Year	mmol per litre
1980	5.248
1981	5.235
1982	5.222
1983	5.208
1984	5.194
1985	5.179
1986	5.165
1987	5.15
1988	5.133
1989	5.117
1990	5.103
1991	5.085
1992	5.061
1993	5.041
1994	5.022
1995	5.001
1996	4.97
1997	4.944
1998	4.923
1999	4.905
2000	4.883
2001	4.859
2002	4.831
2003	4.804
2004	4.783
2005	4.761
2006	4.738
2007	4.723
2008	4.714