Public Health Implications of metals concentrations from prescribed burns: A study adjacent to the Perth Metropolitan Area.

Abstract

Ash is not a homogenous product. It is the solid residue of combustion and contains a complex mixture of chemical products. The ash residue resulting from fires varies and is dependent on its source, and burn characteristics such as the temperature of the fire. Ash comprises particles of carbon, soot and trace elements. Ash presents public health risk to people and communities, through direct and indirect ingestion and inhalation. The health effects of ash exposure are not limited to symptoms affecting the eyes, throat and lungs. They can contribute to chronic disease and increase the risk of cancer. The purpose of this study was to evaluate the public health implications of controlled burns in the Darling Escarpment, adjacent to Perth’s metropolitan area in Western Australia. Concentrations of metals in unburnt vegetative litter and ash (post burn), stratified by size fraction, were determined to assess the potential mobilisation of metals caused by prescribed burns and the potential public health implications associated with burns. These data can also be used in predictive modelling to ascertain the amount of metals likely to be released per hectare when authorities plan future burns in the area. Ash samples were collected immediately following the fire to capture fine material before it was blown away. The ash samples were separated into size fractions to investigate whether there are differences within each sub sample. Australia’s National Environmental Protection Measures were used to establish whether the metal concentrations were above designated thresholds for health and environmental investigation, these being the recognised levels above which metals are deemed to pose a risk to public (or environmental) health. Vegetation and ash samples were collected from three sites immediately after the prescribed burn. Samples were analysed for thirteen (13) metals with nine (9) showing statistically significant increases in concentrations in vegetation as compared to ash. The percentages of metals are higher in ash than in vegetative ground litter. The metals identified are Manganese Vegetation (158.3 ± 89.3 mg/kg), Ash (442.2 ± 462.6 mg/kg); Barium Vegetation (19.4 ± 25.3 mg/kg); Ash (41.8 ± 62.7 mg/kg); Zinc Vegetation (15.3 ± 9.7 mg/kg), Ash (25.6 ± 29.7 mg/kg); Vanadium Vegetation (31.0 mg/kg ± 76.3 mg/kg), Ash (32.2 mg/kg ± 51.0 mg/kg); Copper Vegetation (4.2 ± 1.5 mg/kg), Ash (10.9 ± 9.8 mg/kg); Chromium (Total) Vegetation (8.3 ± 15.6 mg/kg), Ash (9.6 ± 12.4 mg/kg); Lead Vegetation (6.1 ± 8.3 mg/kg), Ash (12.2 ± 8.5 mg/kg); Nickel Vegetation (2.2 ± 2.2 mg/kg), Ash (4.7 ± 4.4mg/kg) and Cadmium Vegetation (0.6 ± 0.9 mg/kg), Ash (0.6 ± 0.9 mg/kg). The ash samples were sieved through three aperture’s (2-4 mm, 1-2 mm, <1 mm) to stratify the samples by size fraction and concentrations in all but three of the metals increased as the ash size decreased. It was demonstrated that the level of metal present within the ash samples did not exceed health investigation levels and, with the exception of Manganese, and did not exceed environmental investigations levels. Metals were unlikely to pose a risk if left insitu. However, modelling estimated the volume of metal released and it was determined that metals can pose a subsequent risk if mobilised by wind or water. The likelihood of such mobilisation is high and this finding therefore has public health implications for surrounding communities that are subjected to increases in their exposure to metals, associated with bushfires. The findings from this study contribute to the management of prescribed burns by providing a better understanding of the composition of ash and the effects of potential distribution via aerial deposition or runoff. Data from this study can be used to do predictive modelling of heavy metal mobilisation that may result from burns of similar vegetation environments. This becomes particularly significant where burns are conducted in water catchment areas. Keywords-Smoke, Ash, Prescribed Burn, Heavy metal.