Dermal Exposure Monitoring

The following list (disregarding surface contamination) compares the advantages and disadvantages of the two categories.

ENVIRONMENTAL MONITORING: Air Sampling and Dermal Dosimeter Measurements
- ADVANTAGES enables one to:
  - differentiate among multiple routes of exposure
  - differentiate exposure by specific tasks
  - identify location on the body of high exposure
  - evaluate protection from clothing
  - sample non-invasively.
- DISADVANTAGES usually require:
  - some uniformity of deposition density onto the skin
  - temporal stability of the compound on the collection media
  - more intervention with the user while working than biomonitoring
  - a controlled dose database to interpret results.
BIOLOGICAL MONITORING: Blood or Urine Measurements
- ADVANTAGES enables one to:
  - integrate all routes of exposure
  - assess near-term historical exposure
  - measure absorbed (albeit metabolized) dose
  - sample with less intervention during work.
- DISADVANTAGES usually require:
  - freedom from biologic interferences or cross reactivity
  - invasive sampling from blood among individuals
  - a more sensitive analytical method than environmental monitoring
  - a dose-metabolism database to interpret results.
It is important to realize that direct and indirect methods are mutually antagonistic; their simultaneous use should be avoided.⁽⁵⁾
In the field, covering all or a large or important portion of the skin with dosimeters will occlude the actual deposition and absorption of a dose expected to be measured indirectly. If both dosimetry and biomonitoring are used, their results are likely to be inversely correlated.⁽⁶⁾
Of their advantages and disadvantages discussed above, perhaps the most important issue is having sufficient sensitivity for either method to quantify dermal exposures at the level of interest. If only one method is sensitive enough, the "choice" is simple. If both an environmental (dermal) and biological method is available, can both detect doses at the level of interest? And which would detect the lowest dose? A hypothetical example demonstrates both the decision method and why environmental methods tend to be more sensitive by factors of at least 3 to more than 30 fold.

Historical Uses and Interpretations of Environmental Monitoring:

Dermal monitoring using gauze pad dosimeters was originally described in 1962 by Durham and Wolfe.⁽³⁾ They used these dosimeters almost qualitatively to assess agricultural pesticide applicator's exposure. It was nearly twenty years before such uses became more quantitative in studies to assess harvester's exposure during "reentry" to pesticide treated fields by Spear et al.^(4-5)
A good summary of dermally absorbable residues was presented by Popendorf and Leffingwell.⁽⁵⁾ Further studies of exposures to pesticide applicators have been summarized by Ness.⁽²⁾
While the early studies by Durham and Wolfe⁽³⁾ showed the dermal route to be important, it wasn't until the methods became more quantitative that the over-whelming ratio became apparent.^(4-5)
A consistent pattern in virtually all such studies is that the dermal dose exceeds the respiratory dose by ratios between 50 and 1000 to 1. A dermal to airborne dose ratio of 100:1 is a good first approximation for the many open uses of low volatility compounds studied thus far.

The reason such monitoring began with pesticides relates to their low vapor pressures and high dermal absorption rates.

Vapor pressure is the intrinsic physical determinant of the rate of evaporation of a chemical; it is equivalent to the maximum concentration of a chemical as a vapor at a given temperature.⁽²⁰⁾
In the workplace, vapors are normally diluted to a fraction of this concentration, at least 100 fold and sometimes by as much as 10⁵. As long as the dilution is equal to or greater than the ratio between a chemical's vapor pressure and its permissible exposure limit (PEL or similar TLV), the worker will not be over-exposed.
This ratio has been called the Vapor Hazard Ratio by Popendorf ⁽²⁰⁾, see Table 1. The Vapor Hazard Ratio for many pesticides in Table 1 is 10 or less; only a few non-pesticidal compounds have a VHR in the same range.
It doesn't take much fresh air to dilute the concentrated vapors at their source 10 or even a 100 fold to below their allowable exposure limits; thus, over-exposures to pesticide vapors are rare (except for fumigants, as their name implies, and in poorly ventilated greenhouses).
It is also difficult to exceed a pesticide's exposure limit when it is sprayed as an aerosol. During agricultural field applications:
- A pesticide is typically diluted with water to a concentration of about 0.1 to 0.2 percent active ingredient.
- Thus, to exceed an 8-TWA exposure limit of 0.1 mg/m³ with a 0.2% solution requires a total aqueous aerosol concentration of 50 mg/m³. While that may occur briefly on rare occasion to any applicator, it is very unlikely to be anywhere near continuous in any workplace.
- Such levels as a spray would approximate a shower; someone exposed to 50 mg/m³ as a solid power or dust would experience rapid clogging of their nose and Irritation of their throat before experiencing an overdose of the active ingredient.
- Heating a pesticide (or any organic compound) without combustion will Increase its volatility⁽²⁰⁾; however, combustion has been shown to breakdown a pesticide's structure.⁽²¹⁾
While a great deal of early research and a lingering emphasis in worker pesticide education has been directed toward the respiratory route of exposure, respiratory doses as later measured were not sufficient to explain the frequency of acute health effects observed following the introduction of organophosphate insecticides around 1950.
Commercial insecticides are selected for their ability to be absorbed by insects. As it turns out, these chemicals are also easily absorbed through human skin.
- Absorption rates of 10% to 30% were reported for some early and common insecticides.⁽²²⁾
- Various mathematical models of skin absorption have been developed based on the physical nature and interaction of skin and individual molecules.^(23-25) Most of these models use a commonly reported ratio between a compound's solubility in octanol versus water.
- Generalizing based on the above relationships, if a compound's dermal absorption is 10% or more, one can be quite sure that dermal doses will predominate in most settings.
- For compounds with absorption of 1 to 10%, the balance will depend upon volatility. The airborne route is more likely to be predominant for compound's with less than 1% absorption.