Example Exposure Limits

When dealing with issues of flammability, let s say that OSHA uses a limit of 10 percent LEL (lower explosive limit) as a safe level. In this case, a more progressive company may believe that the OSHA level will not provide employees (or the company) with adequate protection and chooses to adopt a level of 5 percent LEL or lower. 

As another example, let s use chemical exposure limits. OSHA uses a PEL (permissible exposure level) of 1 ppm (part per million). A company might find it advantageous to choose a level of less than 1 ppm to ensure that exposure levels stay low. Taking a stance that is more stringent than current government standard levels provides companies (and workers) with an extra level of confidence that workers will not be injured or have their health adversely affected by exposure. 

Some companies believe that setting their sights on levels that are more stringent than government guidelines ensures that even if a safety system fails, workers will not be overexposed and the company will not be placed in a compromised situation. 

Some in-house guidelines are not based on government regulations but on company in-house expertise or general experience. Over time, successful companies that repeatedly perform work activities develop expertise. With this developed expertise comes the development of in-house 

OSHA has published a DRAFT rule that deals with safety programs. Under this proposed rule, each employer must set up a safety and health program to manage workplace safety and health to reduce injuries, illnesses, and fatalities by systematically achieving compliance with OSHA standards and the General Duty Clause. The proposed rule is called the DRAFT PROPOSED SAFETY AND HEALTH PROGRAM RULE 29 CFR 1900.1 Docket No. S H-0027. There are five core elements that OSHA proposes the programs have  

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Because pulp bleaching agents are, for the most part, reactive oxidising agents, appropriate precautions must be taken in their handling and use. For example, it is important to ensure that the threshold limit values (TLV) (20) in Table 2 are not exceeded in the workplace air. These are airborne concentrations in either parts per million by volume under standard ambient conditions or mg per cubic meter of air. They "represent conditions under which it is beUeved that nearly all workers may be repeatedly exposed, day after day, without adverse effect" (20). TWA refers to a time-weighted average for an 8-h workday STEL is a short-term exposure limit or maximum allowable concentration to which workers can be continuously exposed for 15 minutes. 

The easiest means for assessing occupational exposure hazards associated with materials used in a process is through the use of Permissible or Occupational Exposure Limits (OEL or PEL) which go by a variety of names for example, TLV (U.S. - American Conference of Government Industrial Hygienists), MAK (Germany), or individual company established values. Occupational exposure limits are usually set based on a combination of the inherent toxicological hazard of a chemical and a series of safety factors such as intraspecies variability in test results, nature and severity of the effect, adequacy and quality of... 

A wide variety of commercial equipment is available for detection of hazardous chemicals, including a number of chemical warfare agents. For example, ion mobility spectroscopy is used to detect nerve, blister, and blood agents. The Chemical Agent Monitor is a portable, hand-held point detection instrument that uses ion mobility spectrometry to monitor nerve or blister agent vapors. However, minimum detection limits are approximately 100 times the acceptable exposure limit for nerve agents, and approximately 50 times the acceptable exposure limit for blister agents. 

These two definitions reflect two sides of the same situation. In this book, the term critical effect(s) will be used for the hazard/effect considered as being the essential one(s) for the purpose of the risk characterization, e.g., for the establishment of a health-based guidance value, permissible exposure level, or Reference Dose. It should be noted that the critical effect could be a local as well as a systemic effect. It should also be recognized that the critical effect for the establishment of a tolerable exposure level is not necessarily the most severe effect of the chemical substance. For example, although a substance may cause a serious effect such as liver necrosis, the critical effect for the establishment of, e.g., an occupational exposure limit could be a less serious effect such as respiratory tract irritation, because the irritation occurs at a lower exposure level. 

Exposure limits for specific activities have been further limited. For example. Appendix I, Title 10, Part 50, Code of Federal R ula-tions recommends a design limit of 0.1 mSv )r (10 mrem jr ) for the maximum calculated radiation dose to any one organ in an individual at the site boundary of a commercial light water nuclear power plant (NRC, 1984). 

TLV The TLV or Threshold Limit Value refers to a safe level of exposure by inhalation. The definition was established by the American Conference of Governmental Hygienists. There are several variations or criteria levels for the TLV. As an example, hydrogen sulfide has a TLV for short-term exposure limits (STEL) of 15 minutes of only 5 ppm. Comparing this to the TLV-STEL of 400 ppm for carbon monoxide provides an indication of the need to be extremely careful when H2S is suspected. Under OSHA Standards, and particularly on MSDS (Material Safety Data Sheets) compounds are associated with a time weighted average (TWA) TLV, which is the allowable concentration for an 8-hour continuous exposure period. For firefighting purposes, the short-term exposure is likely more realistic. 

In this manner, a nearly universal and very nonselective detector is created that is a compromise between widespread response and high selectivity. For example, the photoionization detector (PID) can detect part-per-billion levels of benzene but cannot detect methane. Conversely, the flame ionization detector (FID) can detect part-per-billion levels of methane but does not detect chlorinated compounds like CCl very effectively. By combining the filament and electrochemical sensor, all of these chemicals can be detected but only at part-per-million levels and above. Because most chemical vapors have toxic exposure limits above 1 ppm (a few such as hydrazines have limits below 1 ppm), this sensitivity is adequate for the initial applications. Several cases of electrochemical sensors being used at the sub-part-per-million level have been reported (3, 16). The filament and electrochemical sensor form the basic gas sensor required for detecting a wide variety of chemicals in air, but with little or no selectivity. The next step is to use an array of such sensors in a variety of ways (modes) to obtain the information required to perform the qualitative analysis of an unknown airborne chemical. 

Occupational exposure to chemical substances almost invariably involves multiple chemicals. That situation may result in PK interactions, which may affect the relationship between the atmospheric concentration of the parent chemical and the associated biomarker concentration (Viau 2002). For example, such an interaction is known to occur between ethylbenzene and the xylene isomers (Jang et al. 2001). Commercial xylene contains about 20% ethylbenzene, which modifies the slope of the relationship between urinary methylhippuric acid (MHA) and airborne xylene concentrations. That kind of interaction is unlikely at the subparts-per-million exposure concentrations seen in the general population. But because the BEI for MHA was obtained from the relationship observed after exposure to commercial xylene, thereby taking the interaction into account, the slope of the relationship cannot be extrapolated to the subparts-per-million range. Similar PK interactions have been observed for other mixtures but only at concentrations nearing or exceeding the occupational exposure limits (Viau 2002), so it would be a priori reasonable to consider extrapolation of the relationship between biomarker concentrations and those of their parent chemicals. For example, Tardif et al. (1991) demonstrated that, provided inhalation exposure to a mixture of toluene and xylene was kept below their airborne occupational exposure limits, there were no PK interactions between the compounds that affected the linear relationship between airborne parent-chemical exposure and urinary-metabolite concentrations. However, such an interaction was apparent at higher concentrations. 

Promulgate tables of concentration limits of hazardous substances that cause stochastic or deterministic effects in exempt and low-hazard waste and the rules for using the tables. For example, concentration limits of substances that cause deterministic effects should include an identification of the organ or organs at risk from exposure to each substance, so that the risk index for multiple substances that cause deterministic effects can be evaluated properly. 

The SSD example shows that one model, the SSD, can be considered sufficiently predictive for simple questions (e.g., Is the HC5 protective of community responses ) but of more limited — or even insufficient — predictive capacity for specific assessment questions such as those regarding the effects of exposure on biodiversity. This shows that general conclusions on the validity of a model cannot be drawn. For the extrapolation methods for which validation studies have been done, the examples (and limitations) have been provided in the preceding chapters. 

The quality of an assessment—and the degree of uncertainty associated with it—should be evaluated in relation to the decision to be made. Is it sufficient to answer the types of questions with which we began this document For example, if an assessment predicts exposure to a contaminant to be below the acceptable or other established exposure limit, is the quality of the assessment sufficiently robust to support that finding Is it possible that the assessment outcome is uncertain enough that the true exposure has a substantial probability of being above the limit ... 

Another aspect of OELs is that they tend to decrease gradually over time as they are revised. The half-life1 of OELs displayed in Table 9.2 is an example of that. This has also been shown in other several studies, e.g. Hansson (1998) which includes a review of the Swedish OELs from 1969 to 1992. Greenberg (2004) made a review of the documentation of British asbestos exposure limits from 1898 to 2000 and Markowitz and Rosner (1995) reviewed the TLV for silica from 1935 to 1990. Both these studies show that the OELs are lowered as more information on adverse effects becomes available and the protection of worker s health is given higher priority. This aspect points at the influence of each organisation s time-frame for the update procedures. 

Benzene, toluene, ethylbenzene, and xylenes (known as BTEX) are probably the most widely used aromatics, in abimdant use in automotive fuel, as solvents or as feedstock for more complex compounds. The American Occupational and Safety Administration (OSHA) permissible exposure limit for benzene, for example, is as low as 3.26 mg m (Ippm) due to its carcinogenic nature. This value should be compared with the 10-100 Xgm of BTEX typically foimd in urban outdoor environment (Saarela et al., 2003). The photocatalytic degradation of BTEX might emit a variety of intermediate products and by-products. For example, the photocatalytic degradation... 

PEL Permissible exposure limit a legal limit set by OSHA on the length of time industrial workers may be exposed to a substance during an 8-hour time-weighted average day without adverse effects Persistence The quality of remaining for a long period of time (such as in the environment or the body). Persistent chemical substances—for example, DDT and PCBs, are not easily broken down... 

Some subheadings relate to information that is national or regional in nature, for example EC number and occupational exposure limits . Suppliers or employers should include information under such SDS subheadings that is appropriate and relevant to the countries or regions for which the SDS is intended and into which the product is being supplied. 

Class 3 solvents Solvents with low toxic potential (solvents with low toxic potential to man no health-based exposure limit is needed). Examples acetone, ethanol, and ethyl ether... 

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