Year	mmol per litre
1980	4.164
1981	4.142
1982	4.119
1983	4.094
1984	4.076
1985	4.06
1986	4.044
1987	4.029
1988	4.016
1989	4.004
1990	3.996
1991	3.99
1992	3.989
1993	3.989
1994	3.991
1995	3.993
1996	4
1997	4.007
1998	4.01
1999	4.017
2000	4.023
2001	4.025
2002	4.027
2003	4.03
2004	4.035
2005	4.041
2006	4.049
2007	4.058
2008	4.067