Table 1. Standards of categorization and reference categories for socio-demographic measurements.

Characteristics	Level 1	Level 2	Level 3	Level 4	Level 5
Age	0–14	14–60*	>60	—	—
Race	White*	black	American Indian	Asian	Other races
Education attainment (years)	0–4	5–8	9–12	>12*	—
Income (US$)	<19000	> = 19000**	—	—	—

*Reference category for comparison based on existing studies in the environmental justice literature.

**The classification standard for income is detailed in the text.