Over time, humidifiers can pick up odours because of bacteria, mildew, or just stagnant water. You can get your smelling fresh again with borax.
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All you have to do is dissolve about 1/2 cup of borax in four litres of water and add it to your humidifier. Run it for 10-15 minutes, then rinse out the tank, add clean water, and you're good to go. White vinegar also works well, but if you have an aversion to the smell of vinegar, borax offers a nice alternative.