The Centers for Disease Control and Prevention (CDC) paper bag directive suggests discarding a disposable N95 mask after five uses. As a general rule, filters can be used up to seven times, depending on how often you wash your mask. Filters cannot be cleaned, but they can provide an extra layer of protection. It's important to choose a filter that is breathable and high-flow if you have asthma or COPD.
You can increase filtration efficiency by using a two-layer mask with a pocket where you can insert a filter. The outer layers must be made of tightly woven fabric. Masks that have built-in filters, such as some construction dust masks and filtering face masks, are designed to be disposable. Reusable filters are more the domain of air purifiers, vacuums and other appliances, not individual masks.
Activated carbon filters will extract moisture from the air even when you're not breathing through them. Masks that filter smaller particles clog up faster, because a large particle can block many more access points and because there are much smaller things in the air that can't even be seen with the naked eye. Filters will help you avoid breathing other things you don't want to breathe; things like smoke particles, perfume odors or garbage, dust and dandruff can be filtered out with a mask with a filter. On the other hand, a poorly cleaned filter in an all-room air purifier means the purifier won't work as well.
Some have filters built into the mask itself, so the mask is disposable because the filter cannot be replaced. Devabhaktuni Srikrishna, air quality engineer and founder of Patient Knowhow, told Verywell that when it comes to replacing an N95 respirator, “the loss of seal is the most important factor, because “the seal will come out before the filter and, once the seal is lost, air goes in and out. The masks you buy on Etsy, for example, can filter out the same or more amount of particles with the right type of filter, but because they don't have a face seal, they can't be certified. In the case of new filters, the damage means that the filter may not filter properly and you could risk inhaling filtration material, such as activated carbon, which can irritate the lungs and cause damage.
As Christopher Zangmeister, a researcher at the National Institute of Standards and Technology who tested 32 mask materials in a study explains, the air and the respiratory droplets it contains will seek the path of least resistance and escape through parts of the mask that are not covered by a filter. A poorly cleaned mask filter puts you directly at risk of inhaling what you think should leak out.