Year	0 – 6000 (μg/m³).h	6000 – 12000 (μg/m³).h	12000 – 18000 (μg/m³).h	Greater than 18000 (μg/m³).h
1996	7.63	25.61	23.11	43.65
1997	9.78	29.65	30.69	29.88
1998	15.24	16.01	27.46	41.28
1999	4.47	17.14	40.26	38.13
2000	16.14	24.93	20.0	38.93
2001	10.61	23.57	20.73	45.09
2002	10.63	24.04	22.2	43.13
2003	8.54	13.63	12.64	65.19
2004	20.39	23.88	21.43	34.29
2005	12.73	16.61	25.24	45.42
2006	2.43	7.17	21.66	68.74
2007	23.68	25.32	18.77	32.23
2008	4.72	28.27	31.58	35.44
2009	20.05	33.6	24.06	22.3
2010	15.39	28.83	34.58	21.2
2011	12.77	27.07	42.51	17.66