"prisoner mental health is becoming increasingly important," and has caught the attention of the world health organization, which aims to reduce the "effects of imprisonment on mental health." one study focused on the "prison environment rather than on individual factors." the study tested two time periods, short-term and long-term, that evaluated the "mental state changes in response to changes in the environment or prison setting." it ultimately concluded that solitary confinement was "associated with negative effects on mental health." segregation, though similar to solitary confinement, could not be proven to have lasting negative effects on inmates, although those who were segregated had worse mental health than those who were not segregated. the study also concluded that crowding had its own problems, as "increased levels of social density had negative effects on the mental health of inmates."