are useful in self-cleaning electrical contacts. In particular, the discharge that occurs during electrical switching oxidizes the contacts. However, rhenium oxide Re2O7 is volatile (sublimes at ~360 °C) and therefore is removed during the discharge. Rhenium has a high melting point and a low vapor pressure similar to tantalum and tungsten. Therefore, rhenium filaments exhibit a higher stability if the filament is operated not in vacuum, but in oxygen-containing atmosphere. Those filaments are widely used in mass spectrometers, ion gauges and photoflash lamps in photography. Catalysts Rhenium in the form of rhenium-platinum alloy is used as catalyst for catalytic reforming, which is a chemical process to convert petroleum refinery naphthas with low octane ratings into high-octane liquid products. Worldwide, 30% of catalysts used for this process contain rhenium. The olefin metathesis is the other reaction for which rhenium is used as catalyst. Normally Re2O7 on alumina is used for this process. Rhenium catalysts are very resistant to chemical poisoning from nitrogen, sulfur and phosphorus, and so are used in certain kinds of hydrogenation reactions. Other uses The isotopes 186Re and 188Re are radioactive and are used for treatment of liver cancer. They both have similar penetration depth in tissue (5 mm for 186Re and 11 mm for 188Re), but 186Re has the advantage of a longer lifetime (90 hours vs. 17 hours). 188Re is also being used experimentally in a novel treatment of pancreatic cancer where it is delivered by means of the bacterium Listeria monocytogenes. The 188Re isotope is also used for the rhenium-SCT (skin cancer therapy). The treatment uses the isotope's properties as a beta emitter for brachytherapy in the treatment of basal cell carcinoma and squamous cell carcinoma of the skin. Related by periodic trends, rhenium has a similar chemistry to that of technetium; work done to label rhenium onto target compounds can often be translated to technetium. This is useful for radiopharmacy, where it is difficult to work with technetium – especially the technetium-99m isotope used in medicine – due to its expense and short half-life. Precautions Very little is known about the toxicity of rhenium and its compounds because they are used in very small amounts. Soluble salts, such as the rhenium halides or perrhenates, could be hazardous due to elements other than rhenium or due to rhenium itself. Only a few compounds of rhenium have been tested for their acute toxicity; two examples are