Time matters just as much as concentration. We talked last time about measuring toxicity, the “concentration” part of the equation. But what language should we use when talking about exposure times?

Acute vs. Chronic Exposure

Talk about the time aspect of the dose equation often revolves around two words: acute and chronic. We call one-time, high-level exposure “acute.” Have you ever gotten a headache from bleach fumes in an enclosed space? That’s acute exposure. Symptoms of these types of exposures often show up immediately but can appear several hours later.

We call low-level exposure over a long period “chronic.” Symptoms related to chronic exposure may not show up for 10 to 20 years. Examples of chronic exposures leading to workplace illnesses include mesothelioma, silicosis and black lung disease.

Chemical Exposure on the Job

Toxic substances typically enter our body through one of four routes:

• Inhalation: Breathing in a vapor, gas or dust, typically the most common exposure route.
• Ingestion: Eating or placing things in our mouth that may be contaminated. Most companies have that one drilling assistant or mechanic known for grease marks on his sandwich at lunch.
• Absorption: Taking a chemical into our bloodstream through our body’s biggest organ, the skin. Think of using solvents to wash your hands. They can dissolve the skin’s protective barrier of fats and oils. With natural lubricants removed, the skin is subject to irritation, disabling and possibly disfiguring dermatitis, damage and infection.
• Injection: This could involve needle sticks or lacerations from sharp objects. In our industry, it more often involves fluid injection from fuel lines or hydraulic lines. Depending on the machine, we often see hydraulic pressures about 6,000 psi (and going up). Injector pressures for diesel Tier 3 engines are about 30,000 psi. The new Tier 4 engines are 30,000+ psi. Injections of 7,000 psi and above have a 100% amputation rate. If you ever suffer an injection injury, make sure the attending physician understands that the subcutaneous exploration of the wound should parallel that of a gunshot wound. Make sure to tell the physician exactly what type of fluid was injected. On average, you have less than six hours to have an injection injury treated properly or risk severe infection and the loss of a body part.

Once a substance enters our body, it does one of three things with the chemical. We metabolize it, excrete it or store it. While our body works on that, the chemical often works on a target organ or organs — that is, the parts of the body most affected. Remember, harm may not always be at the point of contact. If a chemical or substance has a “systemic” effect, adverse health impacts may take place far away from the body's initial point of contact (like in one of those target organs). For example, benzene affects bone marrow and lead affects the central nervous system.

Remember, too, that often an exposure may involve multiple chemicals. Some substances have a synergistic effect when introduced together, meaning the total effect can be greater than the effect of each chemical by itself.

Protecting Workers from Toxic Exposures

Industrial hygienists at government agencies have come up with exposure limits, that is, the maximum amount of exposure a worker could experience in an 8-hour shift with no adverse health effects. OSHA calls theirs the permissible exposure limit (PEL). In the U.S., it is unlawful to expose workers to concentrations higher than the PEL. When workplace concentrations exceed the PEL, an employer must take measures to reduce the potential exposure for workers. However, the PEL is not the most protective exposure limit standard. (OSHA even says it hasn’t updated many PELs since 1970.) The National Institute for Occupational Safety and Health (NIOSH) also has recommended exposure limits (RELs). In many cases, NOISH limits are stricter than OSHA.

Usually, we express exposure limits in parts per million (ppm) or milligrams per cubic meter (Mg/m³). How small is one part per million? It’s about:

• 1 minute out of two years.
• 1 inch out of 16 miles
• One penny out of $10,000
• Or four drops in a 55-gallon drum

Before we wrap up, let me share other important exposure-related terms to know:

• Immediately dangerous to life or health (IDLH): OSHA defines IDLH as a situation posing an “immediate threat to life or would cause irreversible or delayed adverse health effects or would interfere with an individual's ability to escape from a dangerous atmosphere.” Think of an enclosed space with noxious gases.
• Short-term exposure limit (STEL): Usually this means up to 15 minutes four times a day with an hour break in-between exposures.
• Ceiling: If a “C” precedes the exposure limit, it means “ceiling.” At no time shall the worker go over the PEL at any part of the workday.

In my next column, I’ll help you take some of this knowledge and use it to develop a HazCom program for the chemical hazards your employees may face.

Until next time, stay safe and keep turning the right.