What part of the SCBA ensures a secure fit for the user?

The mask seal is the component of the SCBA that ensures a secure fit for the user. A properly fitting mask is crucial for the effectiveness of the breathing apparatus, as it prevents the ingress of contaminated air and ensures that the wearer can breathe safely in hazardous environments. The mask seal typically forms a tight gasket around the face, allowing the user to maintain proper airflow while minimizing any leaks. This secure fit is essential not only for comfort but also for safety, as any gaps could compromise the air supply and expose the wearer to harmful substances.

Other components like the pressure regulator, air cylinder, and carabiner play significant roles in the overall function of the SCBA, but they do not directly contribute to the fit of the mask on the user's face, which is why they are not the correct answer in this context.