Using a coffee filter as a filter
Some DIYers are adding their own filters to masks, and some are using coffee filters. Unfortunately, a coffee filter doesn’t allow air to pass through, so you wind up breathing around it, rather than through it, which defeats the purpose of the filter. If you want to add your own filter to a mask, use polypropylene fabric. If you cannot find this, you can use two pieces of facial tissue, folded over. But be sure to switch those out daily.