Porous ceramic "candle" filters are cylindrical filter media that fit into dual chamber gravity-driven delivery systems. They are typically coated with a bacteriostatic agent such as silver (which inactivates bacteria and viruses) and their small (1 micron) pores retain larger microbial pathogens such as protozoan parasites. The effectiveness of these filters at reducing disease prevalence in low-income countries has been demonstrated in a number of published randomized control trials conducted by the likes of the London School of Hygiene and Tropical Medicine, OXFAM, and others.