used when you want to emphasize how small, unimportant, etc. somebody/something is

used when you are saying that the fact that a particular thing is present in a situation is enough to have an influence on that situation

His mere presence made her feel afraid.

The mere fact that they were prepared to talk was encouraging.

The mere thought of eating made him feel sick.

The merest hint of smoke is enough to make her feel ill.

LEARNit Dictionary