Year	mmol per litre
1980	4.104
1981	4.09
1982	4.076
1983	4.059
1984	4.042
1985	4.028
1986	4.019
1987	4.01
1988	4.004
1989	3.998
1990	3.996
1991	3.996
1992	3.999
1993	4.003
1994	4.007
1995	4.012
1996	4.017
1997	4.026
1998	4.034
1999	4.042
2000	4.05
2001	4.06
2002	4.068
2003	4.077
2004	4.088
2005	4.099
2006	4.106
2007	4.116
2008	4.128