Year	mmol per litre
1980	5.334
1981	5.324
1982	5.311
1983	5.301
1984	5.289
1985	5.278
1986	5.267
1987	5.256
1988	5.246
1989	5.239
1990	5.232
1991	5.221
1992	5.201
1993	5.16
1994	5.127
1995	5.103
1996	5.087
1997	5.073
1998	5.061
1999	5.049
2000	5.042
2001	5.032
2002	5.024
2003	5.01
2004	4.993
2005	4.98
2006	4.968
2007	4.956
2008	4.944