Table 1. Characteristics of children with and without amalgam fillings.

	Amalgam Group (N=198)	Referent Group (N=205)
Age, mean± SD (year)	9.9±0.7	9.8±0.8
Gender, %
Male	44.9	54.6
Female	55.1	45.4
Ethnicity, %
Han	97.0	97.6
Others	3.0	2.4
Father's education, %
Less than high school	14.3	20.6
High school	33.2	31.9
College	52.6	47.5
Mother 's education, %
Less than high school	23.2	22.2
High school	33.3	36.0
College	43.4	41.9
Family income, %
<1500 CNY/person/month a	27.6	29.1
1500-CNY/person/month	40.2	36.9
>=3000 CNY/person/month	32.2	34.0
Hot food consumption habit b, %
Yes	78.8	74.1
No	21.2	25.9
Gum chewing habit c, %
Yes	20.7	18.2
No	79.3	81.8
Bruxism habit, %
Yes	5.0	6.0
No	94.5	94.6
Fish consumption, %
<=1 meal/week	17.8	16.3
1 meal/week	23.8	23.6
2-3 meals/week	53.5	48.3
>=4 meals/week	5.0	11.8
Total number of amalgam fillings at the time of participation, median (range)	2 (1-7)
Total number of visible amalgam surfaces at the time of participation, median (range)	2 (0-12)
Time (months) since first amalgam treatment, median (range)	31 (1-96)
Cumulative exposure index (visible surface-months), median (range)	56 (0-514)

Percentages were calculated based on the numbers of subjects with data on this item

^aCNY: Chinese Yuan

^bHot food consumption habit: answered “usually” to the question “How often do you eat foods, soups, and drinks when they are still hot?”.

^cGum chewing habit: Those answered “usually” to the question “How often do you chew gum?”.