In der Anwendung von Bioziden wird die Schaumversprühung zunehmend als sicherere Alternative zur Tropfenversprühung eingesetzt. Bislang fehlen jedoch Expositionsdaten sowie etablierte Ansätze zur Charakterisierung und Bewertung der potenziellen Aerosolfreisetzung bei der Schaumversprühung, sodass bei der Zulassung von Biozidwirkstoffen und -produkten Unsicherheiten bezüglich der Bewertung bestehen. Erste Untersuchungen im Rahmen der vorliegenden Studie zeigen, dass bei der Verwendung von Schäumen eine Aerosolbildung nicht ausgeschlossen werden kann, die Aerosolfreisetzung aber gegenüber der konventionellen Tropfenversprühung signifikant reduziert ist. Die derzeitige Bewertung auf der Basis von Daten für Sprühanwendungen kann jedoch ggf. zu einer Überschätzung der inhalativen Exposition führen. Weitere Untersuchungen sind notwendig zur Ermittlung von Korrelationen zwischen Aerosolfreisetzung und Produkt- und Prozessparametern. Diese können als Grundlage für die Bewertung des Expositionsrisikos sowie ggf. für die Implementierung der Schaumversprühung in Expositionsmodellen dienen.
In recent years, foam spraying of biocides has often been used as a safer method for treatment of surfaces than conventional droplet spraying. This refers to the application of biocides in the occupational section, where e.g. combined disinfectant cleaners have been developed for foam treatment of large surfaces, as well as to cosmetics and household cleaners for consumer use (“convenience products”).
In the context of biocides, foam spraying offers several significant advantages over conventional droplet spraying, in particular better visual control of the application, an increased contact time, a mechanical cleaning during removal and often lower product consumption. With regard to the safe use of biocide foams, it is frequently stated by the industry that inhalation exposure to aerosol is considerably reduced or completely excluded for foam spraying compared to droplet spraying processes. However, up to now there are no data or established models which can substantiate this statement.
The use of biocides in the European Union is in the scope of the so called Biocidal Product Regulation (BPR, Regulation (EU) No 528/2012)  which assures a harmonised procedure for the evaluation and authorisation of biocidal active substances and biocidal products. One aim of the BPR is to ensure a high level of protection for human health and the environment. To this end a well defined exposure scenario which characterises the professional exposure during the product use is an essential part of the human risk assessment. Reliable information on the potential release of particles in the health-related size ranges under conditions of use of the product is thus of vital importance.
Regarding droplet spraying processes, there is a well developed understanding on the physical and technical parameters determining aerosol release and the potential inhalation exposure can be reasonably well quantified on the basis of experimental data or from mathematical exposure models, such as SprayExpo [2; 3].
In contrast, experimental characterisation, modelling and assessment of potential aerosol exposure from foam spray application of biocides has received only little attention so far. The Technical Guidance Documents for biocides [4 to 6] provided by the ECHA and the established exposure models do not provide much information on this kind of application. No data are available neither on inhalation exposure from foams nor on physico-chemical parameters influencing the airborne release fraction and the particle size spectrum.
A simple direct transfer of the approach used for the modelling of droplet spray applications, e.g. in SprayExpo or ConsExpo [2; 3; 7], to foam spray applications is not possible, since e.g. no droplet size distribution can be specified as input parameter for the models. Besides the lack in information on the droplet size, there are further uncertainties regarding the relationship between aerosol release potential and product and process parameters during foam application (e.g. spraying technology, pressure). There is also the possibility that a simple transfer of exposure models for droplet spraying to foam application may lead to an overestimation of the inhalational exposure.
Due to the lack of exposure data, the foam application of biocides is assessed based on droplet spraying data as a worst case scenario in the evaluation procedure. Generation of additional measurement data on the exposure from biocide foams would be a valuable contribution to close this gap. Therefore, the purpose of this survey, initiated by the German Federal Institute for Occupational Safety and Health, is to give an overview on the current state of knowledge on application scenarios and product types of biocidal foams, on the foam spraying technologies used as well as on inhalable aerosol release from foam spraying processes.
For collection of the information on foam spraying technologies and biocidal foam products as well as on aerosol release from foam spraying in this study, an extensive, but non-comprehensive literature research was carried out using various data bases (Scopus, Science Direct and Web of Knowledge) as well as Google. The current state of knowledge and the use of foam sprays for biocide application were surveyed from internet and literature (with a focus on the German market). Details about the claim, the active ingredients and the application devices were gathered either from the Safety Data Sheet (SDS) or from the respective product specification sheets.
In addition, some exploratory aerosol measurements were carried out for typical biocide products to compare the aerosol release potential from foam spraying to conventional droplet spraying.
2 Foam spraying
2.1 Description of foams
In contrast to a conventional droplet spray which is an aerosol mist  a “Foam is a dispersion of gas in a liquid or a solid, whereas the volume fraction of gas in the foam is mostly between 0.5 and 0.9”  (based on ), but can reach values of up to more than 99% . In general, all kinds of foam solutions consist of a hydrophilic liquid continuous phase, containing the solvable active ingredients and amphiphilic substances acting as foaming agents, e.g. surfactants and proteins. Besides these basic substances the formulation may contain a third hydrophobic dispersed phase, for example a lipid phase, and other substances, improving foam formation and stability [9; 12; 13]. The so-called blowing agent, responsible for the gaseous phase, includes physical agents, such as propellants, and chemical agents.
In general, foams can be divided into different types: for example liquid (low viscosity, e.g. disinfectant foam), semi-solid (high viscosity, wasp insecticide foam), solid foams (high viscosity, e.g. polyurethane foam)  or wet foams (high liquid content) such as water based disinfectant foams and dry foams (low liquid content) .
2.2 Foam spraying techniques
Since according to the literature survey, there is no universally valid classification of foam spraying technologies, the distinction in the present study was made based on the generation of the gaseous phase of the foam. According to this approach, two principal foaming technologies can be distinguished: mechanical foam generation and blowing agent foam generation.
Mechanical foam generation by mixing air and liquid is used for production of liquid foams. This technique can be displayed as a continuous, often large-scale process, e.g. generation of disinfectant foams, or as a discontinuous, in general small-scale process, e.g. soap foam generation or consumer disinfectant pump sprays. In this technique, basically the liquid formulation is mixed with air at a desired ratio, before the mixture is pressed through an annular nozzle, small orifices or a (double) sieve/foam net for foam generation and/or foam spraying .
In continuous (low-)pressure foam generation techniques, air is supplied actively by injection of pressurized air at pressures of up to 10 bar, but most commonly in the range between 3 and 5 bar. In high-pressure foam spraying techniques air is passively added to water using the Venturi principle. Here, water is supplied at pressures of up to 200 bar and flow rates of up to 1,300 l/h. Foam generation is often realised by flat-spray/fan-jet nozzles. These mechanical foam spraying techniques are commonly used for continuous large-scale deployment of biocidal foams for disinfection purposes.
In contrast to mechanical foam generation, liquid, semi-solid or solid (dry), highly-viscous foams can be generated by using blowing agents, also known as “pneumatogens”. In this technique, a gaseous product, formed either by evaporation of a compressed liquid propellant or decompression of the gaseous propellant (= physical blowing agent) or by a chemical reaction (= chemical blowing agent) is used for the formation of bubbles out of the liquid formulation containing the active ingredients and the foaming agent (e.g. [15; 16]).
Physical blowing agents typically include hydrocarbons or blends thereof (e.g. pentane, iso-butane, propane), halogenated hydrocarbons and air. Foam generation based on hydrocarbons is especially relevant in the context of liquid and semi-solid foams for small-scale discontinuous applications by aerosol spray cans pressurized to 2 to 4 bars, e.g. for cosmetics application, as well as for deployment of biocides, e.g. against wasps [12; 15 to 17].
2.3 Consequences for exposure assessment
Out of the results of the internet survey two questions arise which may impact the evaluation of the exposure to biocide foams within the scope of the authorisation procedure under BPR :
- Is it feasible to predict possible exposure to the biocide foam based on the physical characteristics of the foam?
- Which influence does the foam application technique have on the level of the aerosol release?
3 Materials and methods
3.1 General approach
For a first assessment of the possible release of aerosols from foam spraying processes, a series of cursory experiments were carried out, including various spraying techniques and types of foams.
To avoid the influence of the (unknown) composition of the biocidal products on the results and to allow for a comparison and evaluation of the results, all aqueous-based products were foam and droplet sprayed. Using the same formulation and the same spraying devices under similar operational parameters, allows for a direct comparison of the results leading to meaningful conclusions regarding the possible aerosol release from foam spraying processes.
3.2 Inhalation exposure characterisation of spray products
Usually inhalation exposure characterisation is based on droplet size distribution analysis of the native spray, e.g. by laser diffraction spectrometry or phase Doppler anemometry [18; 19]. However, foam sprays, providing a non-homogeneous liquid-gas-mixture unlike to the well-known characteristics of a droplet mist from conventional sprays, are not accessible to such in-situ methods. Similarly, exposure scenarios, such as surface treatment resulting in the formation of overspray, cannot be reasonably considered using methods for native droplet characterisation.
In addition, the main mass fraction of the droplets is comprised of solvents and/or propellants. Therefore, in general, droplets generated during spraying evaporate within a short period of a few seconds, so that the spray aerosol will shrink to the non-volatile constituents, resulting in maximum inhalability. This status is assumed to represent the real exposure situation of the user and needs to be considered for inhalation exposure characterisation for sprays containing non-volatile active components.
An approach to characterise spray products in view of the possibility of inhalation exposure during conditions of use is a mass balance method. The method comprises the determination of release fractions for the three health-related size classes according to EN 481  (and ACGIH1)) standards: the respirable, the thoracic and the inhalable fraction. The release fractions, Rres, Rthor, Rinh, are defined as the mass, mres, mthor, minh of the aged spray aerosol generated in the three size classes, normalised to the total mass, M, of the foam product released
As mentioned in the previous section, the data of the release fraction characterise the foam application process. These data are needed to estimate the inhalation exposure for a defined application scenario (amount of spray used during foam application, room size, ventilation, …) using mathematical exposure models. Mass balance-based models usually require the source strengths, Qres, Qthor, Qinh, as input parameters. These quantities are given by
where is the release rate of the total mass of the spray product.
For the determination of the release fractions, a well defined spray bolus is released into a control chamber of known volume, followed by the measurement of the concentration of the remaining aerosol of non-volatile components in the three health-related size fractions at the end of spray release action, c0,resp, c0,thor and c0,inh. A fan installed in the spray cabinet ensures a homogenous distribution of released aerosol droplets during and after the application. The spray droplets released are subject to evaporation of the solvent occurring in time scales smaller than a few seconds for particle diameters < 100 mm and the typical solvents used. Spraying is performed according to the intended use of the spray product. In order to simulate a surface treatment application, the spray is directed against a vertical surface from a defined distance.
The aerosol masses, released in the particle size fractions, are calculated from these initial concentration values by
The release fractions are then calculated from Eqs. (1) and (3).
Aerosol concentration measurement in the health-related size classes is performed using the RESPICON personal aerosol monitor (Helmut Hund GmbH, Wetzlar, Germany) , enabling gravimetric and on-line light-scattering analysis. The instrument directly measures the three health-related size fractions of airborne suspended particulate matter. For stagnant air sampling conditions, the upper end of the size spectrum aspirated by the RESPICON is about 40 mm. The instrument is a combination of a two stage virtual impactor (for aerodynamic size classification), three sampling filter cassettes for gravimetric or chemical analysis of the collected material and three light scattering photometers for on-line concentration monitoring (see Figures 1 and 2).
Figure 2. The scheme shows the principle of operation of the RESPICON. The grey shaded units are the collection units for three different aerosol size fractions. Each size fraction is monitored on-line by light scattering. 1: inhalable particle inlet, 2: optical measuring volume, 3: separation plate, 4: light source/detector, 5: connection to data logger, 6: light trap, 7: filter/XAD cassette, 8: connection to sampling pump.
After the measurements the voltage data are transferred into concentration data based on the average mass concentration determined gravimetrically or, in case of too low filter deposits, by application of standard calibration factors. For a description of the working principle of the RESPICON and validation experiments see .
Applying this mass balance method, effects occurring in real world, such as spray ageing and overspray formation are considered for exposure characterisation. Furthermore, spray applications not accessible to in-situ droplet characterisation, such as foam sprays, can be characterised with regard to their inhalation exposure potential. In addition to the droplet spectrum, the concentration of non-volatile compounds in the individual liquid spray formulations is taken into account in the test procedure.
For both, droplet and foam spraying experiments, this mass balance method was used to quantify the aerosol release.
3.3.1 Pumpsprays for droplet spraying/foam spraying
For both products used in the experiments, droplet spraying is realized via a one-component nozzle. For foam spraying the same application device is used, but additionally equipped with a thin plastic gauze placed before the spray nozzle. The spray products are released into a 1.5 m3 control volume. The following pumpspray products are characterised:
- Household cleaner spray and foam, non-volatile fraction: 9.9 wt%
- Household disinfectant spray and foam, non-volatile fraction: 2.9 wt%
3.3.2 Low-pressure droplet spray gun/Low-pressure foam spray gun
For all formulations under investigation, the pressure sprayer GLORIA FoamMaster FM50  is used at the same operation conditions with internal mixing, but with either a spray or a foam lance, both equipped with a flat spray nozzle. Device pressure is constantly kept at 3 bars. The sprays are directed against a chalk board with the dimensions of 1.5 x 2.0 m in a 41 m3 control volume (Figure 3).
The product investigated was a quarternary ammonium compound disinfection spray and foam (1 % working solution), non-volatile fraction: 0.40 wt% (working solution).
3.3.3 Blowing agent foam generation
Two propellant foam spray applications are investigated, both are available as ready-to-use spray cans and equipped with a conventional circular one-component dosing nozzle and a tubing for well-targeted application. The spray products are released into a 0.16 m3 control volume. Products investigated were:
- wasp insecticide foam,
- biocidal foam with high viscosity.
A search of the web with a focus on the German market resulted in 59 descriptions of biocidal foam products. For disinfectants of the Product types (PTs) 1, 2, 3 and 4 according to BPR  41 foam applications were found; most of the products were designated for the use in the food and feed area (PT 4). For insecticides (PT 18) eight foam products, e.g. wasp foam, were found.
More detailed data about the claim, the active ingredients and the application devices were gathered either from the Safety Data Sheet (SDS) or from the respective product specification sheets. Common active substances found in disinfectant foams were e.g. quarternary ammonium chlorides (QAVs), guanidine derivates, but also hydrogen peroxide, organic peroxoacids and active chlorine representing a high potential for inhalation exposure. Insecticides contain geraniol, pyrethrin, permethrin and natural extracts as active substances.
Different techniques are used for the foam application of biocides, mechanical foam spraying techniques based on mixing of air and liquid for the production of liquid foams are frequently used for e.g. continuous large-scale deployment of biocides for disinfection purposes. Often special spray devices for mechanical foam generation are sold in combination with the biocide product or commercially available low or high pressure foaming devices and spray nozzles are recommended. This is the case for disinfectants, wood preservatives and construction material preservatives.
For rodenticides, insecticides, repellants and attractants usually aerosol cans are sold which work with the principle of foam generation based on hydrocarbons. This is commonly used for formation of liquid and semi-solid foams for small-scale discontinuous applications by aerosol spray cans pressurised to 2 to 4 bars [1 to 4]. Exposure can be modified and reduced by additional equipment such as spray tubes for focused point application or by physical chemical properties of the product as it is given for the highly viscous biocidal foams.
In summary, disinfectants, wood preservatives, construction material preservatives and insecticides can be considered as groups of biocides with a high variation of potential exposure which is worth to be analysed in detail.
For rodenticides, insecticides and repellants and attractants (PTs 14, 18, 19) usually aerosol cans are sold; thus, a more similar application may reduce the variability in exposure with respect to the foam spray.
Since no data on exposure to aerosols from biocide foams were found in literature, some exploratory aerosol measurements were carried out for typical biocide products to compare aerosol release from foam and droplet spraying. In general, the mass balance method [21; 22], as described in the previous section, gives a good characterisation of the inhalation exposure hazard of foam spray processes and droplet spray-processes under real conditions of use. This enables for direct comparison between the release fractions of the foam spraying process and the respective droplet spraying process.
Gravimetrical analysis was not possible for any of the filters (respirable, thoracic, inhalable size regime). Therefore standard calibration factors had to be used for data analysis .
Figure 4 shows an example of the mass concentration pattern for the inhalable and thoracic size fraction as measured by the RESPICON system during the release experiment with the household cleaner foam.
Five consecutive spray actions were carried out each followed by aerosol ageing and ventilation phase.
Figure 5 shows the final results of the release fractions for mechanical foam spray applications and the corresponding droplet spray applications of the different disinfectant or cleaner products containing non-volatile active substances.
Importantly, mechanical liquid foam generation results in relevant aerosol release in the three health-related size fractions respirable, thoracic and inhalable. A direct comparison of foam spraying to droplet spraying shows an average reduction of the release fractions in the three health-related size classes by a nearly constant factor of about 3 for both, the use of discontinuous pump sprays for small-scale deployment and continuous spray guns for large-scale deployment.
Regarding the different foam spray products, a difference for the respirable, the thoracic and the inhalable fraction by about a factor of 5 in average can be observed between the discontinuous pump spray and continuous foam spray application of a disinfectant. For the present investigations, these release fractions for mechanical foam spraying are in the range of 1.0E-03 to 1.0E-02%.
Figure 6 shows only the final results of the release fractions for the foam applications and includes the two propellant foam spray applications from aerosol cans, i.e. the wasp insecticide foam and the other highly viscous biocidal foam.
The two latter products cannot be droplet sprayed. The generation of the semi-solid, highly-viscous foams out of aerosol cans using physical blowing agents results in a more than hundred fold lower aerosol formation compared to liquid, low-viscous foams generated from the spray gun and pump sprays. With release fractions in the range of 1.0E-06 to 1.0E-05%, these two products are assumed to represent only a lower level of exposure.
It is important to note, that the aerosol release fractions determined in the present experiments refer to the total content of non-volatile compounds in the liquid spray formulation and not specifically to the active ingredients. Therefore, besides the foam properties and foam spraying technology, the differences in the release fractions observed between the different products may be partly explained by variations in the fraction of non-volatile components in the spraying formulation.
The literature survey shows that foam spray applications can be found for a broad range of biocidal product types and applications. Furthermore, it shows that despite the variety of biocides and applications there are no data or models to quantify inhalation exposure during the usage of biocide foams. This emphasises the importance to develop approaches for the characterisation and assessment (e.g. measurements and modelling) of inhalation exposure against aerosol from foam spray applications.
In the biocide authorisation procedure currently spraying data are used as worst case scenario to evaluate the exposure from foam applications. The results from the cursory experiments of the present study give a first hint that the exposure during foam application might be reduced in comparison to droplet spray application of biocides. Nevertheless, more investigation and more data are needed to substantiate this finding.
Regarding inhalation exposure assessment, a mass balance approach was successfully established, allowing for direct determination of aerosol release in the three health-related size classes (respirable, thoracic, inhalable). Besides consideration of spray ageing as well as real exposure conditions, this method is universally applicable for droplet and foam spray characterisation.
The most relevant foam spraying technologies for biocides can be classified into two types:
1. mechanical foam generation by mixing (pressurized) air and liquid typically used for production of aqueous, low-viscous foams and
2. blowing agent foam generation using propellants for production of liquid and (semi)-solid, i.e. higher-viscous, foams.
Direct comparison of the results from droplet spraying and mechanical foam spraying under equal process and product parameters shows a reduction in aerosol release during foam spraying by a factor of about 3 compared to conventional droplet spraying for all aqueous-based biocidal solutions investigated. Generation of semi-solid, highly-viscous foams, using physical blowing agents, results in a more than hundred fold lower amount of non-volatile aerosols in the health-related size classes, indicating that increasing foam viscosity seems to reduce the level of aerosol release.
In conclusion, these exploratory experiments show that aerosol exposure cannot generally be ruled out for the foam application of biocides. However, it can be assumed that foam spraying of potentially harmful substances might be a suitable measure to reduce inhalation exposure.
These results suggest that increasing foam stiffness/viscosity reduces aerosol release. Due to the limited number of experiments only some hints on the dependence of aerosol formation on the characteristics of the foam spray technology and the final foam, respectively, could be discovered. Further parameters potentially affecting the amount of aerosol release, e.g. foam expansion ratio or pressure during foam application, as well as especially physico-chemical data of the formulation like surface tension and viscosity, could not be characterised within this study. It is conceivable that especially foam application by high-pressure spraying – which has not been carried out in the present study – might lead to significantly higher aerosol release fractions.
The release fractions determined in the present experiments are substance-unspecific, since they refer to the total content of non-volatile compounds in the liquid spray formulation. Therefore, besides the foam properties and foam spraying technology, the differences in the release fractions observed between the different products could be partly related to variations in the fraction of non-volatile components in the spraying formulation.
More comprehensive experiments are necessary to identify correlations between process and product parameters and in a second step to potentially reasonably predict aerosol release. Therefore it would be straight forward to carry out a parameter study which investigates all relevant foam application technologies in use for biocides application in more detail with regard to their aerosol release fractions, their dependence on the operational parameters such as pressure, nozzle type and diameter, etc., and the physico-chemical properties of the liquid formulation and the respective foam. Furthermore, typical foam application scenarios (fact sheets) have to be identified and described. The results would also provide valuable input data for implementation of foam spraying into indoor exposure models for exposure risk assessment which has not been considered yet.
The actual survey was focused on inhalation exposure. In addition to the reduction of inhalation exposure, it seems to be reasonable that dermal exposure might also be reduced when using foam application of biocides. This could be an additional aspect of future investigations.
1) ACGIH: American Conference of Governmental Industrial Hygienists
- 17. Regulation (EU) No 528/2012 of the European Parliament and of the Council of 22 May 2012 Concerning the Making Available on the Market and Use of Biocidal Products. OJ (EU) (2012) Nr. L 167, p. 1-123.
- Prediction of aerosol exposure during spray application with the mechanistic model SprayExpo. Published by: Federal Institute for Occupational Safety and Health, Dortmund, Germany 2014. www.baua.de/en/Topics-from-A-to-Z/Hazardous- Substances/SprayExpo.html
- Koch, W.; Behnke, W.; Berger-Preiß, E.; Kock, H.; Gerling, S.; Hahn, S.: Validation of an EDP assisted model for assessing inhalation exposure and dermal exposure during spraying processes. Dortmund: Federal Institute for Occupational Safety and Health, 2012.
- Human Exposure to Biocidal Products – Technical Notes for Guidance (TNsG). Published by: European Chemical Agency (ECHA), Helsinki, Finland 2002.
- Human Exposure to Biocidal Product – Technical Notes for Guidance, User Guidance, Version 1. Published by: European Chemical Agency (ECHA), Helsinki, Finland 2002.
- Human Exposure to Biocidal Products – Technical Notes for Guidance. Published by: European Chemical Agency (ECHA), Helsinki, Finland 2007.
- ConsExpo 4.1. Published by: National Institute for Public Health and the Environment (RIVM), Bilthoven, The Netherlands 2014. www.rivm.nl/en/Topics/C/ConsExpo
- Spray. [Online] 2014. http://en.wikipedia.org/wiki/Spray
- Arzhavitina, A.; Heckel, H.: Review: Foams for pharmaceutical and cosmetic application. Int. J. Pharm. 394 (2010) No. 1-2, pp. 1-17.
- Wilson, A. J.: Foams: Physics, Chemistry and Structure. Berlin: Springer 1989.
- Stevenson, P.: Foam Engineering: Fundamental and applications. John Wiley & Sons 2012.
- The Pharmaceutics and Compounding Laboratory: Aerosol Systems. Published by: UNC Eshelman School of Pharmacy, 2013. http://pharmlabs.unc.edu/labs/aerosols/objectives.htm
- Sakai, T.; Kaneko, Y.: The effect of some foam boosters on the foamability and foam stability of anionic systems. J. Surfact. Deterg. 7 (2004) No. 3, pp. 291-295.
- Lemlich, R.: Some physical aspects of foam. J. Cosmet. Sci. 23 (1972), pp. 299-311.
- Polymatrix – Innovative tools for polymers. Cure Terms Glossary. Published by: Polymatrix Ltd., Shrewsbury, Shropshire, United Kingdom 2013. http://polymatrix.co.uk/Cure% 20%282%29.htm
- Blowing agent. http://en.wikipedia.org/wiki/Blowing_agent
- Hoffbauer, B.: Foam aerosols. Aerosol Spray Rep. 35 (1996), pp. 508-515.
- Combellack, J. H.; Matthews, G. A.: Droplet spectra measurements of fan and cone atomiser using a laser diffraction technique. J. Aerosol Sci. (1981) No. 12, pp. 529-540.
- Nuyttens, D.; Baetens, K.; De Schampheleire, M.; Sonck, B.: Effect of nozzle type, size and pressure on spray droplet characteristics. Biosyst. Eng. 97 (2007) No. 3, pp. 333-345.
- EN 481: Atmospheres: Size fraction definitions for measurement of airborne particles in the workpklace. Comité Européen de Normalisation (CEN) 1992.
- RESPICON TM, Technical Description and Instruction Manual. Published by: Helmut Hund GmbH, Wetzlar, Germany 2000.
- Koch, W.; Dunkhorst, W.; Lödding, H.: Design and performance of a new personal aerosol monitor. Aerosol Sci. Technol. 31 (1999) No. 2-3, p. 231-246.
- FoamMaster FM 50. Published by: GLORIA Haus- und Gartengeräte, Witten, Germany 2014. www.gloriagarten.de/de/profiline/spruehgeraete/druckspruehgeraete/foammaster-fm-50.html
Dr.-Ing. Katharina Schwarz, Dr. rer. nat. Annette Bitsch - Fraunhofer Institute for Toxicology and Experimental Medicine ITEM, Hannover, Germany.
Dr. rer. nat. Dagmar Holthenrich, Dr. rer. nat. Kathrin Bissantz, Dr. rer. nat. Markus Ehni - German Federal Institute for Occupational Safety and Health (BAuA), Dortmund, Germany.