The effect of loading on the filtration performance of breathing system filters.

The filtration performance of breathing system filters can be measured by challenging the filter with sodium chloride particles. However, the challenge itself may alter the filtration performance. This effect was determined by repeating a standard challenge test five times on one sample each of 18 different electrostatic filters and seven pleated hydrophobic filters. The penetration of sodium chloride particles through all 18 electrostatic filters increased as the load on the filter increased. In contrast, there was a decrease in penetration through six of the seven pleated hydrophobic filters (p < 0.0001). However, the changes in penetration values were small compared to the large range of penetration values of the filters, and the effect of loading on penetration values using the challenge specified in the European standard is small.