Respirable crystalline silica and feldspar particles airborne and respired in riding arenas: could it be a non-conventional exposure for humans?”

Respirable mineral particles represent a potential health hazard in occupational settings and ambient air. In particular, respirable crystalline silica (i.e. < 4 µm quartz grains and/or other silica polymorphs) are classified as carcinogens. A few studies have been carried out on the potential toxicity of feldspars, but some authors have recently stated that respirable grains of these minerals “may induce cytotoxicity and acute pro-inflammatory responses to a similar or greater extent than quartz” and also that other mineral particles like muscovite may be associated with toxicity and pro-inflammatory effects. Soils used in equine riding arenas, commonly described as riding surfaces, are a mixture composed by several natural rock frequently enriched of organic additives. The most common and abundant minerals composing these soils are quartz and feldspars, but some types of phyllosilicates are also present. During equine activities in the field mineral particles resulting from the continuous crushing of the soil are air dispersed. Therefore, the presence of quartz and feldspars in equine riding surfaces could be a risk factor for the health problems in humans working closely with these animals. Since horses themselves can suffer from pneumoconiosis, they can play the role of sentinel animals. Moreover, the hypothesis that horse riding represents an unconventional occupational exposure to respirable crystalline silica and feldspars is likely. The present study deals with an investigation of inorganic particles present in air and in soils of equine riding arenas, and in equine bronchoalveolar lavage fluid (BALF) samples, distinguishing among RCS species and others. The used techniques, depending on the kind of samples, are: SEM-EDXS, TEM-EDXS, XRPD, and cytopathological analysis. The result highlights the abundant presence of RCS and feldspars in air, soils and biological samples, as well as muscovite and other phyllosilicates are widely present. The present study highlights the problem relating to airborne dispersion of RCS (and feldspars too) in a specific context and a potential increased exposure risk for humans, both equestrian workers and people frequenting riding arenas (respectively professional and recreative contest). In this dynamic context, the horses can be used as a sentinel for human health by a periodic control of the BALF inorganic particles burden.