The filters in these types of masks must eliminate ninety-nine percent of toxic particles while still allowing enough airflow for the wearer to breathe. Because of this, these masks are designed for short-term use; once the filter becomes clogged, the mask can no longer be used. You might wonder why larger filters aren't used. The answer is that using a larger filter would require a correspondingly larger mask, which would increase the weight of the equipment, making it difficult and impractical for individuals to carry and use.