Year	0-6 000 (μg/m3).h	0-6 000 (μg/m3).h	6 000-12 000 (μg/m3).h	6000-1 2000 (μg/m3).h	12 000-18 000 (μg/m3).h	12 000-18 000 (μg/m3).h	> 18 000 (μg/m3).h	Greater than 18 000 (μg/m3).h	Greather than 18 000 (μg/m3).h
1996		7.6		25.6		23.1			43.7
1997		9.8		29.7		30.7			29.9
1998		15.2		16.0		27.5			41.3
1999		4.5		17.1		40.3			38.1
2000		16.1		24.9		20.0			38.9
2001		10.6		23.6		20.7			45.1
2002		10.6		24.0		22.2			43.1
2003		8.5		13.6		12.6			65.2
2004		20.4		23.9		21.4			34.3
2005		12.7		16.6		25.2			45.4
2006		2.4		7.2		21.7			68.7
2007		23.7		25.3		18.8			32.2
2008		4.7		28.3		31.6			35.4
2009		20.1		33.6		24.1			22.3
2010		15.4		28.8		34.6			21.2
2011		12.6		26.8		42.4			18.2
2012		14.2		32.4		26.3			27.1
2013		19.8		36.6		22.9			20.7
2014		15.1		36.1		30.8			18.0