Prediction of aerosol exposure during spray application with the mechanistic model SprayExpo
Commissioned by the Federal Institute for Occupational Safety and Health (BAuA), the Fraunhofer Institute for Toxicology and Experimental Medicine (ITEM) has developed the mechanistic model SprayExpo for prediction of aerosol exposure during spray application of non-evaporating biocidal substances. SprayExpo is included in the TNsG Human Exposure to Biocidal Products - Guidance on Exposure Estimation, Part 2, chapter 3, "Defaults and Models" for the dermal and inhalation exposure assessment in the authorisation process of new and existing biocidal products.
The model calculates the airborne concentrations in various health relevant particle size fractions. It takes into account turbulent diffusion, droplet evaporation and gravitational settling. In addition, it includes an improved droplet impaction module for calculating the overspray during spraying onto a surface. Furthermore, it is no longer necessary to enter primary droplet distributions directly. Instead, for common spraying techniques these are stored in a database from which they can be retrieved by specifying the spraying technique and simple process parameters such as the spraying pressure. During development of the model, sensitivity analysis revealed that besides the substance release rate, the droplet spectrum is the process parameter that has a decisive impact on the exposure level. In contrast, the vapor pressure of the solvent only plays a secondary role for the exposure concentration of the active ingredient within the relevant range of values.
For evaluation of SprayExpo, predictions generated with this tool have been compared with results obtained using the already existing models "ConsExpo" and "BG-Spray" as well as with workplace measurement data. To this end, exposure concentrations were determined by personal sampling and subsequent chemical analysis at workplaces in the area of antifouling treatment (shipyard) and in several scenarios in stored product protection. For both room spraying and spraying onto walls, comparisons between the model and experiments revealed that spray applications can generally be reproduced with an uncertainty of factor 4 or lower. With regard to dermal exposure, the model can only take into account the sedimentation flow of the airborne droplets, but not accidentally occurring splashes. Therefore, the dermal exposure at the workplace is underestimated by SprayExpo in the majority of cases. However, the dermal exposure is represented quite well in the case of room spraying. In summary, the comparison of modelled predictions with measured values demonstrates that SprayExpo is appropriate for assessing exposure during indoor spraying processes. It has to be acknowledged, however, that the model has to be used with the necessary expertise.
Video-clip showing the application pattern wall line
3D animated representation of wall spraying (along a line) showing the released overspray
MS Excel® worksheet for SprayExpo 2.2
To facilitate the use of the SprayExpo model, an MS Excel® worksheet was developed that can be executed under Microsoft Excel® 2003 or later using Microsoft Windows® 2000 Service Pack 3 or later, or Windows® XP or later (recommended). The main input parameters are: the released droplet spectrum, the release rate, the concentration of the active substance, the spatial and temporal pattern of the release process, the vapor pressure of the liquid, the size of the room and the ventilation rate. The path of the sprayer can be explicitly included into the model. For surface treatment by spraying, a droplet deposition module is incorporated in the program package. This module calculates the fraction of non-impacting droplets which are relevant for human exposure.
Since version 2.2, SprayExpo allows switching the interface language between English and German within the program. Thus, separate language versions are not required anymore. In addition, the algorithm used for calculation was substantially accelerated compared to previous versions.
Screenshot from the MS Excel® worksheet for SprayExpo 2.2