Year	mmol per litre
1980	4.255
1981	4.24
1982	4.225
1983	4.207
1984	4.189
1985	4.165
1986	4.142
1987	4.121
1988	4.103
1989	4.086
1990	4.069
1991	4.055
1992	4.039
1993	4.026
1994	4.016
1995	4.004
1996	3.996
1997	3.989
1998	3.982
1999	3.98
2000	3.978
2001	3.977
2002	3.975
2003	3.976
2004	3.98
2005	3.984
2006	3.992
2007	3.998
2008	4.004