Shop-vac, a major manufacturer of HEPA material strongly advises against using their vacuum bags to make a filter
First, the filter materials used in the vacuum cleaners are for no purpose other than to protect the end users of Shop Vac products from particles or debris being expelled back into the ambient air during operation. They are in no way designed or intended to protect humans from bacteria, viruses or other pathogens. Second, direct contact or coverage of the human mouth or nose with the filter materials are strictly forbidden for any purpose. Under no circumstances should anyone attempt to make a mask or mask material from any filters manufactured, sold or distributed in the market for or by Shop-Vac Corporation. These materials were not designed, intended or designated for this type of use. Please consult the CDC website for more information at https://www.cdc.gov/coronavirus/2019-ncov/index.html.
So, if you want to use HEPA material at least make sure it is enclosed by another filter material to stop the fibreglass fragments being breathed in.
Dr. Wang’s group tested two types of air filters. An allergy-reduction HVAC filter worked the best, capturing 89 percent of particles with one layer and 94 percent with two layers. A furnace filter captured 75 percent with two layers, but required six layers to achieve 95 percent. To find a filter similar to those tested, look for a minimum efficiency reporting value (MERV) rating of 12 or higher or a microparticle performance rating of 1900 or higher.
The problem with air filters is that they potentially could shed small fibers that would be risky to inhale. So if you want to use a filter, you need to sandwich the filter between two layers of cotton fabric. Dr. Wang said one of his grad students made his own mask by following the instructions in the C.D.C. video, but adding several layers of filter material inside a bandanna.
So, someone needs to test exactly how many small fibres are potentially released during the pressures of normal inspiration as fibreglass can set up an inflammatory reaction in the lung, or, even cancer in rats
Little information is known about the health effects caused by small fibers. Smaller fibers have the ability to reach the lower part of the lungs increasing the chance of adverse health effects.
People who work with fiberglass or who have worn-out duct work lined with fiberglass in their homes or workplace may have long-term exposure to fiberglass. There is no evidence that fiberglass causes cancer in people. Animal studies have shown an increased risk of cancer when fiberglass fibers were implanted in the lung tissue of rats, but these studies are controversial because of how the fibers were implanted. Based on these animal studies, the International Agency for Research on Cancer has classified some fibers used in fiberglass as possible human carcinogens (cancer causing agents).
Here's an alternative design using a 3D printer - the Montana Mask which uses bits of a surgical mask for the filter.