In the face of rising fear over anthrax and bacteria-related bioterrorism, scientists warn of the dangers of a similar, but often over-looked threat posed by microscopic particles in the air.
Environmental scientists have long known of this threat and have spent years studying the physics of these particles, which are very similar to many forms of biological weaponry, and have similar effects on the body.
Coal dust is one of the most well known examples, resulting in black lung disease. The particles can also carry bacteria, spreading infections such as Legionnaires' disease. Even the detrimental health effects of second-hand smoke and pollution result from tiny air particles.
The particles that cause the most damage are extremely small, ranging in size from about 0.05 microns to 10 or 20 microns, where a micron is a millionth of a meter. Once released into the air, these particles can remain aloft for almost forever, and have the potential to enter into improperly sealed buildings.
Scientists are most concerned with the smallest particles because they are able to pass through the safeguards of the respiratory system, such as nose hairs and mucus deposits, to eventually embed themselves into the most vulnerable regions of the lung.
While in the air, all particles - regardless of what they are carrying - act pretty much the same.
As Dr. William Nazaroff, a professor of environmental engineering at the University of California at Berkeley explains, "The fact that this is a biological issue doesn't really change the behavior of the particles in the building, or the respiratory deposition."
However, once the particle does land in the lung, its chemical or biological toxicity does come into play, as can be seen with tuberculosis and other bacterial diseases that are transmitted on air particles.
The size of the particle is very important due to the physiological structure of the lung. Starting from the trachea or windpipe, the tubes of the lungs branch extensively, and each branch results in a decrease in size of the air pipes. At the end of each branch is the smallest division, called the alveoli, each perhaps 50 microns across. It is in these tiny alveoli sacks that oxygen and carbon dioxide exchange takes place.
"You could just barely see [alveoli]," said Dr. Hobbs, director of toxicology at Lovelace. "It's half the size of a human hair."
There are many millions of these tiny sacks, so many in fact that if they were all flattened out together, their total surface area would cover a tennis court.
The lung has several ways to defend itself against foreign substances, including a coating of mucus to trap particles, and tiny hair-like cells called cilia, which serve to sweep foreign substances up and out of the lung through a wavelike motion. In addition, each alveoli is protected by a single macrophage cell, which eats up any invading substances or bacteria.
When foreign bacteria are brought into the alveoli, there is a battle between the macrophage and the invaders. If the bacteria win, they can begin to do damage to the lungs and the rest of the body. The air particle itself can also cause damage, as seen in lung fibrosis, in which a coal dust particle can become integrated into the lung tissue and cause damage.
"Your lung," said Dr. Morton Lippmann, a professor of environmental medicine at New York University medical school, "is a very good culture medium - you can get enormous replication and damage to the lung."
The amount of potential damage for a given particle is due mostly to its size, because large particles, around 20 microns, are more easily trapped than smaller ones. However, extremely fine particles that are much smaller than a micron are also trapped because they are diffused by the strong air currents in the lung, and do not make a straight path to the alveoli, similar to the difficulty with which a feather is thrown through the air compared to a baseball.
The dangerous particles lie in between, from about one to five microns in size.
Dr. Nazaroff explains that the story does not end just with size. The way in which the particles spread throughout a room is very complex, and must be taken into consideration. Particles can be spread by many sources, including warm air rising from human body heat.
Dr. Nazaroff explained that this heat is, "roughly equivalent to a 75-watt light bulb," and along with other sources, such as ventilation and temperature variations near windows, causes the particles to be rapidly spread throughout a room.
Ventilation systems can spread the particles from room to room as well.
"In a nutshell, the particles not only have to be small," said Richard Spertzel, a former weapons inspector and biologist for the United Nations Special Commission on Iraq. "They have to be small enough so that they are capable of staying airborne, and also they have to be small in order to get down into the lungs."
Many scientists have warned of this threat in relation to environmental pollution, and have suggested improvements to indoor and outdoor air filtration systems. However, since building regulations concerning air purity have been improving in recent years, any regulation would most likely be voluntary.
"We should get the insurance companies to lower your life insurance slightly if you have approved equipment," suggests Dr. Matthew S. Meselson, a professor of biology and bio-weapons expert at Harvard.
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