There are filtered and filter-free humidifiers. Filters can capture bacteria and allergens. However, eventually, they get saturated and need replacement. Depending on how often you run the humidifier, filters may need to be changed every three months. The price of replacement filters adds up over time. Sometimes that can make these models more expensive over the lifetime of the humidifier. Filter-free models have easier maintenance and cost less over time, but their lack of filter is one less barrier for bacteria.