Personal Time-weighted Average Research Articles

Personal exposure to static and time‐varying magnetic fields during MRI system test procedures

To assess personal time-weighted average (TWA) static magnetic field and time-varying magnetic field exposure for several system testing tasks routinely conducted by engineers at an MRI manufacturing plant. Personal exposure was measured using a personal dosimeter that measured B and dB/dt in all three orthogonal directions with a 1 Hz frequency. TWA exposure was calculated and random effects and linear mixed effects models were used to assess exposure levels and variability. Whereas full-body peak (2T) and TWA (200 mT/8 h) exposure limits for the static magnetic field originally proposed by ICNIRP were not exceeded, time-varying magnetic field peaks (dB/dt) did exceed the proposed threshold value (767.9 mT/s) suggested by IEEE on several occasions at the 3.0 Tesla (T) whole-body cylindrical system. TWA exposure levels are well below the proposed limit values, but peak exposure limits are exceeded during certain procedures performed on 1.0T to 3.0T systems.

Characterizing Historical Industrial Hygiene Data: A Case Study Involving Benzene Exposures at a Chemical Manufacturing Facility (1976–1987)

This article describes how nearly 3700 air samples of benzene collected in a typical chemical manufacturing (acetic acid) facility in the United States from 1976 to 1987 were used to characterize daily time-weighted average (TWA) exposure levels. We found that those workers directly involved in manufacturing operations had likely TWA exposures to benzene of about 2.0 ppm from 1976–1981 and about 1.0 ppm from 1982–1987. These results are consistent with the improved industrial hygiene programs at chemical facilities, which often occurred following the adoption of stricter occupational exposure limits. Additionally, about 97% of all personal TWA samples had reported benzene concentrations less than 10 ppm, which was the permissible exposure limit in place prior to 1987. Because one of the primary objectives of historical workplace air sampling efforts was to understand the source of release of contaminants, a large number of short-term (typically about 1 min) area samples were also collected. Although these types of samples are often not useful for predicting human exposure without time-motion information, airborne benzene concentrations were about five- to tenfold higher for many of the short-term area samples than for the personal TWA measurements. The methodology presented here should be useful for evaluating industrial hygiene data collected after the early 1970s (after the promulgation of OSHA), and our findings support prior reports that large corporations in the United States have tended to reduce workplace exposures to airborne contaminants consistent with historical changes in occupational exposure limits.

Successful Reduction of Morticians' Exposure to Formaldehyde during Embalming Procedures

A case study of the effectiveness of upgraded ventilation engineering controls in a military mortuary facility was performed. Worst-case mortician formaldehyde exposures generated during the use of highly concentrated embalming fluid (required to meet a 2-week preservation standard for overseas case processing and return of the deceased to the continental United States) were documented. A detailed exposure evaluation via consecutive short-term exposure limit (STEL) samples facilitated characterization of the hazard potential for each distinct phase of the embalming process. After baseline screening with 3M passive formaldehyde dosimeters, a total of 145 personal and area STEL sorbent tube samples were collected during six embalming cases between 1994 and 1998. Prior to the installation of local exhaust ventilation controls, personal time-weighted average (TWA) exposure values during embalming activities were 3.19-7.69 ppm for a mean of 4.80 ppm (calculated 8-hour TWA exposures for mortician workshifts were 1.32-2.86 ppm, mean 1.93 ppm). Initial STEL exposures ranged from a low of 0.14 during preembalming body preparation to 20.89 ppm during aspiration of arterial fluids (mean = 4.16 ppm). Embalming room general area samples revealed a mean concentration of 0.76 ppm. With ventilation upgrades installed in 1997, calculated personal 8-hour TWA exposure values during embalming procedures were reduced. STEL exposures decreased to between 0.11 to 3.44 ppm (mean of 0.55 ppm); embalming room general area sample concentrations decreased to a mean of 0.089 ppm. Because occasional 15-min peak exposures continued to exceed the 2.0 ppm Occupational Safety and Health Administration STEL during tasks involving large volumes of embalming fluid or direct contact with paraformaldehyde preservative powders, general room ventilation was further upgraded to 25 room air changes per hour.

Time trends in exposure measurements from OSHA compliance inspections of the pulp and paper industry.

Time trends in employee exposures to the air contaminants measured by the Occupational Safety and Health Administration (OSHA) during compliance inspections of pulp and paper manufacturing facilities conducted between 1979 and 1997 were evaluated based on the measurement results stored in the OSHA Integrated Management Information System (IMIS) database. The IMIS database is among the largest sources of occupational exposure measurements available for occupational health research in the United States. The IMIS database contains the results of 3,568 personal time-weighted average (TWA) measurements for 171 air contaminants made at 524 establishments in Standard Industrial Classification (SIC) 26. An analysis of these measurements revealed an overall decrease in the total number of measurements made per year since 1991, and a decrease in the percentage of measurements by year that exceeded the OSHA permissible exposure limits (PELs). Linear regression analyses detected decreasing trends in the geometric mean concentrations by year for 33 of the 36 agents analyzed.

Exposure Comparison of Outdoor Asbestos Abatement Techniques in a Petroleum Refinery

Abstract This study reports on the comparison of four outdoor asbestos abatement methods which have been used at a large petroleum refinery. These methods are: negative pressure enclosure; negative pressure glove bag; and two types of partial enclosures, open top (five-sided enclosure) and windscreen/extended barricade (less than five-sided enclosure). The air monitoring results for each method are compared in terms of downwind area, personal time-weighted average, and personal excursion limits for workers abating asbestos thermal system insulation. The results show that for a large sample set (>11,000) accumulated over a 3-year period, all downwind area asbestos concentrations at prescribed distances from the abatement activity are essentially equivalent for all four abatement methods, with geometric means of 0.004 to 0.005 fibers/cc (f/cc). The data indicate that negative pressure enclosures provide no added protection for bystanders, as compared with the other abatement methods normally used in outdoor petroleum refinery environments. The data support the use of partial enclosure methods in certain situations since they yield downwind area exposure potentials that are statistically equivalent to those of the negative pressure enclosure method. The negative pressure glove bag method was shown to yield the lowest personal time-weighted average exposure potential. The geometric mean for this abatement method was 0.008 f/cc, with a geometric standard deviation of 3.1, in comparison with both negative pressure and partial enclosures, which had geometric means in the range of 0.020 to 0.023 f/cc, with a maximum geometric standard deviation of 5.6. A similar observation was made for excursion limit samples, with the negative pressure glove bag method yielding a lower average concentration than the other methods. The excursion limit geometric mean for the negative pressure glove bag method was 0.017 f/cc, with a geometric standard deviation of 2.6. This can be compared with a geometric mean of 0.031 f/cc, with a geometric standard deviation of up to 4.6 for the other three abatement methods. The results show that trained workers using adequate wetting techniques and partial enclosures can achieve downwind area concentrations equivalent to negative pressure enclosures. All techniques yielded downwind perimeter concentrations well below the proposed Occupational Safety and Health Administration permissible exposure limit of 0.1 f/cc. When it can be used in an outdoor petroleum refinery environment by experienced crews, the negative pressure glove bag method yields the lowest personal asbestos exposure concentration in comparison with negative pressure and partial enclosures.

Exposure of Workers Engaged in Furniture Stripping to Methylene Chloride as Determined by Environmental and Biological Monitoring

Abstract In response to reports of adverse reproductive effects of exposure to methylene chloride, investigators from the National Institute for Occupational Safety and Health conducted an industrial hygiene survey of a cohort of 14 furniture strippers exposed to methylene chloride in five furniture stripping shops. Both environmental and biological monitoring of the workers were conducted. Personal time-weighted average (TWA) concentrations of methylene chloride were determined. Pre- and postshift alveolar breath samples were analyzed for carbon monoxide and methylene chloride. Postshift venous blood samples were analyzed for carboxyhemoglobin and methylene chloride. Methylene chloride TWA exposures of the workers ranged from 15 to 366 ppm, while exposures to other solvents were well below their respective evaluation criteria. The highest average exposures to methylene chloride by job category were strippers (191 ppm) followed by washers (145 ppm) and refinishers (31 ppm). Postexposure breath concentrations of methylene chloride ranged from 2.3 to 167 ppm, while blood concentrations ranged from 0.1 to 8.8 ppm. Linear regression analysis showed that the TWA methylene chloride exposure concentrations were quadratically correlated to postshift alveolar breath (r = 0.98) and blood concentrations (r = 0.87) of methylene chloride. This high degree of correlation between the biological and environmental concentrations suggested that the major route of exposure was inhalation. Serious problems with ventilation of sprayer-based furniture stripping operations were found. Deficiencies in work practices and personal protective equipment were also noted. Recommendations are made for reducing exposures to methylene chloride during furniture stripping.

Characterization of Worker Exposures to Methylene Chloride Resulting from Application of Aerosol Glue in the Asbestos Abatement Industry

Abstract Adhesives containing large percentages of methylene chloride are commonly utilized in the asbestos abatement industry. The adhesive is used to seal polyethylene sheeting to the underlying surfaces, which in turn is used to contain the work area. When completely contained, the work area is essentially a confined space. During subsequent asbestos removal procedures, High Efficiency Particulate Air filter (HEPA)-equipped exhaust systems provide dilution ventilation to the work area, exhausting 1000 to 2000 cubic feet of air per minute per machine. This study characterizes worker exposure levels to methylene chloride when applying these adhesives during the construction of asbestos abatement containments. Personal samples were collected with both 3M organic vapor monitors and standard SKC charcoal tubes. Analysis of the sampling media was conducted by gas chromatography according to NIOSH Method 1005. Exposure periods, ranging from four to six hours, corresponded to a personal time-weighted average (TWA) of 0.09 to 270 ppm. This corresponded to an eight-hour TWA range from 0.06 to 140 ppm, respectively. Short-term exposure limits (STEL), 15 minutes in duration, ranged in concentration from non-detectable to 1294 ppm. Exposure concentrations greatly varied depending upon dilution ventilation, exposure period, adhesive quantity, and room volume. Sample results revealed that use of large quantities of adhesive in a confined area can result in high concentrations of methylene chloride. It is recommended that the HEPA-equipped exhaust systems, primarily designed to control airborne asbestos fiber levels during subsequent removal procedures, be utilized to provide dilution ventilation to the work area when applying these adhesives. It is a current Environmental Protection Agency and Occupational Safety and Health Administration recommendation to start the HEPA-equipped exhaust systems after the work area is completely sealed off and the decontamination unit has been constructed. If these systems are activated at the beginning of the preparation phase, they will create dilution ventilation that can decrease methylene chloride concentrations by a minimum of fourfold. In addition, air purifying respirators equipped with organic vapor cartridges or supplied air respirators should be worn by the workers to reduce their exposure levels even more. Finally, the aerosol manufacturers should substitute the methylene chloride in these products with a suitable, less toxic replacement.