P3 pre-filter clips over STS gas filters to make a combined Particulate and gas filter. Certified and fully compliant with Australian/New Zealand Standard regarding respiratory protective devices AS/NZS 1716:2012. Filters 99.95% of particulates. Mechanical filtration doesn't lose filtration efficiency as it gets full or wet. Filters are water and oil repellent. Hard casing protects filter material from damage. Easy to connect, no filter retainers required, they simply clip onto the gas filter. Pre-filters are limited to single shift use only. Two required per mask. Dimensions: 86mm (dia) x 14.5mm (H) Weight: 20g