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Extreme Heat and Air Quality

Extreme Heat Events

One of the major impacts of climate change is increases in temperature.(1)  Since 1970, the Northeast has experienced almost 2°F increase in annual temperatures (Adaptation report)(1) .  While these changes are modest, even relatively small changes are predicted to increase the average mean temperature and increase the number of excess heat events during the summer months.  The National Oceanic and Atmospheric Administration (NOAA) reports that continued increases in greenhouse gas emissions will result in a projected 4.5°F to 10°F temperature increase by the 2080s in the Northeast; a substantial reduction in global emissions were reduced substantially would result in a projected 3°F to 6°F warming by the 2080s.  One study found that our region is especially prone to an increase in prevalence of extreme summer temperatures due to temperature and urbanization trends.(2)  Scientists predict climate change will lead to more frequent extreme heat events or heat waves in the summer.  Excessive heat stresses the body’s ability to maintain an ideal internal temperature of approximately 98.6°F.  Individuals that cannot remain cool may increase their risk of experiencing a range of potential adverse health outcomes, including dehydration, heat cramps, heat exhaustion, and heat stroke/sunstroke.(3)  Studies have found that extreme heat events can increase emergency room visits, hospitalization, and deaths.(4)  Children, the elderly, and lower income communities are more likely to experience health effects from extreme heat.

The graphic shows the annual mean temperature at the Blue Hills Observatory. Based on available data, the average annual temperature has risen about 3 degrees Fahrenheit (F) in the past hundred years.

NOAA temperature chart for Blue Hills

Source: Iacono, MJ. (2014).

Heat Stress Vulnerability Assessment

In 2010, DPH/BEH was one of five states that participated in the EPHT Academic Partners in Excellence (APEX) project, led by the University of California at Berkeley. The Heat Vulnerability Index (HVI) map was created to locate populations with vulnerability to heat in metropolitan areas across the United States. The HVI contains a number of factors related to heat-related illness and death. These factors include: quantity of green space, prevalence of chronic disease (e.g., heart disease, diabetes), and the percentage of residents who are disabled or live alone. The aim of the APEX study was to characterize the health status of potentially susceptible individuals living in heat-vulnerable areas. While additional research is underway, preliminary findings of the study suggest that the HVI metric may be a useful tool to identify populations vulnerable to heat effects. (Click here for more information).

Changes in Air Quality from Projected Increases in Temperature


"Good" ozone is present at high altitudes in the atmosphere and is beneficial because it shields the earth from excessive ultraviolet radiation. "Bad" or ground-level ozone is the primary component of smog and is harmful to health. Activities such as driving cars and generating electricity are major contributors to pollutants that form ozone.

Union of Concerned Scientists. (2011). Climate Change and Your Health: Rising Temperatures, Worsening Ozone Pollution. Available from:

Projected increases in temperature related to climate change can result in a number of other impacts, including a decline in air quality from increases in ground-level ozone and changes in particulate matter.  Climate and weather patterns also play a direct role in the formation and movement of air pollutants.  For example, warm, stagnant air is associated with an increased formation of ground- level ozone, a component in smog.  Ground-level ozone is formed when nitrogen oxides (NOx) and volatile organic compounds (VOCs) react in the presence of sunlight and hot weather.  Ground-level ozone can damage lung tissue, reduce lung function, and inflame airways.  For those with asthma or pre-existing respiratory conditions, exposure to ground-level ozone can increase hospitalization and premature death. 

Ground level air pollution such as fine particulate matter with a diameter of 2.5 micrometers (about one ten-thousandth of an inch), commonly referred to as PM2.5, is associated with cardiovascular and respiratory diseases. Sources of these fine particles are typically combustion sources, such as power plants, diesel or gasoline engines, wood burning processes (including forest fires), and industrial processes such as smelters and steel mills. During warm, stagnant days, PM2.5 can remain at ground level; individuals with asthma and other respiratory conditions may experience adverse effects. Since climate change is likely to increase drought conditions, forest fires can increase the level of PM2.5 in the air, which can impact healthy individuals as well as those with pre-existing conditions. The US EPA's Air Quality Index (AQI) is used by weather reports to predict "bad air days" for ground-level ozone and fine particulate pollution. The website provides information that helps users determine the AQI throughout the United States. Monitoring data for both ozone and PM2.5 is available here.

Changes in Allergens

Climate change and projected temperature increases are reported to impact pollen production in several ways.  Warmer spring temperatures can lead to earlier pollination in some plants, while warmer autumn temperatures can lead to a longer growing season.  Increases in temperature and carbon dioxide can also result in increased pollen production.  This longer pollen season will result in a longer allergy season.  Ragweed is one of the most common environmental allergens, triggering hay fever and asthma attacks.  The US EPA reports that at least 26 percent of all Americans have sensitivities to ragweed.(5)  A longer allergy season may increase visits to physicians’ offices and medical facilities. Hay fever already accounts for more than 13 million visits annually to health care providers.

Urban Heat Island

testClimate change is projected to impact all populations; however, people living in cities and urban centers may experience greater climate change-related impacts.  The "urban heat island" (UHI) effect occurs in densely populated cities and metropolitan areas dominated by concrete structures and lack natural cooling systems, such as streams and forests.  The US EPA reports that the UHI effect can increase average temperatures in urban environments by as much as 5.4°F during the day and 22°F in the evening over the surrounding rural areas.(6)  Various regions of the state are susceptible to UHI impacts. At least 36 percent of Massachusetts' land area is classified as urban.

Since the built environment magnifies local temperatures, urban areas are more likely to experience excessive heat events and increased rates of ground-level ozone formation.   Heat extremes and urban pollution sources produce air quality conditions that have significant impacts to human health.  Studies have shown that hottest days often coincide with high concentrations of ground level ozone and other pollutants(7),  which can result in major health risks to vulnerable populations.   One peer reviewed study predicts hospitalization and pre-mature death to increase in urban centers(8).  Planning and assessment of existing infrastructure and the current population, including vulnerable groups, are needed to adequately address the unique challenges presented in urban communities.     

  1. EOEEA. (2011). Massachusetts Climate Change Adaptation Report. Available from
  2. Duffy, P.B. and C. Tebaldi (2012) Increasing prevalence of extreme summer temperatures in the U.S. Climatic Change, 111, no. 2, pp. 487-495.
  3. EPA. (2006).  Excessive Heat Events Guidebook.  Available from
  4. CDC (undated). Climate Change and Extreme Heat Events.   Available from
  5. EPA (2014a).    Climate Change Indicators in the United States: Ragweed Pollen Season.  Available from
  6. EPA (2014b) Heat Island Effect.  Available from
  7. US Global Change and Research Program (USGCRP).  (2014).  Third National Climate Assessment , Chapter 16: Northeast.  Available from
  8. Luber, G. and M. McGeenhin. (2008). Climate Change and Extreme Heat Events. American Journal of Preventive Medicine, 35(5), 429-435.
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