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Most households and workplaces all over the world possess furnishings and electronics, all of which contain potentially toxic flame retardant chemicals to prevent fire hazards. Indoor dust is a recognized repository of these types of chemicals including polybrominated diphenyl ethers (PBDEs) and non-polybrominated diphenyl ethers (non-PBDEs). However, no previous U.S. studies have differentiated concentrations from elevated surface dust (ESD) and floor dust (FD) within and across microenvironments. We address this information gap by measuring twenty-two flame-retardant chemicals in dust on elevated surfaces (ESD; n=10) and floors (FD; n=10) from rooms on a California campus that contain various concentrations of electronic products. We hypothesized a difference in chemical concentrations in ESD and FD. Secondarily, we examined whether or not this difference persisted: (a) across the studied microenvironments and (b) in rooms with various concentrations of electronics. A Wilcoxon signed-rank test demonstrated that the ESD was statistically significantly higher than FD for BDE-47 (p=0.01), BDE-99 (p=0.01), BDE-100 (p=0.01), BDE-153 (p=0.02), BDE-154 (p=0.02), and 3 non-PBDEs including EH-TBB (p=0.02), BEH-TEBP (p=0.05), and TDCIPP (p=0.03). These results suggest different levels and kinds of exposures to flame-retardant chemicals for individuals spending time in the sampled locations depending on the position of accumulated dust. Therefore, further research is needed to estimate human exposure to flame retardant chemicals based on how much time and where in the room individuals spend their time. Such sub-location estimates will likely differ from assessments that assume continuous unidimensional exposure, with implications for improved understanding of potential health impacts of flame retardant chemicals.

Purpose: Human auditory nerve (AN) activity estimated from the amplitude of the first prominent negative peak (N1) of the compound action potential (CAP) is typically quantified using either a peak-to-peak measurement or a baseline-corrected measurement. However, the reliability of these 2 common measurement techniques has not been evaluated but is often assumed to be relatively poor, especially for older adults. To address this question, the current study (a) compared test-retest reliability of these 2 methods and (b) tested the extent to which measurement type affected the relationship between N1 amplitude and experimental factors related to the stimulus (higher and lower intensity levels) and participants (younger and older adults). Method: Click-evoked CAPs were recorded in 24 younger (aged 18-30 years) and 20 older (aged 55-85 years) adults with clinically normal audiograms up to 3000 Hz. N1 peak amplitudes were estimated from peak-to-peak measurements (from N1 to P1) and baseline-corrected measurements for 2 stimulus levels (80 and 110 dB pSPL). Baseline-corrected measurements were made with 4 baseline windows. Each stimulus level was presented twice, and test-retest reliability of these 2 measures was assessed using the intraclass correlation coefficient. Linear mixed models were used to evaluate the extent to which age group and click level uniquely predicted N1 amplitude and whether the predictive relationships differed between N1 measurement techniques. Results: Both peak-to-peak and baseline-corrected measurements of N1 amplitude were found to have good-to-excellent reliability, with intraclass correlation coefficient values > 0.60. As expected, N1 amplitudes were significantly larger for younger participants compared with older participants for both measurement types and were significantly larger in response to clicks presented at 110 dB pSPL than at 80 dB pSPL for both measurement types. Furthermore, the choice of baseline window had no significant effect on N1 amplitudes using the baseline-corrected method. Conclusions: Our results suggest that measurements of AN activity can be robustly and reliably recorded in both younger and older adults using either peak-to-peak or baseline-corrected measurements of the N1 of the CAP. Peak-to-peak measurements yield larger N1 response amplitudes and are the default measurement type for many clinical systems, whereas baseline-corrected measurements are computationally simpler. Furthermore, the relationships between AN activity and stimulus- and participant-related variables were not affected by measurement technique, which suggests that these relationships can be compared across studies using different techniques for measuring the CAP N1.

Auditory nerve (AN) function has been hypothesized to deteriorate with age and noise exposure. Here, we perform a systematic review of published studies and find that the evidence for age-related deficits in AN function is largely consistent across the literature, but there are inconsistent findings among studies of noise exposure history. Further, evidence from animal studies suggests that the greatest deficits in AN response amplitudes are found in noise-exposed aged mice, but a test of the interaction between effects of age and noise exposure on AN function has not been conducted in humans. We report a study of our own examining differences in the response amplitude of the compound action potential N1 (CAP N1) between younger and older adults with and without a self-reported history of noise exposure in a large sample of human participants (63 younger adults 18-30 years of age, 103 older adults 50-86 years of age). CAP N1 response amplitudes were smaller in older than younger adults. Noise exposure history did not appear to predict CAP N1 response amplitudes, nor did the effect of noise exposure history interact with age. We then incorporated our results into two meta-analyses of published studies of age and noise exposure history effects on AN response amplitudes in neurotypical human samples. The meta-analyses found that age effects across studies are robust (r=-0.407), but noise-exposure effects are weak (r=-0.152). We conclude that noise-exposure effects may be highly variable depending on sample characteristics, study design, and statistical approach, and researchers should be cautious when interpreting results. The underlying pathology of age-related and noise-induced changes in AN function are difficult to determine in living humans, creating a need for longitudinal studies of changes in AN function across the lifespan and histological examination of the AN from temporal bones collected post-mortem., Competing Interests: The authors declare no competing financial interests.