What is a requirement specifically for ear plugs?

Ear plugs are essential for protecting hearing in environments with high noise levels. Proper fitting is crucial because if ear plugs do not fit correctly, they may not provide adequate noise reduction, potentially leading to hearing damage. A proper fit ensures that the plugs seal the ear canal effectively, blocking out harmful sounds while remaining comfortable during extended wear.

The other options present characteristics that do not pertain to the essential requirements of ear plugs. For instance, ear plugs do not need to be metallic, as materials that are soft and moldable are often preferred for comfort and effectiveness. Additionally, while ear plugs can be reused, they cannot be used indefinitely; they must be replaced after a certain period to maintain hygiene and effectiveness. Finally, while adjustable ear plugs do exist, not all ear plugs need this feature to provide the necessary protection, making them unnecessary as a basic requirement.