Care and Maintenance of a Harmonica
Care and Maintenance of a Harmonica

If you blow into the instrument while it is cold, the moisture in your breath will condense inside the instrument, and the valves will stick to the reed plate and cease to move normally. Notes not sounding immediately after breath is blown in and sputtering out late is caused by condensation on the valves. When your harmonica is cold, place it in your armpit or otherwise warm it up before you play.

Slide levers can become hard to move when clogged with dirt, grime, or the like. The movement will deteriorate soon if you practice while eating or drinking something sugary. If your throat or tongue become dry while practicing, have a glass of water ready.

Stowing a still-moist instrument can cause rust or mold.
However, you should absolutely never dry your instrument in something like a dryer or a stove! The heat will warp the valves or the body, causing breath to leak out and the notes to become muffled. It is best to let your instrument dry naturally after playing. Dry it in a dust-free, well-ventilated place. It is also a good idea to put a desiccant in your harmonica case.

If even the smallest piece of grime gets between the reeds, the instrument will no longer produce its natural tones. When wiping down your mouthpiece after practice, use gauze or another material with which fibers and dust are unlikely to get inside. Avoid rough-textured tissue paper.

If you drop your instrument, the shock may slightly skew the reeds making the notes buzz, or the lever may bend, immediately making it difficult to move.

(Reference: Zettai! Umaku Naru Haamonika 100 no Kotsu, Yamaha Music Media)