Water must be purified of harmful living organisms and chemicals. Some commercial filters can remove most organisms and some harmful chemicals, but they are not 100% effective. Distillation filters, purifies, and removes some harmful chemicals. Chemicals with a lower or about equal boiling point of water are not eliminated by distilling. Iodine or chlorine dioxide solutions or tablets can be used to purify water. It can be boil water in a fire-resistant pot or water bottle. Water can be boiled in some flammable materials like bark because the water absorbs the heat. Pasteurization takes place at temperatures lower than boiling point, but knowing the temperature of the water and calculating the duration of treatment can be difficult. This technique is useful when only non-durable containers are available. Sunlight can be used with a clear container. Filters made from heat-treated diatomaceous earth can also be used.