Filters utilizing large diameter fibre media rely on electrostatic charges to increase their efficiency of fine particle removal. Large diameter fibre media is normally chosen due to low cost and air flow resistance. However, these filters often lose their electrostatic charge over time because the particles captured on their surface occupy charged sites, thereby neutralizing their electrostatic charge.
"Some fibrous media air filters have electrostatic charges that may be either natural or imposed upon the media during manufacturing. Such filters may demonstrate high efficiency when clean and a drop in efficiency during their actual use cycle. The initial conditioning step of the dust-loading procedure described in this standard may affect the efficiency of the filter but not as much as would be observed in actual service. Therefore, the minimum efficiency during test may be higher than that achieved during actual use."
This paragraph clearly indicates that air filter experts recognize the effect that time has on the efficiency of filters which rely on passive electrostatic charges. Moreover, there are concerns about the potentially inaccurate test results which may occur when applying the standard to coarse fibre filters that rely on an electrostatic charge.