When is MMA allowed to be used in salons?

Methyl methacrylate (MMA) is a substance that has been widely recognized as unsafe for use in cosmetology, particularly in the application of products like nail enhancements and eyelash extensions. Its use in salons has been prohibited because MMA can cause serious health issues for both clients and technicians, including allergic reactions, skin irritation, and significant long-term health risks. The regulations regarding MMA are very strict, and the industry guidelines clearly state that it should never be used in salons. This is why the correct response is that MMA can never be used, as the safety and well-being of clients and professionals are of utmost importance. The other options suggest scenarios where MMA might be permissible, which contradicts established safety regulations and health standards in the cosmetology field.