Particulate matter produced during commercial sugarcane harvesting and processing: a respiratory health hazard?

Abstract

Emissions from sugarcane burning are known to impact on the respiratory health of sugar estate workers and local populations. Despite this, there have been few studies on occupational and ambient exposures and risks from airborne particulate matter (PM) associated with field burning and ash re-suspension. From workplace monitoring on sugarcane estates in two different South American countries in 2010 and 2011, median concentrations of airborne PM10 (particulate matter nominally <10 μm in diameter) were found to be statistically much higher during pre-harvest sugarcane burning (1807 μg m−3) than during either sugarcane cutting after burning (∼123 μg m−3) or in the sugarcane processing factory (∼175 μg m−3). Median PM10 measurements in ambient scenarios, for example in the sugarcane fields before the burning or during 24 h measurements in neighboring villages (bordering the sugarcane plantation), were much lower, between 18 and 37 μg m−3. From the analysis of size-selective samples of airborne PM10, collected during sugarcane field burning, cutting and ambient periods, almost all (∼96 wt %) fell within the ‘respirable’ fraction (<4 μm aerodynamic diameter), with a mass median aerodynamic diameter (MMAD) of 1.1 μm. Residual ash from field and bagasse burning, characterised using Scanning Electron Microscopy (SEM) with X-ray elemental analysis, was found to contain carbonaceous and silicate-dominated particles in the PM0.5 and PM0.5-2.5 size ranges and fibres from <10 to over 50 μm in length. Only a small proportion of the field burning ash (average 0.6 vol %) and bagasse ash (average 1.3 vol %) was in the respirable fraction. However, from grinding experiments, which simulate disaggregation as a result of disturbance during harvest or bagasse ash removal, the ash was fragile and easily broken down into thoracic particulate (<10 μm aerodynamic diameter) and, in some instances, created respirable-sized PM. From exposure calculations, the 8 h time weighted average (TWA) concentrations of PM10, during the different measurement scenarios, were found to be below occupational exposure limits (OELs; 5000 μg m−3 for respirable PM). Ambient PM10 exposure of residents surrounding the sugarcane plantations was found to be below the WHO air quality guideline (50 μg m−3 as a 24 h mean). The relative risk calculated for ‘all cause’ mortality from exposure of nearby residents to PM10 generated by sugarcane burning was found to be 3%. The concentrations of PM10 produced during the processing of sugarcane were high (up to 21.5 mg m−3), which is concerning given that re-suspended particles of ash in the fields and processing plant have been previously shown to contain potentially toxic cristobalite. PM produced during sugarcane burning, and during extended periods of local exposure to the smoke and re-suspended ash, therefore, should be considered as both a potential acute and chronic respiratory health hazard. This issue will become increasingly important with the forecasted rise in sugarcane production for biofuels.