year	pollutant_name	index
1990	CO	100.0
1991	CO	97.3
1992	CO	95.2
1993	CO	90.9
1994	CO	83.5
1995	CO	78.4
1996	CO	76.1
1997	CO	69.6
1998	CO	65.0
1999	CO	60.0
2000	CO	52.5
2001	CO	49.5
2002	CO	45.6
2003	CO	41.9
2004	CO	38.5
2005	CO	35.1
2006	CO	31.3
2007	CO	28.4
2008	CO	25.8
2009	CO	23.4
2010	CO	21.3
2011	CO	19.1
2012	CO	17.5
2013	CO	16.2
2014	CO	15.0
2015	CO	14.0
2016	CO	13.7
2017	CO	13.3