How Chemical Mixtures Shape Absorption in Agriculture and Industry
Imagine a farmer spraying pesticides under the hot sun or a firefighter battling blazing chemicals. In both cases, their skin becomes a potential gateway for toxic substances to enter their bodies.
While inhalation risks often grab attention, dermal absorption represents a silent, significant exposure route—especially when chemicals mix rather than act alone. This article delves into the fascinating science of how agricultural and industrial chemicals penetrate the skin, why chemical mixtures alter absorption dynamics, and how researchers use porcine skin to mimic human exposure. Understanding these processes isn't just academic; it's critical for protecting those on the front lines of chemical exposure 7 .
The skin is the body's largest organ, accounting for about 15% of total body weight and serving as the primary barrier against environmental exposures.
The skin is our largest organ, and its outermost layer, the stratum corneum, serves as the primary barrier against external threats. Composed of dead cells and lipids, it functions like a brick wall, limiting the passage of chemicals. However, this barrier isn't impervious.
The "brick and mortar" structure of the skin's outermost layer provides the primary barrier function against chemical penetration.
Real-world exposure rarely involves pure chemicals. Instead, mixtures—like pesticides in solvents or hydrocarbons in fuels—interact in ways that can amplify or inhibit absorption. This complexity drives researchers to explore not just single chemicals but mixture effects 2 .
In dermal absorption studies, porcine skin is the gold standard substitute for human skin. Here's why:
Pig skin closely mirrors human skin in stratum corneum composition and follicular organization.
Easier to obtain than human skin, reducing variability between experiments.
Researchers use dermatomed porcine skin (shaved to 200–500 µm thickness) in diffusion cells to simulate real-world exposure. This approach allows for controlled, ethical, and reproducible experiments 1 2 .
A pivotal study examined how chemical mixtures influence the absorption of arsenic and nickel. Here's how it worked:
Skin Preparation
Dosing
Sampling
Analysis
Fresh dermatomed porcine skin was mounted in flow-through diffusion cells, maintaining physiological temperature (32°C) and hydration. Solutions containing arsenic-73 or nickel-63—alone or mixed with other chemicals—were applied to the skin surface. Receptor fluid (mimicking blood) was collected periodically to measure penetrated radioactivity. Absorption rates were calculated based on cumulative penetration over time 6 .
| Metal | Absorption Alone | Absorption with Mixture | Increase |
|---|---|---|---|
| Arsenic | Baseline | +33% | Significant |
| Nickel | Baseline | +20% | Significant |
Table 1: Effect of Chemical Mixtures on Metal Absorption
Chemical mixtures don't just add up; they interact. For example:
Like acetone or ethanol can extract skin lipids, enhancing penetration of other chemicals.
In formulations may increase solubility or disrupt the stratum corneum.
In jet fuel (JP-8), toluene penetrates 100 times faster than dodecane .
Researchers use Quantitative Structure-Permeability Relationships (QSPR) to predict absorption. These models incorporate descriptors like molecular weight and lipophilicity but struggle with mixtures. Recent advances add a "mixture factor" to account for interactions, improving accuracy 2 4 .
| Property | Effect on Absorption | Example |
|---|---|---|
| Low molecular weight | Higher absorption | Naphthalene (35% absorbed) |
| High lipophilicity | Penetrates lipid barrier but may get stuck | Benzo[a]pyrene (0.03% absorbed) |
| Mixture components | Solvents or surfactants enhance partner absorption | Arsenic + toluene |
Table 2: How Chemical Properties Affect Absorption
Pesticide applicators face high dermal exposure during:
Handling concentrated products
Spraying leads to deposition on exposed skin
Often overlooked source of exposure 7
Studies show that re-entry workers (e.g., harvesters) sometimes absorb more pesticides than applicators due to longer exposure and less protective gear 7 .
Firefighters encounter complex mixtures like polycyclic aromatic hydrocarbons (PAHs) in smoke. Using artificial sweat to simulate sweating, researchers found:
| PAH | Molecular Weight | % Dose Absorbed |
|---|---|---|
| Naphthalene | 128 g/mol | 35.0 ± 4.6% |
| Phenanthrene | 178 g/mol | 6.8 ± 3.2% |
| Benzo[a]pyrene | 252 g/mol | 0.03 ± 0.04% |
Table 3: Absorption of PAHs in Artificial Sweat (Porcine Skin)
Sweat increases absorption of lower-weight PAHs, emphasizing the need for immediate decontamination.
Dermal absorption research relies on specialized tools and reagents. Here's a breakdown of essentials:
Sourced from dorsal areas of pigs; dermatomed to 200–500 µm thickness.
Function: Mimics human skin barrier properties 1 2 .Apparatus with donor and receptor chambers.
Function: Simulates continuous perfusion like blood flow 1 9 .Stabilized at pH 4.5.
Function: Simulates sweaty skin conditions for exposure studies 1 .Understanding dermal absorption—especially with chemical mixtures—is vital for assessing health risks and shaping safety protocols. Key takeaways include:
They can enhance absorption, increasing toxicological risks.
Swift washing reduces exposure significantly.
QSPR approaches improve risk assessment accuracy.
Regulatory agencies like the OECD and EPA now emphasize in vitro human skin studies and tiered testing strategies to set protective guidelines 9 . As research evolves, so will strategies to protect those exposed to chemicals daily—ensuring that their skin remains a barrier, not a gateway.
This article synthesizes insights from cutting-edge studies on dermal absorption, highlighting the critical role of chemical interactions and the tools used to keep people safe.