Descaling agents are typically acidic compounds such as hydrochloric acid that react with the calcium carbonate and magnesium carbonate compounds present in the scale, producing carbon dioxide gas and a soluble salt.  
Strongly acidic descaling agents are usually corrosive to the eyes and skin and can also attack and degrade clothing fibres, so appropriate protection such as rubber gloves and plastic aprons should be used in cleaning operations.
Descaling can also cause erosion and dezincification of brass, causing the brass to leach lead which is thereby released into potable water lines. Descaling of steam boilers such as those found in espresso machines can be difficult as the boilers do not fill completely and the descaling agent is unable to reach the top of the boiler. If this problem occurs the device's safety valves, probes, and pressure sensors may not be cleared of limescale.