Year	mmol per litre
1980	5.002
1981	4.986
1982	4.972
1983	4.961
1984	4.954
1985	4.947
1986	4.935
1987	4.926
1988	4.919
1989	4.913
1990	4.912
1991	4.916
1992	4.922
1993	4.927
1994	4.933
1995	4.94
1996	4.949
1997	4.956
1998	4.963
1999	4.968
2000	4.973
2001	4.975
2002	4.977
2003	4.977
2004	4.981
2005	4.983
2006	4.988
2007	4.992
2008	4.995