Table 1. Demographic characteristics of study subjects.

Variables	Cases (n = 566)		Controls (n = 566)		P
	n	%	n	%
Age (years)					0.879a
Mean±SD	40.52±6.26		40.31±5.85		0.556b
≤ 35	133	23.5	138	24.4
35–45	320	56.5	321	56.7
> 45	113	20.0	107	18.9
Sex					0.646a
Male	527	93.1	523	92.4
Female	39	6.9	43	7.6
Smoking					0.263a
Now	331	58.5	315	55.7
Ever	11	1.9	19	3.4
Never	224	39.6	232	41.0
Drinking					0.898a
Now	233	41.2	235	41.5
Ever	10	1.8	12	2.1
Never	323	57.1	319	56.4
Work time with noise (years)					0.237a
Mean±SD	18.69±7.59		18.06±7.03		0.145b
≤ 16	248	43.8	261	46.1
> 16	318	56.2	305	53.9
Expose level with noise (dB)					0.712a
Mean±SD	87.18±7.72		87.39±7.39		0.651b
≤ 85	253	44.7	249	44.0
85–92	109	19.3	101	17.8
> 92	204	36.0	216	38.2
High frequency hearing threshold shift (dB)					<0.001a
Mean±SD	35.77±9.86		14.02±4.17		<0.001b
≤ 26	59	10.4	566	100.0
> 26	507	89.6	0	0.0

^a Two-sided χ² test

^b Students’ t-test.