Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants Volume 5 (2008) / Chapter Skim
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19 Xylenes
19 Xylenes
Pages 356-386
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From page 356... ...
-- whose structures are shown below. Commercial xylenes (also called mixed xylenes)
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From page 357... ...
for xylene were originally developed and published in Volume 3 of this series, Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, for exposure durations of 1 h, 24 h, 7 d, 30 d, and 180 d (Garcia 1996)
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From page 358... ...
As the NOAEL for throat irritation is much lower than that for eye irritation, eye irritation did not drive the SMAC for 1- and 24-h exposure durations. Similarly, as the NOAEL for throat irritation is much lower, the end point of dizziness did not drive the SMAC for these durations.
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(1975) in which a subchronic intermittent inhalation exposure study of dogs and rats (180, 460, or 810 ppm of commercial mixed xylenes for 6 h/d and 5 d/wk for 13 wk)
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On the basis of these ratings and overall significance and importance, 50 ppm is considered as a minimal LOAEL. Pulmonary function, nasal swelling, inflammatory markers in nasal lavage, and color vision (color confusion index)
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resulted in adverse effects, this concentration would be considered a LOAEL for 4 wk of discontinuous exposure. Because the measurements were made 2 wk after exposure ceased, when all xylene should have been eliminated from the system, it appears that xylene exposure resulted in some potentially persistent neurologic effects on sensorimotor functions.
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inhalation exposure of 8-mo-old male Wistar rats to m-xylene, at concentrations of 100 and 1,000 ppm, on changes in electroencephalogram (EEG) recordings and on spatial learning in an eight-arm radial maze.
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From page 363... ...
(1994) reported that, in adult male Long-Evans rats exposed to mixed xylenes at 1,800 ppm, 6 h/d for 5 d, the reflex modification audiometry data collected 5 to 8 wk postexposure indicated a hearing loss in the middle-frequency ranges (8 and 16 kHz)
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(1989) , in which male Sprague-Dawley rats were exposed by inhalation to mixed xylenes at 1,000 ppm for 61 d, 18 h/d, 7 d/wk, no alterations in testes, accessory glands, or circulating male hormone levels were noted.
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The toxicity literature on xylene exposure discussed in this document is summarized in Table 19-2. RATIONALE FOR THE 1,000-d AC ACs were determined according to the Subcommittee on Guidelines for Developing Spacecraft Maximum Allowable Concentrations for Space Station Contaminants (NRC 1992)
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100 pure xylene 4h humans, male Significant adverse effect on choice reaction time and Dudek et al. 1990 (purity not volunteers, n simple reaction time (psychomotor efficiency)
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100 m-xylene 6 h/d, 5 d/wk, Wistar rat, Gralewicz and Altered passive avoidance test, and delayed acquisition 4 wk male, 11 per Wiaderna 2001 of two-way active avoidance were noted. group 800, 1,000, mixed xylenes 7 h/d, 7 d/wk, weanling 3- Mixed xylenes at 800 ppm caused marked hearing loss Pryor et al.
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0, 60, 250, xylene 6 h/d for 131 d male and No mortalities and no treatment-related effects on Biodynamics 1983 500 mixture before mating female mating, fertility, pregnancy indices, mean duration of and during a Sprague- gestation, mean litter size, or mean pup weight noted. 20-d mating Dawley rats Also, no effects were observed on reproductive organs period, for or sperm count of male rats.
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, 21 7 yr correlation was noted only for a few end points. (arithmetic mean)
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This was based on the decreased Rotarod performance (impaired motor function and coordination) in male rats exposed to
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JUSTIFICATION OF USE OF FACTORS OTHER THAN DEFAULT FACTORS Before deriving the revised ACs for 1 h, 24 h, 7 d, 30 d, 180 d, and 1,000 d, NASA determined some guidelines on what the uncertainty factors applied in the derivations should be. There are no controlled human-exposure studies of subchronic or chronic duration; only one occupational exposure study exists in which the nature of the chemicals the workers were exposed to is known and thus is a study that can be used.
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EPA used the time-weighted blood concentrations of xylene from the discontinuous exposure protocol used in the Korsak et al. study and applied to the human xylene PBPK to derive an exposure concentration of xylene that will lead to comparable blood concentrations in a continuous exposure scenario and result in the same level of neurotoxicity as found in the rat.
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NASA believed that it may not be beneficial to do so with the graphic data included in the studies considered in this document. Revised Derivations of 1-h AC Three human subject studies were considered for deriving ACs for acute exposure (Olson et al.
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, found prolonged reaction time (in both SRT and ChRT) in 10 male volunteers exposed for 4 h.
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could not be used because the very small sample size, lack of details, and complex exposure design made it difficult to identify a NOAEL or a LOAEL. As no other suitable human exposure study for derivation of a 7-d AC was available, a short-term animal study was considered.
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From page 376... ...
were exposed to o-, m-, and p-xylene separately at 450, 900, and 1,800 ppm, 6 h/d, 6 d/wk for 13 wk. Electrophysiologic measurements for brainstem auditory evoked response recordings at threshold frequencies of 2, 4, 8, and 16 kHz revealed increased auditory thresholds (indicating loss of hearing)
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From page 377... ...
. They exposed male Sprague-Dawley rats to 1,000 ppm of mixed xylenes (consisting of 1.5% o-xylene, 65% m-xylene, 32% p-xylene, and 2.5% ethylbenzene)
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The authors had used m-xylene exposure concentrations of 1,000 ppm for 3 mo and 100 ppm for 6 mo. Rotarod performance was measured after 1, 2, and 3 mo of exposure (1,000- and 100-ppm groups)
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A NOAEL of 450 ppm of p-xylene for ototoxicity was identified in this study. For derivation of the 180-d AC, in addition to the adjustment for discontinuous-to-continuous exposure, a species factor of 3 and a time extrapolation factor of 91 d/180 d following Haber's rule were used.
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. Because these effects have been well documented in several studies, and in the absence of robust chronic exposure data in humans or in rodents, it was decided to use a time extrapolation factor from subchronic studies to chronic studies.
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From page 381... ...
Hence, data are needed for neurobehavioral end points (such as ChRT and SRT that can be assessed by a computerized test battery) , so that a physiologically based pharmacodynamic model can be developed to predict reasonable exposure concentrations for various durations of expected exposures using blood levels as surrogates for target (brain)
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:614-624. ACGIH (American Conference of Governmental Industrial Hygienists)
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Pp. 321-344 in Spacecraft Maximum Allowable Concentra tions for Selected Airborne Contaminants, Vol.
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1984. Emergency and Continuous Exposure Guidance Levels for Selected Airborne Contaminants, Vol.
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1987. Hearing loss in rats caused by inhalation of mixed xylenes and styrene.
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From page 386... ...
1991. Electroencephalographic findings during experimental human exposure to m-xylene.
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