> They also looked at a very limited time window. They also note that some teachers weren’t using the air filters. To correct and clarify: They also looked at

> They also looked at a very limited time window. They also note that some teachers weren’t using the air filters. To correct and clarify: They also looked at a very limited time window. They also note that-- after this very limited time window-- some teachers stopped using the air filters. In the words of the authors, this made long-run results "difficult to interpret."