Yun said that the department’s current system is unable to track what medical conditions are complicated by a heat related illness. However, emergency room doctors at hospitals in the greater Boston area told the Globe that the number of patients with heat-related complications rose during the heat waves this summer.

“The difference this year is that the heat wave has been earlier in the season,” said Scott Goldberg, director of EMS at Massachusetts General Brigham. “We’re generally not seeing this type of extreme heat for such a long period of time.”

A Globe analysis of EMS calls during the heat waves this summer shows that there were more calls when it was hotter outside. During the recent heat wave on the week of July 15, EMS calls were 6 percent to 21.6 percent above the daily call volume.

This was a consistent trend over the past four years. During a four-day heatwave that began on June 5, 2021, which EMS said was comparable to the one two weeks ago, calls were about 30 percent to 43 percent above the average daily volume.

Center for Disease Control data also shows that emergency room admission for heat related illnesses picked up this summer compared to previous months this year.

According to the CDC Heat and Health tracker, a heat wave is defined as temperatures of 90 degrees and above each day for two days or more.

A threshold for declaring a heat emergency is a consecutive two days or more of 95 degree or higher heat index. The heat index takes into account both the temperature and relative humidity, which is a marker of how hot it feels outside. According to the National Weather Service, the heat index was 94 F at 12:45 p.m. on Thursday, the most current data available.

“When we’re hitting heat indexes above 90-95, it really does become dangerous,” said Yun.

The problem is poised to intensify.

“Heat waves are unquestionably getting worse and will continue getting worse due to emissions,” said Mathew Barlow, a University of Massachusetts Lowell environmental Earth and atmospheric sciences professor.

Barlow says this heat is not a new normal, because we shouldn’t expect it to stay the same; instead, the next several years will be getting hotter.

Doctors are worried because the rising heat affects some of the region’s most vulnerable people such as the elderly and people with chronic medical conditions. Older people, who may have long term medical problems or take medications, need to be extra cautious with the intense heat, they said.

Some populations are at a higher risk of heat illness than others. There are those who are unable to limit their heat exposure such as outdoor workers, military personnel or unhoused people.

The lasting effects of heat are also known to impact those who do not have proper air conditioning, which represents about 5.5 percent of Boston households, according to a Boston Globe analysis of Census data.

Traditionally vulnerable low-income households are more affected by the heat waves because they often live in homes that are not energy-efficient and are expensive to cool. This “energy insecurity” makes it hard to stay comfortable and healthy during extreme heat, according to the US Environmental Protection Agency.

Heat has been rising across the country in the past decades. Major cities have been encountering heat waves more frequently, according to the agency. The average amount of annual heat waves rose from two a year during the 1960s to six a year during the 2010s and 2020s.

As heat waves become more common due to climate change, Massachusetts sits in an uncomfortable position where days will continue to be over 90 F during the summer. The populated urban areas with little tree coverage and open green space makes the region feel hotter.

There are predictions by the Massachusetts Department of Health Bureau of Climate and Environmental Health that by 2050 there will be a 10 to 28 day increase in extreme heat.

“I’m worried that 10 to 15 years from now, we might be looking back in 2024 and say this was actually one of the coolest summers,” said Yun.

Warmer temperatures lead to more hospitalizations for health issues such as heart disease and heat exhaustion. If heat exhaustion isn’t treated, it can turn into heat stroke, which can cause serious illness, brain damage and death.

In severe cases, the heat waves can cause deaths. Even heat-related deaths have been rising in the U.S., with about 1,602 in 2021, 1,722 in 2022, and 2,302 in 2023.

Dr. Tess Wiskel, an emergency physician at Beth Israel Deaconess Medical Center, said the center saw more heat-related illnesses during the July 15 heat wave than in previous ones this summer.

“There’s no question that it’s adding to the burden of care that we need to give in the emergency department,” she said.

Some doctors say they have experience with burnout and feeling stressed but not more than usual due to the ongoing issue of understaffing at ERs.

When temperatures reach high 80s and 90s, the Boston Public Health Commission recommends people to stay in air-conditioned spaces and limit outdoor activities.

“During heat emergencies, cooling centers are available for vulnerable residents. All Boston residents are encouraged to stay hydrated and check in on their neighbors,” said Dr. Bisola Ojikutu, executive director of the Boston Public Health Commission.

Rachel Umansky-Castro can be reached at rachel.umanskycastro@globe.com.