Over 9% of Havana's population live in ''cuartería'' (''solares, ciudadela''), 3.3% in shanty towns, and 0.3% in refugee shelters. This does not include an estimate of the number of people living in housing in "fair" or "poor" condition because in many cases these units do not necessarily constitute slum housing but rather are basically sound dwellings needing repairs. According to ''Instituto Nacional de Vivienda'' (National Housing Institute) official figures, in 2001, 64% of Havana's 586,768 units were considered in "good" condition, up from 50% in 1990. Some 20% were in "fair" condition and 16% in "poor" condition. Partial or total building collapses are not uncommon, although the number had been cut in half by the end of the 1990s as the worst units disappeared and others were repaired. Buildings in Old Havana and Centro Habana are especially exposed to the elements: high humidity, the corrosive effects of salt spray from proximity to the coast, and occasional flooding. Most areas of the city, especially the highly populated districts, are in urban decay.