Check out the research from Texaas A&M University Research Department - it should answer all of your questions, they tell you which MERV filter to use and how many layers you need depending on the rating.
https://www.expressnews.com/news/health/article/Texas-A-M-researchers-develop-DIY-respirators-15169122.php
Check out the research from Texaas A&M University Research Department - it should answer all of your questions, they tell you which MERV filter to use and how many layers you need depending on the rating.
https://www.expressnews.com/news/health/article/Texas-A-M-researchers-develop-DIY-respirators-15169122.php