The filtration system for vacuum cleaners varies between machines. Most better quality vacuums today use a filter system so that dust and allergens that are sucked into the vacuum aren't put back into the air. Machines that use a bag to collect dust have no filtration while others are sophisticated enough to filter out pollen and particles less that 100 microns. Certain models have a HEPA (High Efficiency Particulate Arresting) filter, which operates at a higher efficiency rate and removes the smallest particles. More than one type of filter may be used in some of these systems, increasing their effectiveness. These units generally cost more but are very effective in reducing allergen in your home.