With at-home Covid-19 tests in high demand and their efficacy in question, health departments from California to Massachusetts are turning to sewage samples to get a better idea of how much the coronavirus is spreading through communities and what might be in store for health care systems.

Experts say wastewater holds the key to better understanding the public health of cities and neighborhoods, especially in underserved areas that do not have equal access to care.

“Every time an infected person uses the toilet, they’re flushing this information down the toilet, where it’s collecting and aggregating and mixing with poop from thousands of other people,” said Newsha Ghaeli, a co-founder and the president of Biobot Analytics, a wastewater epidemiology company based in Massachusetts.

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