Why might a respirator not fit properly?

A respirator may not fit properly if the user has not been fit-tested because fit testing is a crucial step in ensuring that the respirator provides an adequate seal around the face. Each individual has unique facial features, including size and shape, which means that a respirator that fits one person may not fit another person correctly. During fit testing, proper techniques are used to assess whether the respirator forms a complete seal and whether it is the right model and size for the user. Without this specific testing, there's no assurance that the respirator will protect against respiratory hazards, leading potentially to inadequate protection in hazardous environments.

The other options do not effectively address the key aspect of fit testing. A universally adjustable respirator does not guarantee a good fit, as individual facial geometry plays a significant role. While a respirator can have defects or require maintenance, these factors are not the primary causes of poor fit compared to the importance of proper fit testing.