Wall paintings can be damaged by the biochemical and mechanical action of bacteria, fungi, and algae. Although the growth of microorganisms is favored by high relative humidity, warm temperatures, and the presence of organic materials as a food source, the species which infect wall paintings often have minimal physical requirements. Their culture and identification, therefore, can sometimes be difficult. Environmental controls are recommended to discourage microbial development, but biocidal treatment may also be necessary. Few experimental studies have been made assessing the long-term effects of biocides on works of art.