Table 1.

Basis and data source for classifying air pollutants in “definite risks” category in Greater Houston.^a

				Data source
				AQS 2004b			NATA 1999c
	Basis			No. of monitors			No. of census tracts
Air pollutant	Cancer risk	Chronic risk	NAAQS criteria exceedance	Cancer risk	Chronic risk	Days in exceedance	Cancer risk	Chronic risk
Ozone			X		20	46
PM2.5			X			0
Diesel PM	X	X		6d			895	43
1,3-Butadiene	X	X		7	1		9	1
Chromium VI	X			2e			433
Benzene	X			2			66
Ethylene dibromide	X			1
Acrylonitrile	X						6
Formaldehyde		X			2			143
Acrolein		X			3			889
Chlorine		X						31
1,6-Hexamethylene Diisocyanate		X						6

Abbreviations: AQS, Air Quality System; X denotes the basis for risk classification.

^aGreater Houston consists of the 10-county, Houston–Sugar Land–Baytown metropolitan statistical area defined by the U.S. Census Bureau (2003).

^bData from U.S. EPA (2006b).

^cData from U.S. EPA (2006a).

^dDiesel estimated using measured ambient elemental carbon concentrations.

^eChromium VI estimated using measured ambient chromium PM_2.5 concentrations; see Supplemental Material, Appendix 3 (available online at http://www.ehponline.org/docs/2007/10043/suppl.pdf).