Commissioned by the Federal Institute for Occupational Safety and Health (BAuA), the Fraunhofer Institute for Toxicology and Experimental Medicine (ITEM) developed the mechanistic model known as SprayExpo. The model enables the prediction of exposure to aerosols during the spray application of non-evaporating biocidal substances. SprayExpo has been included in the TNsG Human Exposure to Biocidal Products - Guidance on Exposure Estimation, Part 2, chapter 3, "Defaults and Models". It provides assistance with the assessment of the levels of dermal and inhalation exposure for the approval biocidal products.

The model calculates the airborne concentrations of the various health-relevant particle size fractions. It takes the turbulent diffusion, droplet evaporation into account as well as sedimentation movements. It also includes an improved module for calculating the overspray during spraying onto a surface.

It is no longer necessary to enter the droplet distributions directly, since for certain spray devices, they are now stored in an internal database. Therefore, it is only necessary to specify the spraying technique and simple process parameters, such as the spraying pressure. During the development of the model, a sensitivity analysis revealed the droplet spectrum to be a process parameter that has a decisive impact on the exposure level - as well as the release rate. In contrast to the release rate, the vapour pressure of the solvent only plays a secondary role for the exposure concentration of the active ingredient within the relevant range of values.

Dermal exposure in the workplace has been underestimated

For the evaluation of SprayExpo, predictions generated with the tool were compared with results obtained from the existing models of "ConsExpo" and "BG-Spray" as well as with measurement data obtained at workplaces. For this purpose, exposure concentrations were determined on the basis of personal sampling and subsequent chemical analysis at workplaces in the area of antifouling treatment (at a shipyard) and in a variety of scenarios in the area of stored product protection. For the two scenarios of room spraying and spraying onto walls, comparisons between the model predictions and experimental values 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 the sedimentation flow of the airborne droplets into account, but not accidentally occurring splashes. Therefore, in the majority of cases, the dermal exposure at the workplace is underestimated by SprayExpo. In the case of room spraying, however, the dermal exposure is predicted very successfully.

In summary: SprayExpo is suitable for assessing exposure during indoor spraying processes. It is necessary to acknowledge, however, that this particular model must always be used with the requisite level of professional expertise.

The video clip shows the application pattern of "spraying along a wall line".

3D animated representation of wall spraying (along a line) showing the released overspray.

SprayExpo 2.3 as a worksheet for MS Excel®

To facilitate the use of the SprayExpo model, an MS Excel® worksheet was developed. It is compatible with the operating system MS Windows® XP Service Pack 2 or later and with the program MS Excel® 2007 or later.

The key input parameters are as follows:

• the released droplet spectrum
• the release rate
• the concentration of the active substance
• the spatial and temporal pattern of the release process
• the vapour pressure of the liquid, the size of the room
• the air-exchange rate

The path of the sprayer can be explicitly included in the model. For surface treatment with spraying, a droplet deposition module has been incorporated in the program package. This module calculates the fraction of non-impacting droplets which are of relevance to human exposure.

From version 2.2 onwards, SprayExpo allows users to switch the interface language between English and German. This means that separate language versions are no longer required. In addition to this, the algorithm used for calculation has been substantially accelerated.

Version 2.3 contains some corrections for the removal error messages. These can appear on some systems during the export of results into an external Excel file or when entering the droplet spectrum.

For older systems, SprayExpo 2.3 is also available in the old XLS format.

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