Year	mmol per litre
1980	4.307
1981	4.292
1982	4.282
1983	4.266
1984	4.242
1985	4.22
1986	4.199
1987	4.178
1988	4.161
1989	4.143
1990	4.124
1991	4.108
1992	4.09
1993	4.074
1994	4.059
1995	4.046
1996	4.034
1997	4.019
1998	4.005
1999	3.993
2000	3.98
2001	3.969
2002	3.96
2003	3.954
2004	3.95
2005	3.946
2006	3.942
2007	3.941
2008	3.941