At the wellhead, natural gas will probably contain methane and various heavier fractions. It may also contain a number of non-hydrocarbons, some or all of which will need to be removed. There may also be substances introduced into the well as a result of drilling operations, such as inhibitors, mud etc. The principal harmful impurities in the gas itself are likely to be carbon dioxide and sulphur compounds, notably hydrogen sulphide, which can create corrosion in the pipelines in the presence of water and can seriously damage chemical plant units built of aluminium. Hydrogen Sulphide is also extremely toxic. Excessive water vapour can lead to the formation of acids and Hydrates. Some impurities, such as sulphur or helium, can be recovered economically as by-products if in sufficient concentration in the gas. Other impurities, such as inert gases and nitrogen cause no directly harmful effects but dilute the Calorific Value of the gas and mean that pipeline capacity is being wasted. Whether or not they should be removed is simply a question of economics. See also Treatment, Natural Gas Liquids, Condensates.