In normal use, lightly breathing on a lens to fog it before wiping is generally considered safe and has long been common practice. The main benefit is that the condensed moisture can help lift fingerprints or light grime so they wipe away more easily.

The practical downside is not usually “acid in your breath” damaging coatings. Modern lens coatings are far more durable than that concern suggests. A more realistic issue is that breath leaves moisture on the surface, and if you wipe with a dirty or abrasive cloth you could smear contaminants or cause fine scratches. In cold conditions, condensation can also be heavier than intended.

Best practice: use a clean microfiber cloth or proper lens tissue, and only gentle pressure. Remove loose grit first so you’re not grinding particles into the glass. For stubborn marks, use a proper lens-cleaning solution instead of repeated wiping.

So: occasional breath-fogging for light cleaning is widely regarded as OK, but clean materials and careful technique matter more than the fog itself.