The Occupational Safety and Health Administration’s hazard communication standard includes classifications for acute toxicity due to skin contact, skin corrosion/irritation and skin sensitization.

A hazard that is not mentioned is that some chemicals can absorb through the skin at levels contributing to a worker’s overall exposure.

The chemical might not be toxic, irritating or a sensitizer, but skin contact should be considered when evaluating exposures. That’s why OSHA’s standard on air contaminants includes skin notations for some of the chemicals on its tables of permissible exposure limits.

If you use one of these chemicals, it’s a good practice to explain this exposure route when you train on the chemical’s hazards. The National Institute for Occupational Safety and Health estimated that more than 13 million U.S. workers are potentially exposed to chemicals that can be absorbed through the skin.

Skin absorption

The skin is the body’s largest organ, accounting for more than 10 percent of body mass.

Dermal absorption is the transport of a chemical from the outer surface of the skin both into the skin and into the body. The worker might not even notice that the chemical is absorbing through the skin, but in some cases, this may be the most significant exposure pathway.

Many commonly used chemicals in the workplace (i.e., pesticides, organic solvents) could potentially result in systemic toxicity if they penetrate through the skin. These chemicals enter the blood stream and cause health problems away from the site of entry.

The rate of dermal absorption depends largely on the outer layer of the skin, called the stratum corneum. This layer serves an important barrier function by keeping molecules from passing into and out of the skin, thus protecting the lower layers of skin. The extent of absorption is dependent on the following factors:

  • Skin integrity (damaged vs. intact)
  • Location of exposure (skin temperature, thickness and water content of stratum corneum)
  • Physical and chemical properties of the hazardous substance
  • Concentration of a chemical on the skin surface
  • Duration of exposure
  • The surface area of skin exposed to a hazardous substance

Pathways for absorption

Research indicates skin absorption occurs via diffusion, the process whereby chemical molecules spread from areas of high concentration (on the skin surface) to areas of low concentration (cells and blood vessels under the skin surface). Diffusion sets up a flow through the skin as the chemical naturally spreads to even out its concentration level.

Three mechanisms by which chemicals diffuse into the skin have been proposed:

  • Intercellular lipid pathway
  • Transcellular permeation
  • Through the appendages

Intercellular lipid pathway: The stratum corneum consists of cells known as corneocytes. The spaces between the corneocytes are filled with substances such as fats, oils or waxes known as lipids. Some chemicals can penetrate through these lipid-filled intercellular spaces through diffusion.

Transcellular (cell-to-cell) permeation: Chemical molecules diffuse directly through the corneocytes into deeper layers of the skin.

Through skin appendages (i.e., hair follicles, glands): Chemical molecules flow into the spaces around hair shafts and glands. This pathway is usually insignificant because the surface area of the appendages is very small compared to the total skin area. However, very slowly permeating chemicals may employ this pathway during the initial stage of absorption.

Skin disorders

Absorption through the skin helps explain some skin disorders. Contact dermatitis, defined as an inflammation of the skin resulting from exposure to a hazardous agent, is the most common form of reported skin disorders. There are two types of contact dermatitis:

Irritant contact dermatitis: An inflammation of the skin caused by direct damage following exposure to a hazardous agent. The reaction is typically localized to the site of contact.

Allergic contact dermatitis: An inflammation of the skin caused by an allergic reaction triggered by dermal contact to a skin allergen. Once a worker is sensitized to the allergen, subsequent exposures (even to small amounts of the chemical) can result in skin rashes in parts of the body that had no direct contact with the chemical.

As an employer, you want to be aware of these considerations so you can better position your workers to be aware, as well.