However, small residues of lower hydrocarbons in foodstuffs and pharmaceuticals are not generally considered a problem. Water has good environmental and other advantages, although its critical parameters are much less convenient and it gives rise to corrosion problems. Supercritical water is being used as a medium for the oxidative destruction of toxic waste. There is a particular interest in both supercritical and near-critical water because its polarity falls as the temperature is raised. Ammonia has similar behaviour, is often considered and discussed, but not often used.
Many halocarbons have the disadvantage of cost or of being environmentally unfriendly.
Xenon is expensive, but is useful for small-scale experiments involving spectroscopy because of its transparency in the infrared, for example. Although often pursued in practice for environmental reasons, the more fundamental interest in supercritical fluids arises because they can have properties intermediate between those of typical gases and liquids. Compared with liquids, densities and viscosities are less and diffusivities greater. The conditions may be optimum for a particular process or experiment. Furthermore, properties are controllable by both pressure and temperature and the extra degree of freedom, compared with a liquid, can mean that more than one property can be optimized.