We get this question a lot. The simple answer is pressure-treated lumber can be used in any interior application except cutting boards and countertops. Some have also asked, after they’ve found pressure treated lumber installed inside their homes, if there is any danger in having it indoors. The answer is no. There is no danger of either leaching or off-gassing. But these answers beg another question: Why would you use treated lumber indoors? The reason lumber is treated is to protect it from exterior elements that might cause rot, decay or termite infestation. In a protected environment there is no danger of rot or decay, unless the wood is exposed regularly to moisture, such as near a leaking pipe or window.
Some bathroom sub floor panel products are pressure treated as they may be exposed to moisture on a regular basis and some builders use borate-treated lumber, such as ProWood Borate, for sill plates in areas where termite infestation is common. In extreme areas of termite infestation, such as the US Gulf Coast and Hawaii, the whole structural shell may be constructed using ProWood Borate or other borate-treated lumber products. Other than those examples there would not be a need to use pressure treated lumber indoors.