Final answer:
The correct clearance between the head and the shell of a hard hat when adjusting the headband is one inch. This space cushion is crucial for safety and comfort.
Step-by-step explanation:
When adjusting a hard hat's headband, it's important to ensure proper clearance between the head and the shell for both comfort and safety. The correct amount of clearance to leave is one inch. This allows enough space for the shock-absorbing suspension system to work effectively in the event of an impact, reducing the risk of injury. The clearance also provides space for ventilation and can contribute to a more comfortable fit during extended wear.