Year	Concentration
2000	21.19
2001	21.01
2002	20.53
2003	20.86
2004	20.7
2005	20.77
2006	20.79
2007	21.24
2008	21.41
2009	21.1
2010	21.47
2011	20.63
2012	21.25
2013	21.14
2014	21.27
2015	21.31
2016	21.61
2017	21.05
2018	21.15
2019	20.78
2020	20.42
2021	20.51