Product review (4 stars): Humidifier filters
I have had a Holmes tower humidifier for several years and it requires two of the HWF100 filters.  This particular model forces air through the bottom at the back and through the filters out the top.  I guess like most filters, the top of the filter dries out quicker than the bottom where water is absorbed.  To get a longer life from the filters I reverse the filters top to bottom about 2 times per week.  This extends the life of the filter about 2 times.  Other brand filters don't live up to the quality of the Holmes HWF100.  Prices vary so shop around.