Presentation of exposure value and derivation of result

Apart from the obvious choice of sampling method and study design, also a choice concerning the unit of exposure has to be made by the person documenting the results and/or the persons responsible for deriving the efficiency from these values.

Units of exposure found in the course of the project are:

- mass per piece of equipment or per patch - mass per area

- mass per area and time (flux)

- mass per mass substance applied (especially for pesticides) - mass per application or task

Depending on the further evaluation of the raw data, the choice of unit may influence the numerical result.

An excellent example for the variability that may be induced by these choices is a study published by BELLO et al. who have estimated skin exposure via wiping and have given exposure as ng/cm² but also as overall loading per sample. Highly different results are obtained for both approaches which suggests that the wiped skin area, which is used for the derivation of the mass per skin area exposure, is not identical for all wiping processes (e.g. 28.6 vs. 49.8% for gloves, spraying) (BELLO et al., 2008).

In addition, different results may be obtained due to mathematical reasons: If exposure values are evaluated for each person separately (individual efficiencies during intervention studies or simultaneous sampling of actual and potential exposure) the result may differ from the one obtained from averaged exposure values.

These influences are also connected to the units used to report exposure.

As an example, there will not be a difference between average efficiencies for different units (mg/cm², flux / mass per mass substance applied for simultaneous sampling of actual and potential exposure at the same individual) if an efficiency is estimated for each test subject individually and then averaged (units will be cancelled out). If, however, the flux values or mass values per area are averaged and then these averages are used for the efficiency calculation, there may be different results. An example based on hypothetical exposure data is given in Table 5.2 (77.1% for individual efficiency calculation vs. 78.8 and 82.0% for estimation of efficiency from average exposure values).

In case of intervention studies already for individual efficiency estimation and average calculation afterwards different efficiencies may be found for different exposure units (variability between “before” and “after” intervention scenario). An example based on hypothetical exposure data is given in Table 5.3 (77.1 and 76.8% for individual efficiency estimation vs. 78.8 and 78.2% for estimation from average exposures).

For cross sectional studies no estimation of efficiency for one selected individual is possible.

Table 5.2 Hypothetical exposure data and possible efficiency derivation (simultaneous sampling of actual and potential exposure)

Exposure given in mg / cm² (assumption: simultaneous sampling) Item

# duration / h

exposure

below PPE duration / h

exposure

above PPE efficiency (%)

1 0.5 2 0.5 5 60.00 individual Exposure given in mg / cm² / h(assumption: simultaneous sampling)

Item

# duration / h

exposure

below PPE duration / h

exposure

above PPE efficiency (%)

1 0.5 4.0 0.5 10.0 60.00 individual In addition, the use of these units may imply correlations with other parameters such as the duration or the amount of substance. As an example, a flux (mg / cm² / h) implies a linear relationship with the task duration while at the same time ignoring saturation effects or intermittent contamination. Especially in cases, where sampling durations differ very much these effects may significantly bias an efficiency result.

For the project database, if possible and if no efficiency value was provided in the publication, the exposure value in mg/cm² was used. However, not every study has been conducted in the same way and not all efficiency results are reproducible (see Excel database and corresponding comments).

Another example related to inconsistencies probably caused by different evaluation of measured data are the publications of VITALI et al. and PROTANO et al. Although the contextual information (e.g. weather data, wind speed, information about workers etc.) presented in both publications is identical for the sampled individuals and therefore suggests that identical sets of raw data were used, different penetration factors were derived (PROTANO et al., 2009; VITALI et al., 2009). This implies either an error, missing information or different evaluation of the raw data. As a consequence, only

data published by VITALI et al. were entered into the database as a worst case and data published by PROTANO et al. into the “comments” field for information. The transparency of the publications is not sufficient to retrace the origin of the differences.

Table 5.3 Hypothetical exposure data and possible efficiency derivation (intervention study)

Exposure given in mg / cm² (assumption: intervention study) Item

# duration / h

exposure

with PPE duration / h

exposure

without PPE efficiency (%)

1 0.5 2 0.4 5 60.00 individual

exposure: 8.3 78.79 average exposures used for efficiency Exposure given in mg / cm² / h(assumption: intervention study)

Item

# duration / h

exposure

with PPE duration / h

exposure

without PPE efficiency (%)

1 0.5 4.0 0.4 12.5 68.00 individual result. While efficiencies higher than 90% apparently often are reached and may look similar at first sight, seemingly small differences can still have a large effect on the exposure outcome. As an example, two efficiencies of 98.7% and 97.3% mean penetration factors of 1.3 and 2.7%, i.e. a factor 2 difference in terms of exposure (TSAKIRAKIS et al., 2010).

Therefore it can be summarised that a great number of possibilities concerning exposure units exists and is represented in the project database.

No official recommendation seems to exist. However, it seems reasonable to use the most simplistic approach (mg/cm²) and document other parameters, which may influence the result.

Many database entries are based on a simultaneous sampling of potential and actual exposure at the same individual. While having the advantage of having identical scenario conditions for potential and actual exposure samples (→ lower variability), if actual and potential exposure are sampled at the same time it cannot be excluded that the two layers of exposure will influence each other. An often applied equation uses the assumption that exposure “without PPE” (=potential exposure) corresponds to the sum of exposure on skin (=actual exposure) and exposure outside of PPE or clothing (GROßKOPF et al., 2013; SOUTAR et al., 2000b):

(1) Efficiency (%) = 100 · (1- Skin exposure inside PPE / (Skin exposure inside PPE + Exposure outside PPE))

With: potential exposure = Skin exposure inside PPE + Exposure outside PPE It may depend on the exposure loading and the substance in question if this is reasonable or can represent an overestimation of the efficiency, e.g. because in reality the excess of substance would evaporate or drip off the skin (influencing factors:

vapour pressure, skin or PPE permeation, adsorption on skin / PPE).

This equation is not always practiced. Partly the efficiency is instead calculated by comparing the concentrations inside and outside the clothing or PPE without summing up (FENSKE et al., 2002; GARROD et al., 1999):

(2) Efficiency (%) = 100 · (1- Skin exposure with/inside PPE / Skin exposure without/outside PPE)

With: potential exposure = Skin exposure without PPE

In both equations the efficiency is derived by comparison of actual and potential exposure. The main difference is the general way how potential exposure is derived from measured exposure values. In studies where exposures with and without PPE have been sampled separately, i.e. separated either by time (intervention, different sampling time) or space (cross sectional, different individuals), potential exposure is usually measured directly, i.e. on a person without any PPE (equation (2)).

In cases of simultaneous sampling, however, exposure samples inside and outside PPE are often added up in order to derive the potential exposure. This is usually practiced in cases where the PPE has also been used as dosimeter (e.g. extraction of gloves or coveralls), i.e. it is assumed that the substance which can be detected outside of the PPE would have been additional skin contamination without using the PPE (equation (1)). In other words, the chemical that is already present on the skin is assumed to have been removed from the outside of the PPE, therefore exposure outside PPE does not represent the complete potential exposure.

In cases of simultaneous patch sampling (patches outside and inside PPE) it is usually assumed that patches are impermeable and all contaminants are collected on the outside patch, representing potential exposure. Thus, equation (2) is used.

However, not in all cases the calculations are completely transparent (GARROD et al., 1998) and if the patches are made of permeable material equation (1) may be more reasonable (see e.g. (KANGAS et al., 1993)). Some uncertainty cannot be excluded.

While for low penetrations the difference between both approaches is not large, it can be quite significant for high penetration rates. As an example, if the concentrations above and below protective gloves are identical, equation (1) will lead to an efficiency of 50%, while equation (2) will lead to no reduction efficiency at all.