-22 measures in several parts and let the prisoner recover from the negative effects of isolation, before resuming measures in order to enforce them. This is undoubtedly a discretionary practice, which takes place where the limits of solitary confinement exceed those set by international standards, thus seriously endangering the health of isolated people. 2.2.2. Material conditions There is a clear problem with the material conditions of solitary confinement, and this is particularly true for disciplinary isolation. Very often cells used for disciplinary confinement do not meet national and international standards with regard to size, light, ventilation, and furniture. The same also often applies to outdoors areas. In many cases, isolation is carried out in unfurnished cells, with only a bed without mattress, no blankets, no bedlinen, no hygienic facilities besides a squat toilet visible from the peephole or via CCTV equipment (which sometimes is the norm also for non-isolated detainees, without guaranteeing their privacy). There are also cases in which detainees are kept naked or with single-use paper underwear. Harsh conditions may also include no windowpanes and no heating. Facilities are frequently inadequate, narrow, poorly illuminated. If there are attached outdoor areas, they might be small and/or covered by a net. In some countries, no books are allowed other than religious books, and the detainee has to prove that he is a religious person. 2.2.3. Effects on mental health Solitary confinement can have a very detrimental effect on the mental and physical health of detainees. There is extensive research12 on the physical and psychological harm that solitary confinement can cause, and the central harmful feature is the lack of meaningful human contact, which is lack of interactions with other human beings. Since all human beings are different, all people react differently to 12. Smith, P. S., The Effects of Solitary Confinement on Prison Inmates: A Brief History and Review of the Literature, Crime and Justice, 2006, Vol. 34, No. 1, pp. 441-528 Lobel, J. and Smith, P. S., Solitary Confinement. Effects, Practices, and Pathways Toward Reform, Oxford University Press, 2019, (ebook). United Nations General Assembly, The Istanbul Statement on the Use and Effects of Solitary Confinement, addendum dell’Interim Report of the Special Rapporteur on Torture and Other Cruel, Inhuman or Degrading Treatment or Punishment, A/63/175, 28 July 2008.