Year	mmol per litre
1980	5.103
1981	5.105
1982	5.099
1983	5.095
1984	5.095
1985	5.096
1986	5.091
1987	5.08
1988	5.066
1989	5.041
1990	5.019
1991	5.006
1992	5
1993	4.998
1994	4.997
1995	4.998
1996	4.998
1997	5
1998	5.001
1999	5.003
2000	5.002
2001	4.998
2002	4.995
2003	4.992
2004	4.99
2005	4.986
2006	4.981
2007	4.976
2008	4.972