 the mutant gene (distinct from the ancestral allele defined by dbSNP database), respectively, corresponding to general genetic model. The variables included in the analyses were related to the dependent variables with a logit link. The relationship was measured as odds ratios with 95% confidence intervals. The Statistical Package for the Social Sciences (SPSS for windows, version 11.0.1, 2001, SPSS Inc, Chicago, IL) was used for all the analyses.

Characteristics of subjects
All resistant individuals had normal hearing in both ears (with the hearing threshold < 25 dB in every fre-quency). The hearing threshold of 3 kHz in left ear was significant higher in the susceptible workers than in the resistant workers (40.23 ± 9.53 dB vs. 16 33.7%) were significantly different between two groups (P < 0.05, two-sided χ 2 test). Therefore, in the following multivariate logistic regressions, the covariates of these factors were included for adjustment. All the remaining variables were not significantly different. As all the subjects were living in the same community nearby the factory and had a consistent low leisure time exposure to noise, this factor was therefore not analyzed in this study. In addition, almost all the subjects were recent high-school or college graduates, this job was their first employment, and thus they had no history of exposure to noise at previous workplaces.

Distributions of SOD2 SNPs
The distributions of the SOD2 SNP genotypes and alleles in the susceptible workers and the resistant workers are shown in Table 2. The minor allele frequency (MAF) for all SNPs in SOD2 were higher than 5% (or percentage of heterozygotes higher than 10%), indicating that those SNPs were frequent in Chinese Han population. All the five polymorphism loci were in Hardy-Weinberg equilibrium. Linkage disequilibrium (LD) between the SNPs were estimated by using Haploview version 3.0 software, showing that most of the LD values were very small, with the largest (r 2 = 0.77) between rs5746136 and rs4880, indicating that the five SNPs were in separated LD blocks.

Association of SNPs with NIHL
Crude and adjusted ORs for NIHL were estimated separately, derived without or with adjustment for confounding factors. As shown in Table 3, the frequencies of CT genotype and C allele of rs4880 in the susceptible workers were significantly higher than in the resistant workers (P = 0.001 and P = 0.006 respectively). After adjusted for age, sex, smoking, drinking,

Interaction between noise exposure level and SNPs
In order to reveal the interactions between SNPs and noise exposure level, we performed stratified analysis per noise exposure level. Inconsistent genotypic distributions between the susceptible and the resistant group were found for different noise exposure levels, suggesting that SNP genotypic effects on the susceptibility to noise induced hearing loss were dependent on the environmental noise exposure level and CNE. In the studied Chinese Han sample, significant gene-environment interactions between rs4880 SNPs and noise exposure were identified (Table 4). In the 85∼ and 92∼ dB (A) exposure levels, the susceptible workers were more likely to be the carriers of CC or CT genotype, whereas in the resistant workers TT genotype was more frequent, rendering that CC or CT were risk genotypes (respectively with adjusted OR, 2.38 and 95% CI, 1.27-4.46, P = 0.006; adjusted OR, 3.67, 95% CI, 1.10-12.30, P = 0.035). Similarly, in the 82 dB(A) ∼ CNE level, comparing with TT genotype, we found that the CC and CT were the risk genotypes (P = 0.023), with adjusted OR of 2.59 and 95% CI of 1.14-5.90. However, there was no such a trend in the lower noise exposure/CNE levels. These results suggest that hearing loss in these workers were resulted from synergic effects of both genetic and environmental factors.

Association of haplotypes with NIHL
Haplotypes were estimated by using Haploview software, followed by an association analysis by considering a haplotype as an allele ( Table 5). The frequencies of haplotype AGCCG was significantly higher in the susceptible group than in the resistant group (P = 0.020), rendering it to be a risk haplotype. With the haplotype AGCTG as the reference, the corresponding OR and 95% CI were 2.63 and 1.14-6.06.

Discussion
Reactive oxygen species (ROS) play a key role underlying the mechanisms for induction of cochlear Table 2 The distribution of SOD2 genotypes and alleles in NIHL sensitive workers and resistant workers   14, 5.90) * adjusted for age, sex (0, male; 1, female); smoking (1, yes; 0, no); drinking (1, yes; 0, no); organic solvents exposure (1, yes; 0, no); heavy metals exposure (1, yes; 0, no); heat exposure (1, yes; 0, no); dust exposure (1, yes; 0, no). damage under various pathological conditions. Superoxide is readily generated in the inner ear following acoustic overstimulation. Superoxide can react in an iron-catalyzed Fenton reaction to form destructive hydroxyl radicals or combine with nitric oxide to form the highly toxic peroxynitrite [32]. Therefore, the regulation of superoxide levels is important for cochlear degeneration caused by ROS. SOD (superoxide dismutase) is an enzyme that converts superoxide to hydrogen peroxide. In humans (as in all other mammals and most chordates), three types of superoxide dismutase are identified. SOD1 is located in the cytoplasm, SOD2 in the mitochondria and SOD3 is extracellular. The first is a dimmer (consists of two units), while the others are tetramers (four subunits). SOD1 and SOD3 contain copper and zinc, while SOD2 has manganese in its reactive center. Localization of both SOD1 and SOD2 in the cochlea has been reported, and SOD1 is the most abundant in the cochlea, comprising approximately 74% of total SOD activity. Absence of SOD2 has been shown to lead to an increase in hearing loss related to acoustic trauma. In addition, auditory dysfunction due to noise exposure is attenuated by application of SOD2. Furthermore, transgenic mice over expressing SOD2 were protected against amino glycoside-induced hearing loss, which is also mediated by ROS [33]. However, no protective effects of over expression of SOD2 against acoustic trauma were observed. Therefore, the protective effects of SOD2 against acoustic trauma remain unresolved.
In the present study, we found statistically significant difference between the NIHL susceptible workers and resistant ones in the genotypic and allelic distributions of rs4880 in gene SOD2. As revealed by multivariate logistic regression analysis, the CT genotype of rs4880 was also associated with a higher risk of NIHL (adjusted OR, 2.18; 95% CI, 1.34-3.54, P = 0.002). These results suggested that the rs4880 polymorphisms (V16A, or called −9T > C) of the SOD2 may confer to the risk of NIHL in the Chinese Hans. A recent study [34] showed that V16A homozygotes may have higher SOD2 activity than V16A heterozygotes. The −9T > C (V16A) polymorphism in the MnSOD mitochondrial targeting sequence significantly reduced MnSOD catalytic activity in cryopreserved hepatocytes [35]. SOD2 contains a signalling peptide of 24 amino acids, and thus −9T > C (or V16A) describes genetic change (T to C) at the ninth position of the upstream of the DNA sequence coding SOD2 signaling peptide. Another study [36] identified a valine/alanine polymorphism (rs4880 or V16A) in the targeting sequence of SOD2, and an in vitro study [16] showed that valine (Val) instead of alanine (Ala) results in less efficient transport of Mn-SOD into the mitochondrial matrix, which can compromise the ability to neutralize superoxide radicals in the cell. This polymorphism were also associated with diabetic nephropathy in Japanese [37] and Korean patients [38] with type 2 diabetes, breast cancer risk [39], development of Alzheimer's disease, a high degree of carotid atherosclerosis [40], exudative age-related macular degeneration in Japanese individuals, and associated with elevation of prostate cancer risk in Caucasians (Val/Ala versus Val/Val: OR, 1.17; 95% CI, 0.97-1.42; Ala/Ala versus Val/Val: OR, 1.28; 95% CI, 1.03-1.60; P (trend) = 0.03).
However, our positive findings are not in agreement with the results reported by an earlier study using an Italy population of noise-exposed male workers [27] showing that there were no statistically significant difference between the SNPs of rs4880 in SOD2 and the risk of NIHL. However, this study differs from our study in several aspects which may explain the discrepancies. First, there is a large difference in sample size between our study and the previous studies, resulted in different statistical powers achieved. Fortunato et al. analyzed 94 male workers from an aircraft factory [27], while we genotyped > 400 either NIHL sensitive or resistant workers in an air-conditioning factory. Second, this genetic polymorphism may confer a racespecific risk, and our study demonstrated that Chinese Han might be more susceptible to this genetic exposure than the European peoples.
Although sometimes useful, single SNP may not be sufficiently informative for unraveling complex disease [41]. The multipoint haplotype-based association studies by considering linkage disequilibrium pattern and joint effects of several SNPs might overcome this weakness and thus are more powerful for locating the underlying SNP loci for complex disease [42]. Further haplotype analysis using the Hapl