A BREATH OF FRESH AIR

Take a full breath in. Please take a very long inhale. Feels wonderful, doesn't it? Breathing is something you typically pay little attention to, despite its importance to your survival. We cannot survive more than a few minutes without respiration, which involves taking in oxygen to sustain each of our cells and expelling waste, predominantly carbon dioxide. Each resting inhalation is approximately 500 milliliters (approximately one-half of a quart) in volume. But did you realize that you inhale a multitude of other substances along with oxygen with each breath? You inhale air pollutants such as ozone, nitrogen oxides, sulfur dioxide, and carbon monoxide, to name a few of the most common. You've probably heard of some of these gaseous pollutants, particularly ozone, and are aware that they can be detrimental to your health in high concentrations. However, many people are surprised to learn that each inhalation can also contain very minute solid or liquid particles that are also considered air pollutants and can cause severe health issues.

Health Effects

Particles in the atmosphere occur in a broad range of diameters. Dust particles that everyone has seen floating in a shaft of sunlight are typically between 100 and 1000 microns (micrometers) in length, where a micron is one millionth of a meter. During a dust cyclone, the airborne particles are approximately 10 to 100 microns in diameter. Pollen grains, which cause so much anguish for so many people, have diameters of approximately 3-200 micrometers. However, we have not yet reached the worst of it! Larger-than-10-micron particles, such as dust and pollen, do not remain in the atmosphere for long because the constant pull of gravity draws them back to the surface within minutes or hours.

On the other hand, particles with diameters less than 2.5 microns (or less than 1/30th the width of a human hair) are so minute that they can remain in the air for several days and are carried by winds and very small air currents. These so-called fine particles can be inhaled profoundly into the respiratory system, unlike coarse particles such as dust and pollen, which do not travel very far into the body beyond the nasal passages when inhaled. Due to their ability to penetrate deeply into the bronchial system, fine particulates (also known as PM2.5 by atmospheric scientists) have been linked to a number of severe health problems.

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It is well known that exposure to high levels of particulate particles exacerbates respiratory illnesses such as asthma and emphysema. High PM2.5 concentrations have also been linked to an increased risk of lung cancer and birth defects. In the past decade, evidence has accumulated that microscopic particles can reach the deepest portions of the lung and even enter the circulation, where they can affect other organs. Particularly, PM2.5 exposure has been associated with cardiovascular disease and increased heart attack rates. Recent research has demonstrated that ultrafine particles (less than 0.1 micrometers) can travel through cell membranes and infiltrate other organs, including the brain.

In the United States, it is estimated that exposure to atmospheric particle pollution causes more than 20,000 fatalities annually that would not have occurred otherwise. Thankfully, the Environmental Protection Agency's regulations have resulted in a decline in PM2.5 concentrations across the United States over the past two decades. According to the American Lung Association, as many as 4 in 10 U.S. residents continue to be exposed to persistently high levels of PM2.5, which undoubtedly results in the premature deaths of thousands of U.S. citizens in these areas.

Particles in the air also reduce atmospheric visibility, resulting in the typically cloudy, smoky conditions associated with air pollution. The greater the number of particles per cubic centimeter in the atmosphere, the more solar radiation is scattered. With high particle concentrations, the sun's beams are dispersed in all directions, as opposed to passing directly through the atmosphere and then reflecting off an object in our field of view and into our eyes. When the amount of dispersed sunlight exceeds the amount of direct sunlight entering our eyes from an object, we perceive haze. ATDD scientists serve on a national scientific steering committee that administers an air quality monitoring network (IMPROVE) dedicated to preserving and enhancing atmospheric visibility in U.S. National Parks and other protected areas.

 Additionally, atmospheric particles influence the weather and climate of Earth. Higher particle concentrations cause more incoming sunlight to be dispersed back into space, thereby decreasing the Earth's surface's net heating. Moreover, particulates are required for the formation of clouds and precipitation. As the number of particles in the atmosphere fluctuates, so do the physical properties of clouds, which can alter the quantity, spatial extent, and radiative reflectivity of clouds, thereby influencing the net heating (or cooling) of the Earth's surface. The interactions between atmospheric particles and clouds are a complex topic that is the subject of active scientific study.

Sources

Then, where do these atmospheric particles originate? Numerous sources, such as gasoline and diesel vehicle exhausts, commercial furnaces, coal-burning power plants, and industrial and manufacturing processes, directly discharge a number of pollutants into the atmosphere. Additionally, natural sources such as forest fires and volcanoes contribute directly to the number of airborne particles. Large portions of atmospheric particles, however, are produced by complex chemical and physical transformations of gaseous pollutants into solid and liquid forms. These gaseous precursors to particles include nitrogen oxides and sulfur dioxide from vehicle exhausts and coal and natural gas power plants, hydrocarbons from gasoline evaporation during refueling and other industrial sources, ammonia from agricultural operations, and metals from waste-burning activities. These gaseous pollutants react chemically with one another and other naturally occurring substances in the atmosphere, transforming into solid or liquid particulates. Due to the abundance of sources and pathways leading to their formation, micro particles in the atmosphere are typically composed of a complex mixture of chemical compounds.

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