Production of potentially hazardous respirable silica airborne particulate from the burning of sugarcane

In some areas of the world where agricultural burning is practised, the airborne particles produced have been linked to respiratory disease in humans. Here, we investigate the abundance and form of silica (SiO2) minerals found within ash and aerosol produced by the experimental burning of sugarcane. Samples of sugarcane leaf were incinerated over a range of temperatures, time scales and airflow conditions, the latter to investigate the effects of wind and updrafts during natural fires. The silica content of the residual ash (from still air simulations) was measured using an improved wet chemical methodology, described here. This indicated that the release of silica from the plant material into the atmosphere increases with increasing temperature of combustion. Airborne particulate, sampled using air-pump-filter apparatus, was characterised using scanning electron microscopy (SEM) with automated image and elemental analysis. For airborne particulate formed at 1100 °C (with airflow), 17% of the particles are in the respirable size fraction (<4 μm in diameter) and contain silica. From X-ray diffraction (XRD) analysis, a component of this silica is present as the potentially toxic polymorph cristobalite. For the residual ash, samples produced with no additional airflow were found to contain cristobalite, however none could be identified in ash formed with an airflow. It is considered likely that this is due to release of cristobalite to the atmosphere (as sampled on filters). This pilot study shows that potentially toxic particles could be released during sugarcane burning and reinforces the need for further study into the emissions and re-suspension of ash from the burning of biomass.