There is some (inconclusive) evidence that biodiversity conservation, replacing dirt flooring, and reducing environmental noise exposure might be much more cost effective areas for global health funding than we currently price in. These findings are, in my view, 'big if true' - with a big 'if'.
If you know of additional important research on these topics or are working on any of these areas, I'd be interested in your thoughts!
Deena's post only mentioned "of at least one large RCT underway, with results expected in a few years" without further reference, but on cursory googling it might be the CRADLE trial?
While GiveWell doesn't seem to have looked into this specifically, this 2015 review of GiveDirectly mentioned that lack of cement floors was in one of GiveDirectly's two sets of eligibility criteria for its standard campaigns:
Happier Lives Institute's 2021 annual review did mention cement flooring among the "micro-interventions" they wanted to look into (alongside deworming, cataract surgery, digital mental health interventions, etc), but I haven't seen anything by them since on this, so I assume it didn't pass their internal review for further analysis.