The deterioration of works of art is related to their component materials and environmental factors such as relative humidity, temperature, ventilation, contaminants and light quality and intensity. The ideal values for paintings are 55 % HR at a temperature 18-20 °C and maximum light levels of 200 lux for paintings and polychrome and 50 lux for fabric and paper. These values can vary significantly depending on whether the objects are moveable or immovable, the characteristics inherent to each object and the conditions of the site where they are being housed. However, as a general rule, it is important not to subject artworks to brusque changes in these values. With restoration interventions on mural paintings or polychromed wooden altarpieces, which are usually located in churches, it can be difficult to take preventative measures that comply with museum criteria. C.R.A. Conservación y Restauración de Arte offers periodic maintenance services by qualified professionals who examine the artworks and control their evolution, acting only when and where necessary, performing surface cleaning, eliminating dust deposits and making periodic inspections to detect and control any signs of deterioration.