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EC number: 265-198-5
CAS number: 64742-94-5
A complex combination of hydrocarbons obtained from distillation of aromatic streams. It consists predominantly of aromatic hydrocarbons having carbon numbers predominantly in the range of C9 through C16 and boiling in the range of approximately 165°C to 290°C (330°F to 554°F).
XYLENE No. 1
condition: no exposure-related mortality, and animals in all five groups
remained in good health throughout the study.
weight: no statistically significant differences.
evoked response: thresholds were increased at all four frequencies from
wk 4 onwards in animals exposed to 2000 ppm or 1000 ppm mixed xylene no.
1. The effect was greatest increase in the 2000 ppm group where deficits
of 35 dB at 2 kHz, 39 dB at 4 kHz,42 dB at 8 kHz and 48 dB at 16 kHz
were recorded at the end of the exposure period with no recovery after 8
wk. Thresholds in the 1000 ppm were increased non-significantly by 13 to
19 dB. No shift was observed in the controls or the groups exposed to
250 or 500 ppm mixed xylene no. 1.
Morphology: exposure to 2000 ppm mixed xylene resulted in an almost
complete loss of hair cells in the three rows of the OHC (greatest in
row 3) with an approx. 6% loss of inner hair cells (IHC) also.
Pronounced loss of OHC was also apparent at 1000 ppm (approx. 50%
reduction in cells of the third row) but no of IHC. Exposure to 250 and
500 ppm caused significant losses (up to 60% in the 5-8-kHz region) in
the third row of the OHC in one animal from each group. There was no
significant hair cell loss in the controls. 250 ppm mixed xylene no.1
was therefore considered a LOAEL by the study authors. The calculated
EC50 was 1114 ppm.
XYLENE No. 2
condition: there was no exposure-related mortality and animals in all
five groups remained in good health throughout the study.
evoked response: thresholds were higher from wk 4 at all four
frequencies in animals exposed to 2000 ppm, increasing slightly over the
course of the study to reach 15 dB at 2 kHz, 17 dB at 4 kHz, 19 dB at 8
kHz and 22 dB at 16 kHz by wk 13. There was no recovery following 8 wk
with no exposure. No threshold shifts were present in the controls or in
animals exposed to 250, 500 or 1000 ppm mixed xylene no. 2.
Morphology: exposure to 2000 ppm mixed xylene no. 2 lead to hair cell
loss in the three rows of the OHC which was greatest in row 3 (average
loss approx. 65%). Four high dose animals also showed substantial IHC
losses in the medium part of the organ of Corti. Exposure to 1000 ppm
caused OHC losses but no loss of IHC. There was no significant hair cell
loss in the controls or in the 250 ppm or 500 ppm groups. The calculated
EC50 was 1636 ppm.
measured exposure concentrations (GC-FID):
based on Gagnaire F et al, 2007, Arch Toxicol 81, 127-143,
There was irrerversible loss of hair cells
in the organ of Corti and basiliar membrane for mixed xylene sample 1 at
250 ppm and for mixed xylene sample 2 at 500 ppm.
xylene no. 1: 20% o-xylene, 20% p-xylene, 20% ethylbenzene, 40% m-xylene
xylene no. 2: 30% o-xylene, 10% p-xylene, 10% ethylbenzene, 50% m-xylene
auditory-evoked responses were recorded following 4, 8 and 13 wk of
exposure and again after an 8 wk recovery period. Cytocochleograms
(total hair cell counts) were made at the end of the recovery period.
The results demonstrated an irreversible, functional loss of hearing (as
assessed from auditory evoked response) in male rats with a NOAEC of 500
ppm for mixed xylene no. 1 and a NOAEC of 1000 ppm for mixed xylene no.
2. Microscopic examination of the organ of Corti and basilar membrane
demonstrated clear reductions in hair cells with a LOAEC of 250 ppm and
a NOAEC of 500 ppm for the two samples, respectively. The calculated
theoretical concentration of mixed xylene causing a 50% loss of hair
cells (EC50) was 1114 ppm for mixed xylene no. 1 and 1636 ppm for mixed
xylene no. 2. The TLAELs calculated
from the 95, 99 and 99.9% statistical upper confidence limits are 242,
254 and 273 ppm respectively or mixed xylene 1 and 417, 446 and 469 ppm
respectively for mixed xylene 2.
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