There are two main IV filter pore sizes; the 0.2 micron filter is used for aqueous solutions, and the 1.2 micron filter is recommended for larger molecule solutions such as lipids. The 0.2 micron filter has also been reported to remove air, microorganisms and particulate matter. In addition, endotoxin retention is reportedly achieved by using a positively charged filter membrane; toxic macro-molecules are released by gram-negative bacteria and are claimed to be effective for up to ninety six hours (Bethune 2001).