Year	mmol per litre
1980	5.006
1981	5.002
1982	4.996
1983	4.988
1984	4.982
1985	4.978
1986	4.972
1987	4.969
1988	4.965
1989	4.962
1990	4.957
1991	4.94
1992	4.917
1993	4.9
1994	4.894
1995	4.901
1996	4.905
1997	4.901
1998	4.897
1999	4.896
2000	4.899
2001	4.903
2002	4.906
2003	4.907
2004	4.905
2005	4.901
2006	4.897
2007	4.894
2008	4.891