Understanding Air Pollution in Winter  
Dr. Roby Greenwald, Rollins School of Public Health at Emory University
Chairperson, Mothers & Others for Clean Air Steering Committee 

For residents of Atlanta, the May to September months mean, among other things, smog alerts and high ozone afternoons. Unfortunately, the advent of shorter days and cooler weather does not mean that air pollution has gone into hibernation until the spring. Changes in weather patterns and environmental conditions from summer to winter lead to differences in the type and distribution of air pollution that exists in the city at these times of year. In the next couple of paragraphs, we’ll discuss some of these differences as well as steps you can take to minimize your exposure to winter air pollution. 

The most obvious differences, of course, between summer and winter are that summer days are long, hot, and receive hours of very intense sunlight. All of these attributes contribute to the formation of ozone during the summer. Also, the demand for electricity is much higher in summer than winter, largely as a result of air conditioner usage. Since most of our electricity in Georgia comes from coal-fired power plants, this means that more coal is being burned and that the concentration of sulfate particles (which come from coal combustion) is higher in summer. In addition, it is not uncommon for the air to be stagnant for days on end in summer, which gives pollutants time to accumulate in the atmosphere.  

In winter, the short days and weak sunlight mean that ozone is not much of a problem. Lower electricity usage leads to fewer sulfate particles, and the frequent passage of cold fronts means that the concentration of pollutants does not have a chance to build up. On the other hand, cool temperatures and weak sunlight also means that pollution emissions from vehicles, power plants and industrial sources have a tendency to stay within a few hundred feet of the ground (in summer, emissions can rise thousands of feet or more into the atmosphere). In fact, the concentration of particles coming from cars and trucks is usually higher in winter than summer because not only does the exhaust stay close to the ground, but also because people tend to drive more when children are in school.

Research by our group at Emory as well as by other scientists around the country and around the world has found that levels of ultrafine particles (particles smaller than 100 nanometers) are often 50% higher in winter than in summer.   Ultrafine particles are emitted from diesel and gasoline engines. They are extremely small (hundreds of times smaller than sulfate particles) and can be inhaled into the deepest regions of your lungs. Once inhaled, these particles are very difficult for your body to remove. In addition, these types of particles are made of materials that can lead to inflammation and oxidative stress in your airways. For these reasons, it is difficult to say at this time which type of air pollution is the biggest public health risk: ozone or ultrafine particles.   The good news is that unlike ozone, ultrafine particles tend to be very localized. When ozone levels are high in the summer, they are high across the entire metro region. Ultrafine particles, on the other hand, are highest near their sources. The most important sources of these particles are vehicles, especially diesel trucks. The concentration of ultrafines is highest on major roadways and quickly diminishes in a few hundred yards distance downwind. Roadways that are lined with trees have reduced concentrations downwind. The highest concentrations in the Atlanta area are found near the major interstate highways, particularly I-285 (since it carries a higher percentage of diesel traffic). 

To reduce your exposure to ultrafine particles during winter months, drive less and spend less time on major roadways. When you are on the road, particle concentrations inside your car are lower when you drive slower, keep the windows closed, and use recirculated ventilation on a higher setting. It is important to point out that ultrafine particles are able to work their way inside the car cabin of even new and well-maintained cars.  

Dr. Roby Greenwald is Assistant Research Professor in the Department of Environmental Health at the Rollins School of Public Health at Emory University.  Together with Dr. Jeremy Sarnat and Dr. Anne Fitzpatrick, he represents Emory on the steering committee of Mothers & Others for Clean Air.

AIR QUALITY 101

What Is Air Pollution?
Air pollution is a variety of substances and gases in our air that pose risks to health.  Important air pollutants include ozone, nitrogen oxides (NOx), sulfur dioxide (SO2), particulate matter (PM), volatile organic compounds (VOCs), toxic substances such as mercury and some naturally occurring substances such as pollen. Read more.

What is Smog or Ozone?
Smog is another term for ground level ozone, which is formed when nitrogen oxide (NOx) combines with volatile organic compounds (VOCs) in the presence of sunlight. NOx are a by-product of fossil fuel combustion (oil, natural gas, coal) and so come from cars and trucks, power plants, and industrial processes. 

What is Soot or Particulate Matter?
Particle pollution, or particulate matter (PM), is a mixture of solids and liquid droplets that vary in size. PM includes aerosols and fine solids, such as dust. These particles can get trapped in the lungs and cause pulmonary and circulatory problems, as well as trigger asthma attacks.  High levels of PM has also been linked to Sudden Infant Death Syndrome (SIDS) and slow lung function growth in children.  Read More about how exposure to particulate matter exacerbates respiratory and cardiovascular diseases.

What is Mercury?
Methylmercury (MeHg) is a developmental neurotoxin that interferes with brain development. Human exposure results principally from consumption of fish and other seafood contaminated by mercury. Significant amounts of mercury are emitted from Georgia’s coal-fired power plants.  Read about Georgi's 2007 rule that requires tighter controls for mercury emissions.

Health Effects of Bad Air
Exposure to many pollutants in the air can result in a range of adverse health effects from shortness of breath and coughing to heart attacks and lung cancer.

Medical Perspectives on Air Pollution

Read Scientific and Medical Expert Perspectives on Smog and Health

Mothers & Others for Clean Air partner organization Rollins School of Public Health at Emory is home to a number of leading air pollution scientists. Read Dr. Jeremy Sarnat's publication linking different kinds of fine particulate matter to emergency department (ED) visits and Dr. Matthew Strickland's paper demonstrating a clear connection between ozone and traffic pollution and pediatric ED visits in Atlanta.

Read about a major study in California showing that air pollution is not just a problem for children and adults with asthma.  Children in polluted areas developed asthma at higher rates and had slower lung function growth than children in less polluted areas.

Read the latest Report on Childhood Asthma from the Centers for Disease Control and Prevention (CDC).

Protecting Children from Air Pollution

In 2007, Mothers & Others for Clean Air united a group of air quality, public health and medical experts to update public health guidelines that outline appropriate levels of outdoor activity on smog alert days. This document serves as the basis for educating schools, athletic directors, pediatricians and other health professionals, day care centers, other child care providers and the general public on the appropriate precautions to take when air quality is poor.

Mothers & Others for Clean Air thanks Kaiser Permanente for its generous support in making possible the production and distribution of thousands of guidance documents for Georgia Schools and Georgia Families.

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Additional Information

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More information on air pollution and health

American Lung Association

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Georgia Agency responsible for monitoring ambient air quality

The Georgia Environmental Protection Division (EPD), Air Protection Branch

National Collaborative Work Group on Green Cleaning and Chemical Policy Reform in Schools