If a humidifier has a reusable filter, it can be cleaned with a solution of one part white vinegar to two parts water. First, rinse the filter to remove any loose debris, then immerse it in a bucket with the vinegar solution for 10 minutes to dissolve mineral deposits.
The water that evaporates out of a humidifier leaves behind mineral deposits that were once dissolved in the water. Harder water leaves behind more minerals, but over time almost any filter picks up enough minerals to impact its performance. White vinegar dissolves these minerals and allows them to be rinsed away, leaving the filter clean and clear.
If there are colored deposits on the humidifier filter, this may indicate a mold infestation. Before using vinegar, immerse the filter in a solution of one part bleach to six parts water for at least 10 minutes to kill microorganisms. Rinse to clean, then proceed with the vinegar solution to remove any minerals. Particularly dirty filters may require multiple treatments to remove all contaminants.
If the humidifier uses HEPA filters or other paper filters, these cannot be cleaned and are meant to be replaced. Dispose of the used filter and install a new one according to the manufacturer's instructions.