Most I've seen cited by mask proponents either don't use control groups (and thus can't really distinguish btwn other factors that correlate with mask wearing) or are laboratory tests just testing what % of particles are blocked by masks--which ignores almost every relevant question about mask mandates and relies on suspect theoretical models about how covid actually transmits between people.
I'm mystified why people on both sides are so up in arms about masks, but nobody gives one shit about uncontroversal and more effective mitigation techniques like air filtration and circulation.
An infected kid in classroom without proper air circulation could fill the classroom with enough virus to cause a super spreader event--mask or no-cotton/surgical mask.