year	pollutant_name	index
1990	CO	100.0
1991	CO	97.1
1992	CO	93.0
1993	CO	88.8
1994	CO	81.2
1995	CO	77.4
1996	CO	74.8
1997	CO	68.8
1998	CO	64.4
1999	CO	59.5
2000	CO	53.7
2001	CO	50.1
2002	CO	46.0
2003	CO	42.5
2004	CO	39.4
2005	CO	34.7
2006	CO	31.7
2007	CO	28.5
2008	CO	25.6
2009	CO	23.4
2010	CO	21.0
2011	CO	19.5
2012	CO	17.9
2013	CO	16.4
2014	CO	15.3
2015	CO	14.8