Year	mmol per litre
1980	4.884
1981	4.885
1982	4.884
1983	4.884
1984	4.885
1985	4.885
1986	4.888
1987	4.892
1988	4.894
1989	4.895
1990	4.896
1991	4.896
1992	4.897
1993	4.897
1994	4.898
1995	4.897
1996	4.897
1997	4.896
1998	4.893
1999	4.89
2000	4.885
2001	4.878
2002	4.871
2003	4.863
2004	4.854
2005	4.843
2006	4.833
2007	4.82
2008	4.807