San Francisco Bay Area Air Basin 											
Toxic Air Contaminants - Annual Average Concentrations and Health Risk											
TAC*	Conc./Risk	1990	1991	1992	1993	1994	1995	1996	1997	1998	1999
Acetaldehyde	Annual Avg	1.3	1.4	1.03	1.31	1.17	0.42	0.83	0.73	0.65	0.76
	Health Risk 	6	7	5	6	6	2	4	4	3	4
Benzene	Annual Avg	2.18	1.82	1.49	1.49	1.4	1.26	0.71	0.61	0.71	0.6
	Health Risk 	202	169	138	138	129	116	66	57	66	55
"1,3-Butadiene"	Annual Avg	0.36	0.29	0.28	0.37	0.29	0.28	0.22	0.19	0.22	0.17
	Health Risk 	135	108	103	138	108	104	82	70	82	64
Carbon Tetrachloride	Annual Avg	0.13	0.13		0.11		0.1	0.08			
	Health Risk 	34	33		29		27	21			
Chromium (Hexavalent)	Annual Avg			0.23	0.2	0.19	0.25	0.13	0.12	0.1	
	Health Risk 			34	30	29	37	19	17	16	
para-Dichlorobenzene	Annual Avg		0.12	0.12	0.12	0.11	0.13	0.14	0.12		
	Health Risk 		8	8	8	7	8	9	8		
Formaldehyde	Annual Avg	1.87	1.73	1.43	1.56	1.66	2.06	2.62	1.85	1.76	2.09
	Health Risk 	14	13	11	12	12	15	19	14	13	15
Methylene Chloride	Annual Avg	1.04	2.32	0.65	0.72	0.59	0.6	0.58	0.55		
	Health Risk 	4	8	2	3	2	2	2	2		
Perchloroethylene	Annual Avg	0.2	0.23	0.17	0.13	0.08	0.09	0.07	0.07		
	Health Risk 	8	9	7	5	3	4	3	3		
Diesel Particulate Matter**	Annual Avg	2.5					1.9				1.6
	Health Risk 	750					570				480
Average Basin Risk***	Without Diesel PM	403	355	308	368	296	315	224	174	179	139
	With Diesel PM	1153					885				619

*   Concentrations for Chromium (Hexavalent) are expressed as ng/m3 and concentrations for Diesel PM are expressed as ug/m3. Concentrations for all other TACs are expressed as parts per billion.											
"**  Diesel PM concentration estimates are based on receptor modeling techniques, and estimates are available only for selected years. Estimated concentration for 1999 reflects the year 2000."											
*** Health Risk represents the number of excess cancer cases per million people based on a lifetime (70-year) exposure to the annual average concentration. It reflects only those compounds listed in this table											
and only those with data for that year. There may be other significant compounds for which we do not monitor or have health risk information. Additional information about interpreting the toxic air contaminant air											
"quality trends can be found in Chapter 1, Interpreting the Emission and Air Quality Statistics."