Year	mmol per litre
1980	4.202
1981	4.189
1982	4.177
1983	4.164
1984	4.151
1985	4.138
1986	4.122
1987	4.108
1988	4.093
1989	4.076
1990	4.06
1991	4.042
1992	4.019
1993	3.998
1994	3.977
1995	3.956
1996	3.932
1997	3.901
1998	3.874
1999	3.849
2000	3.83
2001	3.816
2002	3.81
2003	3.806
2004	3.804
2005	3.805
2006	3.809
2007	3.814
2008	3.819