Isha Ray, writing with and channeling the late Kirk R. Smith, in Lancet Global Health:

In the past 40 decades, there have been many innovations in the development of low-cost and efficacious technologies for WASH and household air pollution, but many of these technologies have been associated with disappointing health outcomes, often because low-income households have either not adopted, or inconsistently adopted, these technologies. In this Viewpoint, we argue that public health researchers (ourselves included) have had an oversimplified understanding of poverty; our work has not focused on insights into the lived experience of poverty, with its uncertainties, stresses from constant scarcity, and attendant fears. Such insights are central to understanding why technologies for safe water or clean cooking are unused by so many households that could benefit from them. We argue that, rather than improved versions of household-scale delivery models, transformative investments in safe water and clean cooking for all require utility-scale service models.

In the months before Kirk passed away, this topic — the combination of WASH and HAP interventions, the merging of decades of thinking about usage, service delivery, affordability, and quality of interventions — was a common theme. See also ‘Let the “A” in WASH Stand for Air: Integrating Research and Interventions to Improve Household Air Pollution (HAP) and Water, Sanitation and Hygiene (WaSH) in Low-Income Settings.’

an aside: I think they must have meant “the past 40 years” or “past 4 decades” — but also Kirk worked on these issues with such mental intensity and productive output it may as well have been 40 decades.