The summer of 2020 brought fear of Covid-19, social distancing – and heat-related health problems that affects tens of millions of Americans.

During those months, more than a quarter of the US population suffered from the effects of extreme heat, according to a study released this week. People reported health symptoms like nausea and cramps, as well as a decreased ability to focus, and the effects hit low-income households the hardest.

The study focuses on last summer but given the brutal heatwaves that have hit the US in 2021, especially in western states, it raises the prospect that the impact of extreme heat is highly likely to be worse this year.