As millions of people across the country continue to broil under soaring temperatures, many cities are investing in a practical way to help residents beat the heat: cooling centers.

A smattering of studies indicate that cooling centers can help reduce heat-related health emergencies and fatalities. They are a common-sense remedy for residents dealing with an unrelenting heat wave, but one whose efficacy is hard to measure.

The facilities are not always accessible to those who need them — particularly at night, which can be a problem when the temperatures stay stifling after the sun goes down.

Those who frequent cooling centers tend to be older and have lower incomes than the general population, studies show, and many of them have no shelter at all. That, in turn, has led to a sort of “social stigma,” according to Kathryn C. Conlon, an environmental epidemiologist and associate professor at the University of California, Davis, leading people to see cooling centers as a last resort rather than a reprieve.

But with the heat wave that has engulfed much of the Western United States for more than a week lingering on — triple-digit temperatures are expected to stretch into the weekend for millions — the cool, communal spaces have been welcome amenities.