25 Large-capacity dormitories inevitably imply a lack of privacy for prisoners in their everyday lives. Moreover, the risk of intimidation and violence is high. Such accommodation arrangements are prone to foster the development of offender subcultures and to facilitate the maintenance of the cohesion of criminal organisations. They can also render proper staff control extremely difficult, if not impossible; more specifically, in case of prison disturbances, outside interventions involving the use of considerable force are difficult to avoid. With such accommodation, the appropriate allocation of individual prisoners, based on a case by case risk and needs assessment, also becomes an almost impossible exercise. All these problems are exacerbated when the numbers held go beyond a reasonable occupancy level; further, in such a situation the excessive burden on communal facilities such as washbasins or lavatories and the insufficient ventilation for so many persons will often lead to deplorable conditions. The CPT must nevertheless stress that moves away from large-capacity dormitories towards smaller living units have to be accompanied by measures to ensure that prisoners spend a reasonable part of the day engaged in purposeful activities of a varied nature outside their living unit. Access to natural light and fresh air 30. The CPT frequently encounters devices, such as metal shutters, slats, or plates fitted to cell windows, which deprive prisoners of access to natural light and prevent fresh air from entering the accommodation. They are a particularly common feature of establishments holding pre-trial prisoners. The CPT fully accepts that specific security measures designed to prevent the risk of collusion and/or criminal activities may well be required in respect of certain prisoners. However, the imposition of measures of this kind should be the exception rather than the rule. This implies that the relevant authorities must examine the case of each prisoner in order to ascertain whether specific security measures are really justified in his/her case. Further, even when such measures are required, they should never involve depriving the prisoners concerned of natural light and fresh air. The latter are basic elements of life which every prisoner is entitled to enjoy; moreover, the absence of these elements generates conditions favourable to the spread of diseases and in particular tuberculosis. The CPT recognises that the delivery of decent living conditions in penitentiary establishments can be very costly and improvements are hampered in many countries by lack of funds. However, removing devices blocking the windows of prisoner accommodation (and fitting, in those exceptional cases where this is necessary, alternative security devices of an appropriate design) should not involve considerable investment and, at the same time, would be of great benefit for all concerned.