Year	mmol per litre
1980	4.022
1981	4.021
1982	4.022
1983	4.024
1984	4.03
1985	4.029
1986	4.027
1987	4.024
1988	4.025
1989	4.031
1990	4.03
1991	4.03
1992	4.03
1993	4.027
1994	4.024
1995	4.017
1996	4.003
1997	3.992
1998	3.983
1999	3.976
2000	3.969
2001	3.96
2002	3.954
2003	3.95
2004	3.948
2005	3.945
2006	3.946
2007	3.947
2008	3.948