HEPA filters cut covid-19 sick days but we've been slow proving this
Covid-19 sick days were 20 per cent lower in schools with air-cleaning HEPA filter machines, researchers on an eagerly-awaited study have found. But why has it taken so long to discover how well they work, asks Clare Wilson
![HEPA filters cut covid-19 sick days but we've been slow proving this](https://images.newscientist.com/wp-content/uploads/2023/10/30152545/SEI_177403767.jpg)
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