The role of electrostatic charge accumulated by respirable sized allergens with regard to thunderstorm asthma

Asthma is a chronic respiratory condition that renders the sufferer susceptible to attacks of airway inflammation characterised by wheezy breathlessness. Hospital asthma admissions of epidemic proportions can be triggered by numerous events including thunderstorms. Allergens consisting of starch granules with diameters ≤ 1μm have been implicated as the causal agent of these thunderstorm epidemics and are released as pollen grains rupture via osmotic shock upon contact with moisture. The meteorological conditions associated with thunderstorms therefore produce an atmosphere with a sufficiently high concentration of respirable allergens that too great an inhalation challenge is presented to sensitive individuals. The wind and rains required to produce this atmospheric abundance of allergens are, however, also found in association with other phenomena not coincident with a rise in asthma incidence and therefore a more specific thunderstorm related cause has been sought. There has been little systematic investigation into the electrical effects of thunderstorms and in particular the accumulation of charge by atmospheric particulates. The effect of particle charge on deposition within the lungs has therefore been investigated using numerical modelling techniques. Initial simulations were carried out using a one-dimensional implementation of Weibel´s lung model of the complete airway. Applied particulate concentration was estimated based upon the levels of intact pollen available for rupture at the time of previously investigated asthma epidemics. Inhalation flow rate corresponded to 0.5 litres per second. The Rayleigh charging limit for particles with a 1 μm diameter was calculated and deposition simulated using proportions of this unipolar charge level. It was found that increasing charge levels up to the Rayleigh limit affected a shift in peak deposition upward from the alveolar region. Particles of various diameters were then input into the simulation with charge equal to the half their Rayleigh limit. Reducing the particle size from 2 μm to 0.25 μm again affected a shift in peak deposition but this time into the alveolar region. Further simulation was then carried out using a two-dimensional implementation of Weibel´s mode- l of the lower airway only, modified to incorporate the branching structure and alveolated walls found in this region. Multiple breath simulations were performed showing the complex nature of particle residence and deposition within the alveolar region. Deposition efficiencies in the lower airway regions were found to increase with particle charge. This effect was observed to be greater with smaller particle diameters due to their increased mobility. The results of both simple and more complex simulations of the inhalation of particles with diameters appropriate to the respirable allergens thought likely to be the causal agent with regard to thunderstorm asthma have been obtained. An increase in the charge resident on these particles was found to alter the deposition throughout the entire airway and promote proportional deposition efficiency within the alveolar region where provocation of an allergic response is most likely. These results have implications not only for an increased understanding of thunderstorm related asthma epidemics but for the deposition of deliberately introduced particles such as respiratory drugs.