classifier). The size measurement capability of this instrument ranges from 10 - 1100 nm. Total flow through the lung is 16 l min-1 corresponding to the normal breathing rate at rest. Several runs were averaged during experiments to reduce noise in the data due to natural variability of aerosol concentration.

The ioniser experiments were carried out using a commercially available corona discharge ioniser. Ambient air was used as the control and after sufficient data was collected the ioniser was switched on and left running throughout the experiment. The ion density was approximately 5 x 1010 per m3 as measured by a hand held ion counter. Field measurements were taken concurrently downwind and upwind of high voltage powerlines in a variety of weather conditions, in rural environments.

Theoretical calculations of laminar flow diffusional, electrostatic, inertial and gravitational deposition in cylindrical tubes representing the first six generations within the lung were undertaken, based on the equations of Yu and Diu (1983), in order to determine the enhancement effect of image charge on inhaled aerosols carrying different charge distributions. The tube geometry and air flow rates were chosen to allow comparison of the present calculations with the experimental data of Cohen et al. (1998). Charge distributions on aerosols of size 1 - 1000 nm corresponding to experimental measurements made downwind of powerlines and during the ioniser experiments in the laboratory, in addition to those calculated from arbitrary concentrations of positive and negative ions in air (Dr A. P. Fews, unpublished) were examined

Results and Discussion

In the larger bin sizes the number of particles counted are few and hence the data become less reliable. Figure 1 compares the ICRP 66 lung model (1994) with laboratory measurements in ambient air taken over a period of about a day. There is reasonable agreement with the ICRP model, although the smaller particles are not deposited as efficiently as in the ICRP model. This may be due to the fact that the mechanical lung model only has one way air flow.

Graph showing depoition in model lung compared with ICRP model
Figure 2: Comparision of deposition for ambient aerosols and charged aerosols

Measurements taken downwind of powerlines show a small increase in deposition compared with upwind results in six out of seven measurements (table 1). In the seventh reading the increase may be due to a main road that was close to the upwind site. These readings are provisional and show only a small increase which is not statistically significant. Further readings need to be taken using the SMPS-C over a longer time scale.

Table 1: Results of lung deposition downwind and upwind of various powerlines
Date Location Corona Polarity Precipitation % Deposition Downwind % Deposition Upwind % Difference
18/11/04 Latteridge -ve Moderate rain 39.0 34.0 5.0
02/12/04 Lower Godney +ve Dry 31.0 25.2 5.8
13/01/05 Little Sodbury end +ve Dry 28.8 23.2 5.6
13/01/05 Little Sodbury end +ve Dry 26.0 21.6 4.4
27/01/05 Lower Godney +ve Dry 29.1 26.2 2.9
27/01/05 Lower Godney +ve Dry 50.7 49.7 1.0
02/11/04 Little Sodbury +ve/-ve Fine Drizzle 12.3 15.6 -3.3

Figure 3 shows total deposition from the theoretical calculations made for different aerosol charge states. It is clear that image charge effects can significantly enhance the amount of deposition in the upper generations of the lung even for a modest ion charge imbalance, although the absolute deposition efficiency is still low. An ion imbalance of at least 500 cm-3 is typically seen downwind of a powerline producing corona discharge and the maximum concentration observed in field measurements was 6000 cm-3 (Fews et al., 2002).

Graph showing theoretical deposition calculations for aerosol charge states
Figure 3: Theoretical deposition calculations for different aerosol charge states

Table 2 shows ratios of deposition efficiencies for the charge states and particle sizes examined by Cohen et al. (1998). The present calculations find a smaller increase in deposition for more highly charged particles than was observed in Cohen et al., but corroborate their findings qualitatively. It is hoped to extend this work to examine the effect at deeper regions in the lung.

Table 2: Ratio of deposition efficiencies for charged/uncharged paticles - Comparison between deposition calculations and data from Cohen et al. (1998)
THIS WORK Singly charged/charge neutralised Singly charged/zero charged Charged neutralised/zero charge
20 nm 1.25 1.35 1.08
125 nm 1.14 1.74 1.53

COHEN et al. Singly charged/charge neutralised Singly charged/zero charged Charged neutralised/zero charge
20 nm 3.4 5.3 1.6
125 nm 2.3 6.2 2.7

The present work provides evidence for greater deposition in the lung due to increased charge on the particles. Further work is required to show that this effect can be observed in field measurements downwind of powerlines.