I bought a Zimpure purifier over a year ago and took it apart last month, planning to replace the filter. I was surprised to see filters used for respirators inside and wondered if I had been bamboozled. Zimpure is intended to draw fumes by being positioned close to the nozzle, but I do not feel the suction power of the fan is enough to do so adequately.
After researching different filters, I wanted to know what was truly necessary for handling VOCs and UFPs emissions. To get a definitive answer, I contacted one of the authors of the blog post I listed under the reports section, Kevin L. Dunn.
I sent an email to Kevin, asking about face mask filters and Brownian motion. I received permission to quote the following information:
NIOSH [The National Institute for Occupational Safety and Health] has conducted a lot of research on the effectiveness of respirator filters for the filtration of nanoparticles. As you mention, Brownian motion does play the primary role in the removal of the smallest particles (<100nm) by filtration. And tests of N95 respirator filters against nanoparticles have shown that at the most penetrating particle size (MPPS), filtration efficiencies range between 90-98% showing that they are highly effective at removing particles of all sizes, including nanoparticles. Several studies showed that P100 filters offer higher filtration efficiencies typically greater than 99% (Shaffer and Renagsamy 2009). So either of these filter types should provide good filtration for particulates. The filtration of VOCs is more complicated and requires the use of an additional type of filtration like carbon beds. The effectiveness of these filters depend on many factors including the airflow rate (affects residence time in the carbon bed), inlet mass loading, the working capacity of the carbon bed, and the temperature and humidity of the gas stream….
As far as using filtering facepiece (FFR) filters on a fan outlet to remove particles, they are not made for that purpose and may not be feasible. However, some companies have made ventilated enclosures using a fan and high efficiency particulate air (HEPA) filter on the outlet which should provide good removal of particles and may be appropriate for your application.
With this information, I was able to determine that filters blocking 95 to 99.9% of particles .3 microns or larger are sufficient for use in air filter systems to block nano particles, due to Brownian motion. “The ‘N95’ designation means that when subjected to careful testing, the respirator blocks at least 95 percent of very small (0.3 micron) test particles.”link “A P100 rating is the highest for personal respiratory protection. As long as your mask fits properly a P100 filter will block 99.9% of particles .3 microns or larger.”link
The Environmental Protection Agency states the following: “HEPA is a type of pleated mechanical air filter… This type of air filter can theoretically remove at least 99.97% of dust, pollen, mold, bacteria, and any airborne particles with a size of 0.3 microns (µm). The diameter specification of 0.3 microns responds to the worst case; the most penetrating particle size (MPPS). Particles that are larger or smaller are trapped with even higher efficiency. Using the worst case particle size results in the worst case efficiency rating (i.e. 99.97% or better for all particle sizes).”
Click here for information on the history of HEPA filters
Sarah Aquirre, author of a blog post entitled, “What is a HEPA filter?” suggests confirming that a filter is true HEPA before purchase. I saw many ads claiming filters were true HEPA, but further investigation proved otherwise.
The Summary of the 2nd report listed near the beginning of this post relayed the success of an enclosure made by the NIOSH team. The enclosure was “highly effective” in decreasing particulate and TVOCs in the print room. The enclosure covered 10 printers.
“There were 10 “desktop-scale” 3-D printers arranged on shelves 2 high by 5 wide and one freestanding industrial-scale 3-D printer (not in use at the time of our survey). A recirculating wall-mounted air conditioning unit was in the print room but no ventilation.”
The enclosure was custom-built by NIOSH based on dimensions of the shelving units used to hold the 3-D printers. As shown in Figure 12, the enclosure consisted of acrylic panels for doors that were held closed using magnetic strips. The enclosure was then ventilated using a portable floor fan (Model SS-400-PYT, Sentry Air Systems Inc., Houston, TX) connected to 6-inch diameter flexible hose (Model W1036, Woodstock International Inc., Bellingham, WA) to exhaust the air around the printers. The portable floor fan had a variable speed controller rated to move air at up of 700 cubic feet per minute (CFM) and the air was passed through a high efficiency particulate air (HEPA) filter (Model SS-400-HF, Sentry Air Systems Inc., Houston, TX) and an 8 lb carbon bed (Model SS-408-CF, Sentry Air Systems Inc., Houston, TX) to remove particles and organic vapors, respectively before discharging the air back into the room.
Regarding filtering VOCs, I am not clear on what would be needed for a single 3D printer. Kevin Dunn mentioned carbon bed, airflow rate, mass loading, capacity, temperature, and humidity. This requires further research and investigation. I will update this topic in February or March 2020. At this time, the outtake filter on my 8 cf3 enclosure has two new fiber sheets of activated carbon, one HEPA-like filter, and a fan with max air flow of 52 CFM (cubic feet per minute). It is time to replace the HEPA-like filter. I want to be as sure as I can be that I make changes producing optimal results.