That's the one. From the conclusion: > Indeed, with a benefit-to-cost ratio of 0.2σ increase in test scores per $1,000 of expenditure, air filter installation

That's the one. From the conclusion: > Indeed, with a benefit-to-cost ratio of 0.2σ increase in test scores per $1,000 of expenditure, air filter installation is one of the most cost-effective education policies available to policymakers today. > Specifically, economically disadvantaged students disproportionately attend schools in highly-polluted regions. Given this, installing air filters in polluted regions should both raise student achievement and reduce the pervasive test score gaps that plague public education.