The respiratory tract is the primary entry portal for airborne contaminants. It comprises three zones: (1) the upper respiratory tract (including the nose and throat airways), (2) the bronchiole airways of the upper lung, through which air is breathed, and (3) the alveolar region, where oxygen exchange between the lung and the circulatory system (bloodstream) occurs across a very thin layer of cells between the air in the lung and the blood in the bloodstream. Where and to what extent a particular air pollutant affects the lung is dependent upon whether it is a gas or a particle, as well as its water solubility or (in the case of particles) its composition and size.
Particles that are inhaled and then not exhaled are deposited in the lung. Larger particles tend to be deposited in the upper respiratory tract or the bronchiole airways. Particles larger than approximately 10 mm in diameter get caught by the nose and throat such that they do not get into the lung, but are instead swallowed or sneezed out of the body. For comparison, a human hair is approximately 70 mm in diameter. Particles less than 10 mm in diameter but more than about 2.5 mm in diameter are largely deposited in the upper airways of the lung, where the insoluble particles are rapidly cleared out of the lung by the tiny hair-like cilia that line these airways, being carried up the lining of the lung to the throat, where they are swallowed and cleared from the body by the digestive tract. The fine particles (those less than approximately 2.5 mm in diameter) more readily reach the deepest recesses of the lung, where they can be deposited in the sensitive alveolar region, and are also not as rapidly cleared as from the upper airways and therefore can cause significant damage.
The extent to which gaseous air pollutants reach deep into the lung is defined largely by the solubility of the gas. Sulfur dioxide, for example is highly soluble in water, and so it is readily absorbed by the mucous lining of the nose, throat, and upper airways and does not usually reach the alveoli in the deepest parts of the lung. In contrast, less water-soluble gases, such as NO2, are not as readily absorbed in the upper airways and are more likely to reach the alveoli and have the potential to do damage there.
In recent years, interactions of particles and gases have been found to modify the toxicities of each other. For example, if SO2 is present along with particles, it can be absorbed onto and react with the surface of the particle, convert into sulfur oxides such as sulfuric acid (H2SO2–) or ammonium bisulfate (NH3HSO4), and thereby bypass the defenses of the lung and reach the deepest parts of the lung. Conversely, the addition of sulfur oxides to a particle can cause that particle to become more acidic, thereby increasing the solubility of the metals in the particle such that, upon impaction on the lung, these metals can be more bio-available, thereby getting into the body before it can be cleared. In the case of transition metals (such as vanadium and iron), for example, this can cause these metals to cause greater oxidative stress in the body than they would without the co-presence of sulfur oxides on the particle (e.g., see Costa and Dreher, 1997).
Air Pollutant Health Effects
Background
The earliest convincing evidence that very serious health consequences can result from exposures to air pollution comes from historical pollution episodes, notably the Meuse Valley and the Donora disasters discussed previously, and the London fog episodes of the 1950s and 1960s. A landmark event in the recognition of the disastrous impacts of air pollution on health occurred on December 1–5, 1952 in London. Trapped coal smoke in the River Thames valley accumulated as a result of a stationary high-pressure cell accompanied by wind speeds near zero. Approximately 4000 excess deaths occurred in London that week, and excess deaths continued for weeks afterward, thus indicating that there were both immediate and delayed effects.
More recently, there were regional air pollution episodes of particulate matter from massive forest fires in Southeast Asia and Mexico in the 1990s, shutting down businesses, schools, and airports. As recently as 1998, schools in Tehran, Iran were shut down because automobile exhaust made O3 air pollution levels rise to six times above acceptable levels, causing some residents to wear masks when venturing outdoors. Thus, pollution episodes can still occur today, especially in rapidly developing nations.