The longevity of filters in reusable masks depends on a variety of factors, such as dust levels and environmental pollution. Generally, the filters can last up to 16 to 24 hours of normal use, but should be replaced within 8 hours if you are engaging in activities with a lot of dust. It is also possible to use the filters up to seven times, depending on how often you wash your mask. NIOSH-certified N95 FFRs are designed to filter 95% of particles when properly fitted to the user's face.
Recent studies have been conducted to determine the effectiveness of certain fabrics in filtering particles, as well as the effectiveness of inserting a HEPA filter into a face mask made or made by yourself, according to Kenneth Méndez, executive director of the Asthma and Allergy Foundation of America. These reusable masks come in many different styles and are designed to be used with a filter, such as a HEPA filter, inserted inside the fabric. These filters are specific to masks with front valves, and they even have a small hole on the side to accommodate one. 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 cannot be certified.
It is important to remember that disposable masks and filters should not be used for too long, as it can become increasingly difficult to breathe through them after extended use. When buying both masks and individual filters, two key considerations are fit and breathability. PM2.5 filters can be powerful protective equipment, but their adsorption properties also have their limits. Masks that have built-in filters, such as some construction dust masks and filtering face masks, are designed to be disposable.
A filter is made of five layers and can effectively filter most small allergens, such as dust and pollen, down to the micron level. The filters are made of six layers of PM2.5 material and are designed to “effectively block air pollutants” according to Vida. The accessories brand Vida offers non-medical face masks for the general public, as well as insertable filters. This filter pack from the Ministry of Supplies is tailored to the brand's 3D printed cloth masks, but it also fits any similarly sized reusable mask with a filter pocket.
It is recommended that you change your filter after a week of use if you decide to use a filter mask, or more often if you use it daily.