More than 100 million Americans will be exposed to extreme heat by 2053, mostly in the South and Midwest

A pedestrian uses an umbrella to get some relief from the sun as she walks past a sign displaying the temperature on June 20, 2017 in Phoenix, Arizona.

Scientists think that climate change will make the problem worse in the future.

According to a new model released on Monday by the First Street Foundation, about 50 U.S. counties will experience heat index temperatures over 125 degrees next year.

Those counties are at the highest level of the National Weather Service's heat index, which combines temperatures with humidity readings to estimate how hot it really is outside.

Thirty years from now, those extreme temperatures will be experienced in more than a thousand counties. The 30-year model is used by First Street because it is the most popular mortgage product in the US.

The southern part of the country is seeing the biggest increase in heat. The number of extreme heat days in Texas and Florida will double in the next 30 years. The work-from-anywhere culture emerged and people fled for sunnier climates in recent years.

Big change is going to happen in the Midwest. There is a "Extreme Heat Belt" going from Northern Texas and Louisiana to Illinois and Indiana. The Midwest has less water to deal with the heat than Florida. The heat in higher humidity areas is not as pleasant.

Matthew Eby, CEO of First Street Foundation, said that a quarter of the country will fall inside the Extreme Heat Belt with temperatures over 125F.

First Street uses high-resolution measurements of land surface temperatures, canopy cover, impervious surfaces, land cover, and proximity to water to calculate the current heat exposure, and then adjusts for future predicted emissions scenarios from the United Nations Intergovernmental Panel on Climate Change. The number of days a property will experience dangerous levels of heat is determined by this.

Flood and fire predictions for every home in America can be found on the group's website and on the website of the real estate company. There is a heat factor on First Street.

Climate migration is already underway in some flood and fire-prone areas because of the heat.

He said that if people move then there will be an impact to the tax base and changes to demand for properties and values.

Depending on the outcome of emissions pledges, the models could change. A moderate model is used in First Street's scenario.

Everyone else would have to be cutting in order for us to be low on our forecast. We are not going to be high. The emission scenarios will get worse.