Fumigants have limited use but have been used for rodent control in situations where conventional methods, such as baits and contact poisons, are either ineffective or impractical. Great care is required in the application of all these formulations and, in many countries, only pest-control professionals are permitted to use them. One of the compounds most commonly used for fumigation is phosphine (PH3), derived from aluminium phosphide and mainly used for the control of infestations of stored product insects but these applications are also efficient against rodents. Another fumigant used is carbon dioxide (CO2) in a specially designed delivery device designed for use against commensal mice. Fumigants have also been used for gassing rodent burrows. In these operations, either pellets or tablets are inserted in rodent burrows, which are then sealed with soil. The gases evolved build up to concentrations lethal to the burrow's occupants. These techniques are common for rabbit control and less frequently for the control of rats.