Since 1800, the human population has increased from one billion[19] to over seven billion,[20] In 2004, some 2.5 billion out of 6.3 billion people (39.7%) lived in urban areas. In February 2008, the U.N. estimated that half the world's population would live in urban areas by the end of the year.[21] Problems for humans living in cities include various forms of pollution and crime,[22] especially in inner city and suburban slums. Both overall population numbers and the proportion residing in cities are expected to increase significantly in the coming decades.[23]