Even then, you cannot see the microscopic particles that the filter catches. Hunter has two models with an auto shutoff feature. All models are shipped with filters. For example, if your new Hunter unit is a replacement for another unit, then the room is probably cleaner than most. However, if you are putting it in a room that has never had an air purifier, then the first filter will need changing sooner.
Most air purifiers use disposable filters.
Hunter units use a combination filter, which is designed to be easily replaced. This pre-filter can be washed with a hose and used again once it has had time to dry. The light comes on after 1,000 hours of use, which is the average time for needing to change the filter. CADR ratings should be used when selecting your air purifier. These are scores from independent laboratory testing which simulates real world conditions.