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Ever since our early ancestors gathered around their first fire, humans have been releasing fine particulates into the air. Only recently have scientists discovered the bad news; fine particulates are so small that they can easily be inhaled into the deepest reaches of our lungs, causing serious breathing disorders. Fine particulates are also a visual blight, capable of reducing visibility so much that beautiful views are blotted out, and road and air travel is made difficult. But if humans have managed to live with fine particulates for thousands of years, why all the fuss now?

Well, the truth is that lung disease has always been a major health problem throughout the world, and a leading cause of death. Just as bacteria were causing serious illness long before they were discovered, its likely that fine particulates have always contributed to lung disease- we just did not know it.

Recent studies have found that fine particulates at current levels can pose a greater danger to our health than the better-level ozone, sulfur dioxide and carbon monoxide. Fine particulates are linked with all sorts of health problems- from a runny nose and coughing, to bronchitis, asthma and even death. In fact, new research here in the U.S. suggests that fine particulates are responsible for tens of thousands of deaths in our country each year.

Its time to clear the air! As well as taking action to control fine particulates, we also need to understand the nature of the problem. This learning module will introduce you to fine particulates- what they are, where they come from and why they are so effective at impairing breathing and visibility. It will also explain what the government is doing to get fine particulates out of the air, and how you can become part of the solution.


Fine particulates may sound like a heavy scientific expression, but it breaks down into simple concepts. Particulates are tiny solid or liquid particles that come in many shapes and sizes, and from many different sources. They are also called particulate matter or PM for short.

Fine particulates are 10 micrometers or less in diameter (PM10). A micrometer is a millionth of a meter, so PM10 is roughly the same size as bacteria, and like bacteria, PM10 is invisible to the naked eye and small enough to be inhaled into our lungs.

Not all PM10 is created equal. It can be composed of very small particulates of about 0.1-0.2 micrometers in diameter, likeyou'dd find in automobile exhaust or fireplace emissions. It can also include particulates at least 10 times this size, likeyou'dd find in sea salt spray or road dust.

To simplify things, we often refer to a fine and coarse fraction of PM10 since they generally differ in chemical composition source and behavior in the air. The fine fraction (PM2.5) contains particulates 2.5 micrometers or smaller- the size most often generated by combustion processes and by chemical reactions taking place in the air. The coarse fraction contains particulates greater than 2.5 micrometers, the size most closely associated with natural sources. As well find out next, the smaller the particulate, the more serious a health risk it poses.


From our lungs point of view, bigger particulates are less harmful. Because of their weight, particulates larger than 10 micrometers settle to the ground quickly. If we do inhale them, they tend to collect in our throat and nose, and are eliminated from our body by sneezing, coughing, nose blowing or through the digestive system.

Particulates in the coarse fraction of PM10 are removed in the upper respiratory system. That is, they don't travel very far into our lungs. They contain materials common to the earth's, crust and the ocean, reflecting the fact that natural sources such as windblown dust and sea salt spray are big contributors to the coarse fraction.Vegetationn is another large natural source. Human activities that involve grinding or pulverizing, such as mining, quarrying and cement manufacturing, are also important. These particulatesdon'tt stay in the air too long, settling to the ground within a matter of a few hours to a few days.

In contrast, particulates in the fine fraction (PM2.5) can remain in the air for days to weeks. They can penetrate especially deep into our lungs, collecting in the tiny air sacs (called "alveoli") where oxygen enters into the bloodstream. As a result, they can cause breathing difficulties and , sometimes permanent damage to our lungs.

A number of potentially harmful substances have been found in PM2.5. Sulfates produced from sulfur dioxide emissions are acidic in nature, and may react directly with our lungs. Elemental carbon produced during wood and engine combustion can pick up cancer-causing chemicals like benzo(a)pyrene and give them a free ride into our lungs. Hundreds of organic carbon compounds in addition to benzo(a)pyrene have been identified inexhauste from vehicles, combustion processes and even meat-cooking operations. Several studies have shown that toxic trace metals such as lead, cadmium and nickel are more concentrated in PM2.5 than in bigger particulates.


PM10 is produced from a wide variety of sources, both natural and human-caused, large and small. Some PM10 is released directly from sources into the air. These are called primary particulates. Other PM10 is formed from physical and chemical reactions mainly involving gases emitted into the air. These are called secondary particulates.

Diesel vehicles (trucks and buses), industrial combustion sources like power plants, incinerators, furnaces, gasoline vehicles and road dust are the largest sources of primary PM10, with diesel and gasoline vehicles thought to be the major sources of PM2.5. In more forested regions of the U.S., forestry burning and wildfires are among the major contributors of both primary PM10 and PM2.5 emissions. Also significant are wintertime emissions from wood stoves and fireplaces, as these sources produce very fine particulates right under our nose.

At present, wedon'tt know how much secondary particulates- sulfates, nitrates and organic carbon compounds- contribute to PM10 levels. However, we do know quite a bit about the gases they com from (the precursors) as shown in the table below. We also know that secondary PM10 is found mostly in the fine fraction and its potential effects of visibility are significant. For instance, sulfates are responsible for most visibility problems in Eastern Canada and United States, while nitrates are important in central and western cities such as Denver and Los Angeles.

Secondary ParticulatesPrecursorSource of Precursor
SulfatesSulfur dioxideIndustry, diesel vehicles, marine vessels
NitratesNitrogen oxidesMotor vehicles, combustion sources
Organic Carbon CompoundsVarious hydrocarbons Motor vehicles, industry, vegetation

Its important to realize that not just large industries contribute to PM10 levels in the air. There are also many small sources that we come in contact with on a daily basis, like barbecuing hamburgers on asmokyy grill, or motor vehicles, releasing soot and hundreds of different organic carbon compounds in their exhaust and kicking up dust as they travel down the road. Fireplaces and cigarette smoke are also important sources that are individually small but numerous. And remember: these particulates are not being released from a stack 300 feet high and 50 miles away.They'ree being released on our streets, in our backyards, our living rooms, and are finding their way into our lungs.


PM10 is now considered one of the worst air pollution problems in the U.S., as hazardous as ground-level ozone and all other outdoor air pollutants. The findings are quite alarming: exposure to high levels of PM10 can play a role in the development of many kinds of respiratory diseases, including asthma, bronchitis, pneumonia and emphysema. Even more serious, this kind of air pollution is associated with a significant rise in the number of premature deaths. Senior citizens and people who already have lung or heart problems are most at risk, but normal healthy adults and children can also be affected.

Many medical researchers are also concerned that PM10 may be linked to heart disease. This possibilityhasn'tt yet been thoroughly studies, but if true, it would mean that PM10 plays an even greater role in making people sick than is now believed.

The link between asthma and PM10 is noteworthy, since millions of Americans suffer from this condition. Acute asthma is now considered the most common cause of medical emergencies in children, and a higher rate of school absenteeism have also been observed when PM10 levels increase.


PM10 has another unwelcome talent: the smaller particulates are effective at getting in the way of light reflected from distant objects to your eye. They can both absorb light, or reflect (scatter) it in all sorts of directions. The net effect is that these particulates can blot out the view, making it difficult for residents and tourists alike to "enjoy the scenery" or even to travel by road or air.

What types of particulates are mostly to blame? Those of approximately 0.1-1 micrometer in diameter are about the same size as the wavelength of visible light. This makes them perfect for interfering with light, which means that you cant see as far or as well. Soot is good at absorbing light, while secondary particulates such as sulfates and nitrates are good at scattering light. This isespeciallyy true under conditions of higher humidity, when their scattering ability is enhanced by water vapor in the air. An entire region may experience a reduction in visibility due to large sources such as forest fires, or in major metropolitan areas during the hot, humid summer season.

  1999, KanCRN Collaborative Research Network