THIS COLUMN AND THE NEXT are a quick review of industrial hygiene—about how the human body can defend itself against bodily contact with hazardous chemicals and what happens when it doesn't.

The key issue, bodily contact, refers to the three chief routes through which a chemical can enter the human body: inhalation, skin contact, or ingestion. In the workplace, the most common are skin absorption and inhalation.

Our bodies were not "designed" to protect us from hazardous chemicals. We were designed with an immune system to protect us from pathogens (infecting agents) and from intrusion by foreign substances. Reflexes (such as vomiting) and sensation (such as itching or burning) support the body's effort to protect itself from harm.

Inhalation
Odor, a property of nearly all chemicals and certainly solvents commonly used in industrial cleaning, is the first line of defense for humans. Normally, humans can smell the presence of solvent at levels well below the point at which the solvent can start to produce harm. The cardinal rule: If you can smell a chemical, use protective equipment to prevent yourself from being exposed to it.

Skin Contact
The skin consists of two layers, the relatively thin outer epidermis and a thicker layer, the dermis. The epidermis consists of several layers of tightly packed epithelial cells that are dead and filled with keratin, a waterproof protein. The epidermis acts as the primary barrier to entry of hazardous chemicals through the skin.

Sebaceous glands in the skin produce an oily secretion called sebum, which consists of a number of organic acids that maintain the pH of the skin between three and five. With its low pH, intact skin not only prevents the intrusion of hazardous chemicals but also inhibits the growth of most pathogenic bacteria due to its low pH.