Year	mmol per litre
1980	5.017
1981	5.008
1982	5.001
1983	4.998
1984	4.998
1985	4.997
1986	4.992
1987	4.989
1988	4.987
1989	4.989
1990	4.996
1991	5.009
1992	5.025
1993	5.042
1994	5.062
1995	5.085
1996	5.11
1997	5.132
1998	5.155
1999	5.175
2000	5.193
2001	5.206
2002	5.217
2003	5.224
2004	5.235
2005	5.242
2006	5.25
2007	5.256
2008	5.261