Time Weighted Average

Permissible Exposure Limits (PEL's) are actual legal limits determined by OSHA that limit the amount or concentration of exposure to certain chemical substances or physical agents (i.e noise) to employees while working. American Conference of Governmental Industrial Hygienists (ACGIH) as many of the permissible exposure limits that were set into place by OSHA were the actual threshold limit values established by ACGIH.

As mentioned in my previous post, most of the chemical exposures are expressed in parts per million. While OSHA is behind the actual adoption of these regulations I think it is important to give credit to the

Now as I also mentioned in previous posts, there are several ways exposure can be expressed. Be it Short Term Exposure Limits, Ceiling Limits, or what I plan to further discuss, Time-Weighted Averages. A Time-Weighted Average (TWA) is basically the average exposure to any hazardous material that an employee could expect to receive under standard working conditions and considering an 8 hour work day or 40 hours per week work week. Time weighted average allows for peaks and valleys to generate a single average concentration.

The average exposure is compared overtime to the occupational exposure limit to check for excursions. Excursions in exposure levels may exceed 3 times the TWA for no  more than a total of 30 minutes during a workday, and for no reason should they ever exceed 5 times the TWA, considering that the TWA is not exceeded. Basically if put simply, a worker can be exposed to higher concentrations so long as it does not exceed a ceiling, the exposure is not occurring for a long period of time, and the number of times that the worker is exposed to that dose of the hazard is kept to a bare minimum.

To calculate the TWA you multiply the concentration (C) by time (T) and then divide by this time.

The goal of having these permissible exposure limits and observing the time weighted averages is to limit the potential for adverse health effects while working on the job. Just to get a glimpse on how these limits are really protecting workers you can look as OSHA's ruling that limits exposure to respirable crystalline silica. OSHA estimates that the rule will save over 600 lives and prevent more than 900 new cases of silicosis each year, and that's by limiting exposure to merely one substance. 

Occupational Exposure Limits

In my last post I discussed Dose-Response and how exposure to certain substances (be it chemical, drug, or general toxic waste), can cause an effect on the exposed individual. In the past several decades we began to realize that many individuals have been exposed to certain chemicals in the workplace and that a number of these exposures have led to very serious and negative implications on the individual’s health. One such example involves asbestos and the development of a deadly form of cancer known as Mesothelioma in those individuals who were exposed to this material. This is merely one of many different examples and it was at this time that government knew it was time to take action. This led to the development of several standards geared towards protecting future generations of working class men and women.

When referring to chemical or other hazardous substances, performance standards for exposure limits are better known as Occupational Exposure Limits or OEL's. There are several types of OEL's, the most typical being the Time Weighted Average. This allows for ups and downs but considers the single average concentration and number of excursions that take place during worker exposure. Other OEL's include Ceiling or Short Term Exposure Limits (STEL) and standards for Vibration, Sound, Heat/Cold, Radiation, Strain, and Biological Exposure Indices. By definition, "An occupational exposure limit is an upper limit on the acceptable concentration of a hazardous substance in workplace air for a particular material of class of materials." 

The two biggest organizations created and tasked with developing these standards are the Occupational Safety and Health Administration (OSHA) and the American Conference of Governmental Industrial Hygienists (ACGIH). The American Conference of Governmental Industrial Hygienists are known for publishing their Threshold Limit Values or TLV's while the Occupational Safety and Health Administration refers to their limits as permissible exposure limits or PEL's. For more information on TLV's you can visit here. According to OSHA's website, 4,679 workers died on the job in 2014. As previously stated, all of these limits, standards, and guidlines are created with the intention of reducing this number of accidents and deaths in the workplace.

For both TLV's and PEL's exposure is generally expressed as parts per million (ppm) for gases and vapors and milligrams per cubic meter (mg/m3) for solids. When considering the OEL you are observing airborne concentration that workers may be exposed to and you want to know how long they can be exposed to this without adverse effects considering 8 hour shifts 5 days per week.

Dose-Response

When discussing dose response and its effect on human life, I think it is important to first understand the concept of the dose-response relationship. The dose-response relationship basically explains how the toxicity of a substance is not only dependent on that of its toxic properties, but also on the amount of time and dose that one is exposed to this substance. The medical definition of Dose-Response can be found here on Merriam Webster. It is common sense if you really think about it, the longer you are exposed to a toxic substance the more harm it will do to your body regardless of the concentration.

One note to add while discussing concepts, is the term known as the threshold dose. This is the dose or exposure level, below which he adverse effects of a substance are not expressed by the exposed population. When we refer to exposure, we are referring to the actual amount of the substance that will be absorbed into the body when exposed and not just the amount of the substance that is in the surrounding environment. Dose in some ways also takes into account the mass of the individual being exposed. Although it may seem that I am discussing dose-response in a negative light there are also many positives that come from studying this concept. Response to a substance may lead to the curing of a disease or simply relieving pain but in order for these to take place, the exposure levels must fall within the range of certain upper and lower bounds regarding the dosage. An example of a dose response curve is provided below.

The problem with illustrating the threshold dose is that a dose-response curve is not always a linear relationship. In fact, in most cases the relationship is a complex logarithmic or hyperbolic curve. One example of a Dose-Response Curve can be found here. On the other hand, the nice thing about dose-response curves is that they can contrast two or more different substances with varying toxicities. From here comparisons of the graphs can be useful when evaluating toxicity. In general, the greater the slope of the graph, the more toxic the material is. For a little further explanation on how the graph is plotted and what it contains you can click here.