Extreme heat

Climate change and extreme heat

Climate change is driving more frequent, intense, and lasting extreme heat events in Massachusetts and across the country. Since the early 1900s, Massachusetts has warmed by nearly 3.5°F, and experts project a continued rise. By 2050, the number of days above 90°F in the state is expected to double or even triple compared to historical averages. These rising temperatures are not only uncomfortable; they also pose serious health and infrastructure challenges, especially in urban and underserved areas.

In Massachusetts we are not as used to or prepared for extreme heat the way southern and western states are. While we are more used to cold weather and preparing for snow, extreme heat is a newer public health concern for Massachusetts residents.

The First is the Worst

In Massachusetts, extreme heat is becoming a more serious health concern as summer temperatures rise year after year. Data show that the first heat wave of the season often the worst for people’s health, leading to more heat-related illnesses and hospital visits than later heat waves. This is because our bodies need time to adjust to the heat. That’s why it’s important to prepare early—before the first hot days arrive.

When it is hot, it is usually humid

In Massachusetts, heat often comes with high humidity, which makes hot weather feel even hotter. This "feels like" temperature is known as the heat index. It usually tracks the actual temperature closely, rising and falling together about 97% of the time. Because temperature data is easier to access and understand, it’s a reliable way to track heat risk.

Unhealthy Heat Threshold

When we compared heat waves over several prior summers, our data show that when temperature rises to 85°F or more for three days in a row, there is a significant increase in emergency room visits and heat-related illnesses.  The Unhealthy Heat Threshold (85°F for 3 or more days) identifies predictably significant health impacts and was consistent across summers.

Unhealthy Heat Alerts and Forecast Page

Temperatures of 85°F for 3 or more  days in a row will trigger an unhealthy heat alert notifications for healthcare professionals, local governments and other community and state organizations so they can plan, prepare and act. Our Unhealthy Heat Forecast page has a status banner that tracks daily and weekly levels of heat in each city and town in the state. The banner turns red when an unhealthy heat alert is active and shows when high temperatures may pose a greater risk for vulnerable people like children, older adults, and those with chronic medical conditions.

Exposure to extreme heat can overwhelm your body’s ability to regulate temperature, leading to heat-related illnesses such as heat cramps, heat exhaustion, and heat stroke. These conditions can get worse quickly and may be fatal if not treated. Extreme heat can also worsen chronic health conditions, including heart disease, respiratory disorders, kidney disease, and diabetes.

Symptoms of heat-related illness can include dizziness, rapid heartbeat, nausea, confusion, muscle cramps, and excessive sweating or lack of sweating. It's important to stay hydrated, seek shade or air-conditioned environm

Vulnerable Populations

While extreme heat can affect anyone, certain groups are more:

• Older adults (65+)
• Infants and young children
• People with chronic medical conditions
• People with disabilities or limited mobility
• Pregnant individuals
• People experiencing homelessness
• Individuals without access to air conditioning
• Outdoor workers and athletes
• People with limited English proficiency
• Communities of color, due to the effects of systemic racism and environmental injustice

Targeted outreach and planning are essential to ensure these populations are protected during heat waves.

Extreme heat and poor air quality

High heat days often coincide with poor air quality conditions, particularly in urban areas or from wildfire smoke. Extreme heat increases the formation of ground-level ozone, a pollutant that can worsen asthma, bronchitis, and other respiratory and cardiovascular conditions. During heat waves, air can become stagnant, trapping pollutants and allergens and making breathing difficult—especially for children, older adults, and those with pre-existing health conditions. Public health alerts during heat waves may also include air quality advisories to inform residents of added health risks.