Researchers created a test to determine which masks are the least effective

The quest began when a professor at Duke’s School of Medicine was assisting a local group buy masks in bulk to distribute to community members in need. The professor wanted to make sure the group purchased masks that were actually effective.
https://www.cnn.com/2020/08/08/us/duke-university-face-mask-test-trnd/index.html

The actual study:
https://advances.sciencemag.org/content/early/2020/08/07/sciadv.abd3083
In the study published Friday, researchers with Duke’s physics department demonstrated the use of a simple method that uses a laser beam and cell phone to evaluate the efficiency of masks by studying the transmission of respiratory droplets during regular speech.

Researchers tested 14 commonly available masks including a professionally fitted N95 mask, usually reserved for health care workers. First the test was performed with a speaker talking without wearing a mask. Then they did it again while a speaker was wearing a mask. Each mask was tested 10 times.

Neck fleeces, also called gaiter masks and often used by runners, were the least effective. In fact, wearing a fleece mask resulted in a higher number of respiratory droplets because the material seemed to break down larger droplets into smaller particles that are more easily carried away with air.
Folded bandanas and knitted masks also performed poorly and did not offer much protection