The advice has changed more than once, which has left a lot of people covering their faces on empty footpaths and uncovering them in busy shops, precisely backwards.

What the distance actually does

Outdoor transmission at distance is very low. Risk rises sharply with proximity, duration and enclosure, none of which describe a quiet walk with two metres of clearance.

The sensible rule is contextual rather than binary: carry a covering, use it where you cannot keep distance, and do not treat the walk itself as the risk.

For anyone exercising hard, a damp covering restricts airflow enough to matter. That is a comfort argument rather than a safety one, but it is a real one.