Anti-allergy bagged systems avoid most of this risk with a bag that will contain dirt and contaminants, and wet filter bagless systems also allow emptying without exposure to airborne particles as it is contained in a 'mud'. While Wet filter systems do avoid the cost and environmental waste of needing new bag for every filling, like wet/dry and shampooing machines they do generally require a very small amount of anti foaming agent, though this is generally environmentally benign. Some Hypoallergenic vacuum bags differ a great deal from non-hypoallergenic bags. Hypoallergenic bags often come in two pieces, an inner and outer bag. The inner bag (which captures the dirt) will be disposable and completely contained. Outer bags will sometimes take the place of a filter all together filtering out dust and allergens out of the air. Some modern hypoallergenic vacuum cleaners will use the two bag system with the outer bag made up of high-performance filtration material treated with antimicrobials to impede the growth of mold, bacteria and other parasites. A two bag system will still require a final HEPA filter to be able to comply with the standards of HEPA filtration. All good Hypo-allergenic systems are designed, not just to contain 99+% of containments (99.97% in the case of HEPA systems), but to clean the air while cleaning the floors, some, including water filter models, achieving retention of particles over 0.3 micrometer diameter of 99.99% (1 particle in 10,000 released). HEPA filters alone are not enough for even the best vacuum cleaner to work effectively. Whether it’s bagged or bagless, a truly hypoallergenic vacuum requires that it have a completely sealed system. All intake air must pass through the dirt capture and filtration systems before it exits the vacuum, any leaks will allow dust and allergens to be ejected back into the air you breathe before the HEPA filter has a chance to do its job, meaning the system itself is no longer HEPA grade despite the presence of such a filter. However, some users who suffer from dust mite allergies, evidence a preference for bagless technology; as most of the dirt is trapped inside the chamber via the use of cyclonic technology, which is, by design, a potentially inherently cleaner technology. Despite progress in filtration systems for vacuum cleaners, central vacuums that vent outdoors or into a nonliving space are preferable for individuals with allergies or asthma, because they prevent indoor emissions altogether.