The Zip and Block encasement prevents soiling from spills. A soft knit fabric with an ultra-thin breathable polyurethane membrane provides protection from allergen molecules of less than 1.0 microns. The top surface boasts a silky smooth polyester knit and is 100% waterproof. Based on expert recommendations, we designed our Zip & Block series to have an outer zipper flap, an interior zipper flap and reinforced inner seams for the most effective barrier to related allergens. We also included an extra-long rustproof zipper for ease of use, an outer and an inner flap closure and binding on the inside seams for extra strength and protection!