EFFECT OF INDUSTRIAL AEROSOLS ON THE PERFORMANCE OF ELECTRICALLY CHARGED FILTER MATERIAL

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Abstract
Recently the use of high-efficiency mechanical filters in respirators has been shown to be potentially hazardous, because under certain circumstances the respirable fibres they contain can be shed; electrically charged filters are therefore likely to become even more widely used than at present. In general these perform well, because their high internal electric field enhances filtration efficiency without causing any increase in airflow resistance, but they do have a possible drawback in that their performance may deteriorate when they are exposed to aerosols. The practical importance of this depends on its extent. Both this and the nature of the process of deterioration have been investigated by exposing respirator filter materials, on site, to industrial aerosols, comprising aerosols produced during foundry fettling operations, foundry burning, lead component manufacture, lead smelting, silica sand quarrying, refractory brick production, coking and asbestos textile manufacture. In addition the effect of coal dust has been investigated by loading the filters in a dust tunnel. The exposures were carried out in increments in order to obtain quantitative data on filter performance as a function of the mass of aerosol deposited. Initially filter efficiency usually decreases with increasing aerosol load, but at higher loads clogging may compensate, causing filter resistance to increase. Equal masses of different aerosols arc found to cause widely different changes in performance. In general dry dusts cause little loss of performance, but foundry aerosols are more damaging to all types of filter, and coal-tar fume to some. A simple theory of degradation is outlined, predicting that loading filters with homogeneous aerosols causes the penetration to increase exponentially, with a constant of proportionality that quantifies the ability of the aerosol to degrade the filter. Aerosols sampled during static experiments give results that support this prediction: though results on respirators themselves suggest that these collect a heterogeneous aerosol, which can obscure the exponential relationship. In general, different types of filter material are degraded at about the same rate, and this is shown to be consistent with a charge-screening process. The correlation between the behaviour of different materials means that it should be possible to devise a laboratory test to enable filter performance to be estimated. The behaviour of coal-tar fume does not fit the simple pattern, and its effect is consistent with solvent action on soluble polymers, which makes quantitative prediction more difficult. Corrective measures are suggested for any situations in which loss of performance is unacceptable.

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