The feasibility of various filtration and sorbtion processes for use in collective protectors is studied with a view to reducing the overall cost of collective protection. Reports on electrostatic precipitation, venturi scrubbers, catalytic oxidation, fluidized beds, and other techniques are reviewed. It is concluded that pleated paper and charcoal filters still offer the most advantages in simplicity, reliability, durability, and cost, and that no new techniques or materials offer an improvement in overall effectiveness. Redesign of the present collective protectors should permit a useful weight and size reduction, and possibly a slight cost reduction. However, it does not appear to be possible within the present state of art to build a collective protector for 0.25 per cfm capacity.