Calculation of Patch Dosimeter Results

  • The resulting measured levels of contaminant can be used in several ways, several of which are presented in Tables 3a and 3b. Most of the following calculations would apply equally well to pads placed either outside of clothing or inside. The example given in these tables is for deposited dose inside the clothing and is simply called "deposition" herein.
  • The first example is to calculate the dermal deposition density. To do this, the total mass of contaminant found during analysis (column B, which may or may not be adjusted for collection efficiency) is divided by the surface area of the pad(s) at each body location (column C).
    Dermal Deposition Density (ug/cm2)  =
    • Chemical Mass
    • Dosimeter Area
    • In the example, pairs of 28 cm² dosimeters were used at most locations except for the chest and back (which had only one pad) and the hands which used gloves.
      • Glove dosimeters are treated like a full body dosimeter; the area of the dosimeters corresponds to the body area covered as shown near the bottom of columns C and D.
      • Dermal deposition density (column E) is useful without further modification to assess the risk of dermatitis. However, there is no rationale for adding these density values.
    • To calculate someone's total dermal deposition (column F), it is necessary to assume that the deposition density on the local body part is accurately represented by the deposition density measured on the dosimeter. A set of standard body surface areas is typically used such as reported in column D. Such an assumption allows the local dermal deposition to be calculated by the following proportion:
      Dermal Deposition (mg) = Chemical Mass   x
      • Anatomic Area
      • Dosimeter Area
    • The validity of this assumption can be qualitatively judged based on observation of the setting. There is as yet no quantitative test for deposition uniformity. A semi-quantitative judgment can be made by comparing the proportion of dose measured at each location among different users doing nominally similar tasks. Some variation is expected in both the distribution and total of all locations.
      • The cited example found intra-personal variation in total dose was ± 2x. However, when trying to measure very low doses of antimicrobial chemicals applied in industrial settings, geometric deviations ranged from 4x to 9x.(15) The variation in localized deposition is probably greater when the physical doses (later called equivalent dose) are smaller and contributed to the larger geometric deviations.
      • Because variability interferes with identifying important inter-personal differences in work practices, it may require some replication in measurements to establish a reliable central tendency such as a mean or geometric mean or an estimate of clothing penetration.
      • Patterns of inter-location differences are quite discernable and inter-personal differences were statistically significant even with only four replicate measures(8); EPA usually requests 12-15 replicates for product registration data.
    • If dosimeters were only placed outside the clothing, then an assumption would also need to be made as to how much of that "potential dose" would have penetrated the worker's clothing. The cited study reported 3 to 7% penetration (the results of outside pads are not shown herein). The inter-location distribution of the dermal dose will allow one to interpret the impact of hypothetical changes in work practices or protective clothing on the user's total dose.
    • It is also only at this stage and beyond that the set of dermal depositions at each location can be meaningfully added to yield a total.
      • The total dose to the cited mixer-loader is 29 mg of the measured chemical.
      • As can be seen in Tables 3a and 3b, hands were the major site of deposition because no gloves were worn. The data indicates that had protective gloves been worn (assume for the moment that hand doses were near zero), the upper legs would have been the next major site of skin deposition for these mixer-loaders; the same could also be said for applicators, although head and lower arms were also major contributors.
      • This scenario suggests the next most important change in work methods or protective clothing beyond gloves if the total dose is judged to be too large.
    • One additional common step is to compare the dermal dose to the airborne dose. This comparison can include adjustment factors for both dermal adsorption and/or for respiratory retention that which is usually also well less than 100% either for particles (where the site of deposition and retention varies by particle size) or for gases and vapors (where retention varies by chemical).
      • As a first approximation, nominal doses or dose rates are often compared without adjustments. The nominal airborne dose can be calculated by multiplying the airborne concentration (in mg/m³) times the respiratory minute volume (Lpm or the equivalent m³/hr) times the exposure duration.
      • Typical respiratory minute volumes are 21 Lpm (corresponding to 10 m³/8-hours for light work rates) or 30 Lpm for moderate work. Thus far, most reported comparisons have involved low volatility compounds and ratio of nominal dermal doses to airborne doses is on the order of 100:1.
    • The remaining steps are various options to adjust or normalize the data for some denominator common across multiple settings.
      • One option is to divide by the amount of material applied or used during the assessment; for instance, mg dose per kg or pound applied.
      • A more common practice, used particularly in the agricultural esticide sector is to divide by the duration of the assessment to calculate the dose rate (mg/hr in Table 3a column G).
        Dermal Deposition Rate (mg/hr)   =
        • Dermal Deposition
        • Task or Exposure Time
      • The final option if the measured chemical is part of a mixture (e.g. diluted with water or impregnated on an inert particulate carrier) is to adjust the amount of measured chemical to the equivalent amount of the entire solution or solid matrix that wouldhave been deposited.
        Equivalent Dermal Deposition (mg or g)   =
        • Dermal Deposition
        • Contaminant Concentration
      • Equivalent dose is useful to compare results if data are collected in settings in which the solution concentrations vary or to apply results from one mixture to another mixture at a different concentration.
        • In the latter case, the known equivalent dermal deposition would be multiplied times the new concentration to yield exposures or doses expected in the new setting.(15, 16) The equivalent dose of formulated product for mixer-loaders (labeled dust in column H of Table 3a) is not much different than the dose of active ingredient because the concentration of the measured active ingredient was 80% of the compound being used and deposited.
        • However, in the second cited case (Table 3b) where the measured chemical comprised only 0.12% of the aqueous mixture to which the associated spray applicator was exposed, the resulting equivalent dose is most appropriately expressed in grams of mist (although a portion of the dose could have come indirectly from contaminated surfaces).