When our camera lenses get dusty, we naturally want to blow on them to get rid of the dust. This method is not only ineffective, but actually bad for the lens because your breath contains acidic elements that can cause damage.

DIY Photography posts a reply from Nikon support that explains what you should do:

The best way to clean a lens is to use a piece of lint free lens tissue and a small amount of Lens solution. Do not use anything containing abrasives or solvents, only use Lens Solution.

First we recommend taking a small blower brush to blow off or brush away loose dust or debris. Next, place a drop or two of cleaner on the tissue (never directly onto the lens) and then wipe the lens in a circular motion, beginning in the center and working your way outward, removing any marks or smear.

If the above supplies are not available a clean, dry, soft, lint free cloth can be used to clean the lens. Do not breathe on the lens to fog it for cleaning. There are harmful acids in breath that can damage lens coatings. Just use the blower bulb, then brush, and wipe the lens in a circular spiral from the center outward.

The same method can be used to clean the viewfinder eyepiece of Nikon cameras. You may want to immediately remove dust particles on your lens, but it's better to let them sit than to try and blow them away. Instead, just wait until you've got the eight minutes required for a proper cleaning. Your lens will thank you for it.

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No, You Shouldn't Blow on a Camera Lens to Clean It