It’s not the heat, it’s the … killer heat index.

As the Big Apple bakes with higher-than-normal temps this week, a new study suggests that the number of dangerously hot days each year will skyrocket this century if climate change is not addressed.

In fact, even the dreaded “heat index” – the “feels-like” temperature that includes the relative humidity – could be broken, according to a report released Tuesday by the Union of Concerned Scientists.

“Our analysis shows a hotter future that’s hard to imagine today,” study co-author Kristina Dahl, a climate scientist at the Union of Concerned Scientists, said in a statement.

“Nearly everywhere, people will experience more days of dangerous heat in the next few decades,” she added.

The study — titled “Killer Heat in the United States: The Future of Dangerously Hot Days” – is the fist to use the heat index, instead of just temperature, into account when determining the impact of global warming, Dahl said.

For the study, UCS scientists used four heat-index thresholds, each of which brings increasingly dangerous health risks — above 90 degrees, above 100 degrees, above 105 degrees and “off the charts.”

“Off-the-charts days are so extreme they exceed the upper limits of the National Weather Service heat index scale, which starts topping out at or above a heat index of 127°F, depending on the combination of temperature and humidity,” according to the report.

By 2050, hundreds of American cities could see an entire month each year with heat-index temps topping 100 degrees if nothing is done to rein in global warming, the scientists found.

On some days, the conditions would be so extreme that they’d exceed the upper limit of the heat index, making it “incalculable,” according to the study.

“We have little to no experience with ‘off-the-charts’ heat in the U.S.,” said Erika Spanger-Siegfried, lead climate analyst at the Union of Concerned Scientists and report co-author.

“These conditions occur at or above a heat index of 127 degrees, depending on temperature and humidity. Exposure to conditions in that range makes it difficult for human bodies to cool themselves and could be deadly.”

Rachel Licker, senior climate scientist at the UCS and another report co-author, said: “The rise in days with extreme heat will change life as we know it nationwide, but with significant regional differences.

“For example, in some regions currently unaccustomed to extreme heat – those such as the upper Midwest, Northeast and Northwest – the ability of people and infrastructure to cope with it is woefully inadequate,” she said.

The Northeast would see an average of 32 days per year with a heat index above 100 degrees, 20 days per year with a heat index above 105 degree and three “off-the-charts” days per year by the end of the century if rapid action is not taken to reduce global warming emissions, the study found.

In New York state, there have been four days per year on average with a heat index above 90 degrees. This would increase to 29 days per year on average by mid-century and 58 by the century’s end, it found.

And historically, there have been zero days per year on average with a heat index above 100 degrees. This would increase to eight days a year on average by mid-century and 23 by the century’s end.

Of the cities with a population of 50,000 or more in the Empire State, New York City, Kingston, Middletown and Poughkeepsie would experience the highest frequency of these days.