Experimental studies on dust in the London Underground with special reference to the effects of iron on the toxicity of quartz

Introduction: Airborne dust in the London Underground railway system can contain significant quantities of quartz (a type of silica), which is known to cause silicosis (scarring of the lungs). However, previous investigations of dust in the London Underground have suggested that the quartz content may be relatively non-toxic because it also contains iron. This hypothesis led to the commissioning of the study to examine (a) the effects on the lung of fine dust collected from the air of an underground station and (b) the extent to which iron may modify the harmful effects of quartz on the lung.The Dusts Studied: In this study the test dusts included a sample of fine dust collected from the tunnel air of a selected London Underground station. The particles were small enough to be breathed into the deep parts of the lung. Analysis showed that most particles contained iron, and that many of the quartz particles were contaminated with iron. Comparisons were made with prepared dusts containing pure quartz to which iron was added in varying proportions. For some experiments quartz particles were also coated with iron using solutions of iron salts.The Biological Tests: The possible damaging effects of these dusts were investigated in several ways. They included: (1) injecting a dose of dust into the windpipe of rats, then measuring various responses of the lung itself and of the scavenger cells and other cells found within the lung which cause inflammation; (2) examining the effects of iron coating of quartz particles on their armfulness.Results and Conclusions: From the results of these investigations we have concluded that the London Underground tunnel dust was considerably less harmful to the lung than pure quartz, at doses equivalent to the quartz content of tunnel dust. This amelioration of the effects of quartz is likely to be due to the presence of iron in the dust, as shown by the modifying effects of iron on the activity of pure quartz in quartz/iron artificial mixtures. Tunnel dust, and also mixtures of quartz with particulate iron, have less of an effect than quartz on the release from the lung scavenger cells (macrophages) of tumour necrosis factor, a substance shown to be important in the development of inflammation and fibrosis.However, tunnel dust was not completely harmless, as higher doses caused some inflammation, aggregations of dust and dust-filled scavenger cells in deep lung air spaces, and some thickening or breakdown of the walls of these air spaces. Tunnel dust was also more harmful than pure quartz to isolated lung scavenger cells in doses equivalent to the quartz content of the tunnel dust, although this effect may simply be the result of larger numbers of particles. It should also be noted that the effects of the tunnel dust were only studied over 60 days, and it is not known whether prolonged residence in the lung causes the quartz particles to lose their associated, protective iron coating.The implications for health risks are that airborne tunnel dust appears to be considerably less toxic than the quartz content would suggest, but is not completely without harmful effects in man if persistent exposure to high dust concentrations were to occur. We have only studied one selected sample of dust and thus possible differences in the compositions of other tunnel dusts should be taken into account before extrapolating the results to the London Underground system as a whole.