A potentially lower cost alternative to using iodine-based water purification tablets is the use of iodine crystals, although there are serious risks of acute iodine toxicity if preparation and dilution are not measured with some accuracy. This method may not be adequate in killing Giardia cysts in cold water. An advantage of using iodine crystals is that only a small amount of iodine is dissolved from the iodine crystals at each use, giving this method of treating water a capability for treating very large volumes of water. Unlike tetraglycine hydroperiodide tablets, iodine crystals have an unlimited shelf life as long as they are not exposed to air for long periods of time or are kept under water. Iodine crystals will sublimate if exposed to air for long periods of time. The large quantity of water that can be purified with iodine crystals at low cost makes this technique especially cost effective for point of use or emergency water purification methods intended for use longer than the shelf life of tetraglycine hydroperiodide.