A midsummer heat wave covering much of the US, Midwest, Great Plains, and South has led to the deaths of at least two dozen people and is now spreading to the Great Lakes and Northeastern states. The National Weather Service said that 141 million people were under a heat advisory because of the above-average temperatures.

At least 17 states reached temperatures of more than 100 degrees Fahrenheit on Tuesday, with many more areas experiencing temperatures of 90 degrees or more. The National Weather Service declares a heat wave when there are at least five days of 90-degree weather.

The death toll is expected to rise as the heat spreads over major population centers on the East Coast. The impact of the heat is compounded by high humidity levels, which makes it hard for the body to cool itself. Most at risk are the poor, the very young, the sick and the elderly.

In a statement to the press, Accuweather meteorologist Jim Andrews said, “With the number of days of extreme heat and humidity of the current heat wave, it may be more significant and impact a larger area than the deadly 1995 heat wave.” That heat wave claimed more than 700 lives and hospitalized thousands.

The National Weather Service warned of dangerous levels of heat and humidity moving eastward with no relief expected until Sunday. Eli Jacks, a meteorologist with the National Weather Service told the BBC, there’s no good place to be, adding, “heat is the number one weather-related killer in the US.”

In Oklahoma, at least 12 deaths have been blamed on the heat that has blanketed the state for the past several weeks. At least three deaths were attributed to hyperthermia, overheating of the body, and excessive heat was the factor in the death of a fourth person. Eight other deaths in the state are also thought to be due to the heat. Many other people have suffered heat-related illnesses. Paramedics have responded to 144 heat-related calls in the Oklahoma City metro area, and 92 have been taken to the hospital.

Oklahoma Governor Mary Fallin declared a state of emergency Wednesday, due to the prolonged heat and drought.

In the Kansas City, Missouri, metropolitan area, heat may have contributed to 13 deaths, with the high temperatures expected to persist through the end of the month. The deaths include a 75-year-old woman and two men ages 66 and 47.

At least one Detroit-area death is blamed on the heat. A 37-year-old man was found dead in his non-air-conditioned home in Highland Township Wednesday afternoon. The medical investigation indicated he was sent home from his job at a restaurant after he became disoriented.

Fire department medics reported a spike in emergency calls in Chicago from people overcome by the heat.

In Wichita, Kansas, doctors treated 25 heat-related illnesses, and in Des Moines, Iowa, 16 people were hospitalized.

Medical professionals stressed the importance of an air-conditioned environment. According to one health official, a big concern is the “furnace effect.” People whose only source of cooling is a box fan run the risk of literally baking, because in an enclosed room the humidity can get so high sweat doesn’t evaporate.

While the extended hot weather is not all that unusual for July, its impact has been compounded by the decrepit state of the US electrical infrastructure, which has failed in many parts of the country, leaving tens of thousands to bake in the heat.

Baltimore Gas and electric issued a warning to its customers of potential blackouts due to the high demand for energy, and New York State is expecting record electrical demand this week.

Power outages were reported in the Chicago area, with 7,300 homes and businesses without electricity in the region. Some 5,100 were without power in Chicago proper, including 2,000 customers in the city’s Lakeview neighborhood.

Meanwhile, 29,000 people in the Detroit metropolitan area are without power due to widespread failures in the electrical grid, with temperatures expected to reach 100 degrees Fahrenheit with high humidity in the coming days. The city of Ferndale was hardest hit, with 5,000 losing power on Wednesday due to a heat-related malfunction at a substation. Areas of Detroit are also facing selective rolling blackouts (see accompanying article, “Ferndale residents speak on Power outages”).