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It is a common feature of solid/gas filtration, and, to a lesser extent, solid/liquid filtration, that the particles in suspension may carry an electrostatic charge, and therefore that a filter medium carrying the opposite charge will be more effective in their removal. Many different media can be given a charge for this purpose, but one in particular has had a long history of effectiveness in this way.

The filtration efficiency of wool felts against sub-micrometre aerosols can be greatly increased by the addition of a special resin, which produces a very long-lived electrostatic effect. The electrostatic charge is generated during processing, the resin powder being agitated in the wool matrix, enabling charge transfer to occur. The wool then has a positive charge and the resin a negative charge, the filter being overall electrically neutral. The random distribution of the resin powder on the wool fibres, and random array of the wool fibres in the filter, means that the electrical field is not uniform and is therefore very effective at capturing both charged and uncharged particles (non-uniformity induces a dipole on to neutral particles, thereby facilitating electrical capture).

The electrical charge thus imparted gives wool resin a high efficiency for filtration of sub-micrometre particles of better than 99.5%, whilst having a very low resistance to air flow. The very high electrical resistivity of the resin means that once generated the charge and filter efficiency are maintained for many years, although tropical conditions can reduce its life.

Wool resin was first developed for use in respirators for combat use in World War I, and today it is still used extensively in the respirator industry, 90 years after its development. Its combination of low breathing resistance and high filtration efficiency is very competitive against more recent materials. Vacuum cleaners and other freestanding dust collectors take advantage of the high efficiency of wool resin against asbestos and other harmful dust. Wool resin is also employed in heating and ventilation work, for example in making clean rooms for computer suites, and prefilters for HEPA filters.

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