Almost all current filtration technologies are evaluated on the concept of “capture” rate, measuring the amount of particulates captured on the first pass through the filtration device. The higher the efficiency of a filtration device, the greater the number of particles captured. HEPA filters have a capture rate of 99.97 percent. HEPA and other high-end filters use dense filter media to capture particles physically; the denser the filter media, the more particles captured.
Today’s high efficiency filtration technologies do not solve the airborne pathogen problem. Although offering excellent capture rates, they still do not kill or inactivate the organisms they collect. Once captured on an HVAC or portable filtration system, pathogens have a breeding ground for colony formation, which can lead to the spread of microorganisms and possibly their endotoxins. Dust, dead skin cells, and other matter collected on the filter provide nutrients for trapped organisms to grow and thrive. During the course of their life cycle, many of these microorganisms also produce waste products, which can be carried downstream in the airflow.
Certain bacteria, for example, produce endotoxins to which many people are highly allergic. As long as the bacteria are alive, this process continues. Killing or inactivating these bacteria is the only way to stop the spread of harmful organisms and their waste products in the air stream.
In addition, certain environmental conditions, such as high humidity common to HVAC systems in certain parts of the country, actually enable organisms to multiply and grow through filters; a phenomenon known by the unsavory term, “filter ripening.” Even in high-end HEPA systems, pathogens can grow and are then pushed into the airstream by the HVAC blower system. These pathogens can be present in the air stream in far higher numbers than they might have been without the filter’s intermediation. Mold or other fungi growing on a filter will generate high numbers of spores, which are then carried throughout the indoor environment and can be inhaled by the occupants.
Finally, in addition to the airborne threat posed by pathogens growing on and through the filter media, additional spores or colony-bred pathogens trapped on the filter pose a threat to anyone encountering the contaminated filter media. Routine maintenance, disposal, or inspection of the filter can expose personnel to any of the original active, captured pathogens as well as any colonies breeding on the filter.
From: “Airborne Security”