When sucking up dust and pet hair, it’s not what you can see that can cause irritation and allergic reactions, but the microscopic dust that you can’t.
Pet dander, for example, are microscopic skin cells shed from animals that cause reactions in people with allergies.
HEPA filters are designed and rated to prevent these particles from escaping the vacuum cleaner, sealing in the dirt that’s collected. This is very important in a pet vacuum cleaner, particularly if anyone in the house (or a regular visitor) has sensitivities to your animals.