1999 – 3.9 kg 2000 – 3.9 kg 2001 – 3.6 kg 2002 – 3.5 kg 2003 – 3.1 kg 2004 – 3.9 kg 2005 – 3.3 kg 2006 – 3.5 kg 2007 – 4.3 kg 2008 – 3.7 kg 2009 – 3.1 kg 2010 – 5.9 kg 2011 – 6.0 kg 2012 – 7.3 kg 2013 – 7.8 kg 2014 – 7.8 kg 2015 – 9.1 kg 2016 – 9.6 kg 2017 – 10.3 kg (OECD)
