While convenient, bagless vacuums do have some drawbacks. They use a removable cup that collects dirt and dust through centrifugal force. Once it's full, you remove the cup, carry it to the trashcan and empty it. Doing so, however, means that a cloud of dust and dirt now floats around your trashcan. To avoid this, you have to empty it directly into a bag, and then tie off the bag to prevent leakage. Of course, dust and dirt will still escape as you make the transfer from cup to waste bag. By now, collecting dust and dirt directly into a disposable bag may not seem like such a bad idea.
People with allergies won't appreciate breathing in a cloud of allergens every time they empty the dust cup. The best way to avoid a sneezing fit is to empty the dust outside. You should stand upwind, cover your mouth and nose with a dust mask or other breathable barrier, and empty the cup onto the ground away from the house. Seems like a lot of trouble, compared to remembering to buy a package of vacuum bags now and then.
Regardless of whether you use bagged or bagless vacuums, the best way to protect yourself against household allergens is to purchase a quality, well-designed vacuum. Cheaper models may have loose seals or poor designs that result in either low suction power or uncontrolled dust dispersal within the vacuum itself, requiring cleaning. Vacuums with HEPA filters (High Efficiency Particulate Air filters) use tightly woven fibers -- usually fiberglass -- to trap almost all allergens, even tiny particles of 0.3 microns or larger that would normally pass through either a bagged or bagless vacuum. While these will need replacing occasionally, a HEPA filter -- especially when used in a quality, bagged vacuum -- will provide the best level of household relief for the allergy sufferer.
Keep reading for lots more information about vacuums and allergies.