Year	mmol per litre
1980	5.262
1981	5.26
1982	5.252
1983	5.242
1984	5.234
1985	5.232
1986	5.228
1987	5.223
1988	5.212
1989	5.194
1990	5.183
1991	5.171
1992	5.15
1993	5.123
1994	5.101
1995	5.082
1996	5.062
1997	5.041
1998	5.02
1999	4.995
2000	4.97
2001	4.949
2002	4.934
2003	4.919
2004	4.905
2005	4.895
2006	4.888
2007	4.888
2008	4.888