- 20 Moreover, by letters of 31 October 2001 and 12 February 2002 (cf. paragraph 8), the Greek authorities provided information on action taken upon the delegation’s immediate observation under Article 8, paragraph 5, of the Convention, concerning Kozani and Heraklion Police Headquarters, and in respect of other police detention facilities visited in 2001. The CPT welcomes the steps taken to remedy certain of the shortcomings observed by the delegation. In particular, as had been the case following previous visits, instructions have been issued to reduce cell occupancy levels in the establishments visited, and measures have been taken to improve hygiene and sleeping arrangements, and to ensure that, in certain of those establishments, detained persons can be offered outdoor exercise. However, it is necessary to ensure that lasting improvements are made throughout law enforcement agency detention facilities in Greece. 34. The CPT wishes to reiterate that depriving persons of their liberty brings with it the responsibility to detain them under conditions which are consistent with the inherent dignity of the human person. At present, this responsibility is not being fulfilled vis-à-vis many persons held in law enforcement agency detention facilities in Greece. Current arrangements are particularly unacceptable as regards persons who can remain in custody for weeks in ordinary law enforcement agency detention facilities such as those described in paragraphs 24 to 28, in particular immigration detainees. As indicated in previous visit reports, conditions in police stations/headquarters will frequently - if not invariably - be inadequate for prolonged periods of detention (cf. CPT/Inf (2001) 18, Part I, paragraph 13). 35. In short, the information gathered in the course of the 2001 visit shows that, despite improvements made, many of the recommendations made by the CPT after its previous visits to Greece, some in the form of immediate observations, have yet to be implemented. Having regard to Articles 3 and 10 of the Convention, the CPT trusts that the Greek authorities will take without delay the further action identified in the following paragraphs. 36. Cells should be of a reasonable size for the number of persons they are used to accommodate. The CPT considers that cells intended for single occupancy for stays in excess of a few hours should be in the order of 7 m², 2 metres or more between walls and 2.5 m between floor and ceiling; as regards collective occupancy cells used for extended periods of detention, the objective should be to offer at least 4 m² per person (e.g. cells measuring 20 m² should hold a maximum of 5 persons overnight). Cells should have adequate lighting (i.e. sufficient to read by, sleeping periods excluded) and ventilation; preferably, cells should enjoy natural light. Cells should be equipped with a call system. The CPT recommends that the situation as regards cell sizes and occupancy rates in the establishments visited in 2001 be reviewed, in the light of the above remarks and the observations made in paragraphs 24 to 32.