Year	mmol per litre
1980	5.174
1981	5.157
1982	5.14
1983	5.123
1984	5.105
1985	5.088
1986	5.07
1987	5.051
1988	5.032
1989	5.012
1990	4.99
1991	4.962
1992	4.928
1993	4.891
1994	4.847
1995	4.802
1996	4.763
1997	4.726
1998	4.697
1999	4.668
2000	4.642
2001	4.621
2002	4.604
2003	4.587
2004	4.572
2005	4.556
2006	4.537
2007	4.521
2008	4.507