In 2010 the dispersion models actually worked very well. The concentration measurements that were taken verified the model results. The main problem at the time was that engine and airframe manufactures and ICAO had not developed standards relating specific particle concentrations to likely damage (or not), so the initial reaction was to prohibit flying where there was any non-zero predicted level - hence the large 'prohibited areas'. Hopefully there will have been some research since 2010 to enable more considered decisions to be made if required in 2014?