The basic idea of climate control is to keep the temperature and relative humidity at a steady rate so as to avoid expansion and contraction of materials, which can cause serious damage. Climate control is typically achieved by utilizing an HVAC system, though depending on weather patterns, these systems can often require a great deal of energy to run, which is a concern for many institutions. Generally, keeping a lower temperature is better for collections storage, because chemical and biological reactions tend to increase as the temperature increases and structural damage is possible to objects made of materials such as wax, at high temperatures. The standard practice for temperature control for general collections objects is a temperature of 70 °F +/-2 °F, or 21 °C +/-2 °C. For some collections, there are specified temperature suggestions to protect delicate materials. For example, furs, paper archives, and textiles may be stored at a lower temperature between 41 °F to 50 °F (5-10 °C), and cellulose nitrate film is often isolated and stored below freezing, as it has the potential to ignite at 106 °F. The standard practice for humidity control for collections objects is a relative humidity of 50% +/-5%, though the expenditures required to keep such a tight relative humidity have been shown to only offer modest prevention benefits. As with temperature control, the main goal of the control of relative humidity to is avoid major fluctuations which can cause physical damage to objects, with long term seasonal changes causing greater damage than short term changes. Hygroscopic materials such as wood, textiles, and bone, are especially sensitive to changes in humidity, as they swell and shrink according to the environment, which will cause deterioration over time. Mold growth occurs only when the relative humidity is about 70% or higher, and insect infestation is more common with a high relative humidity. A very low relative humidity, of about 35% or lower, can cause embrittlement of paper and adhesives, as well as cracking and warping of wood and ivory. Many museum objects are constructed of multiple materials, so in order to provide an appropriate relative humidity for the object as a whole, certain materials may be compromised. An ongoing monitoring system is essential to the preventive care of objects in collections storage, as it helps to identify any problems, evaluate the effectiveness of corrective measures, and document the effect of extraordinary events such as water leaks, long periods of drought, or heavy rains. Hygrothermographs are a tool that constantly monitors both the temperature and relative humidity, and are fairly reliable if properly calibrated, a task that must be completed at least every three months if not more frequently.