There are about 42 million prostitutes in the world, living all over the world (though most of Central Asia, the Middle East and Africa lacks data, studied countries in that large region rank as top sex tourism destinations).[6] Sex tourism refers to the practice of traveling to engage in sexual relations with prostitutes in other countries. Some rich clients may pay for long-term contracts that may last for years.[7][8] Some view prostitution as a form of exploitation of or violence against women[9] and children,[10] and helps to create a supply of victims for human trafficking.[11] Some critics of prostitution as an institution are supporters of the Swedish approach, which has also been adopted by Canada, Iceland, Northern Ireland, Norway and France.