Air Pollution

WHAT lS AIR POLLUTION?

Most respiratory diseases are caused by airborne agents. These may be bacteria viruses, chemicals in cigarette smoke, allergens, various gases, or dusts. Our lungs are uniquely vulnerable to contamination from the air we breathe. To this extent most lung disease is caused by air pollution or air contamination. However, the term air pollution is usually used in a narrower sense to mean contamination of the air with noxious material given off by machines (factories, automobiles, and so forth). This type of air pollution can affect people in a large general area such as a city or valley. Of course one can also define air pollution in the context of inhaling your own or someone else's cigarette smoke (see Section S) or of inhaling contaminated air unique to your work area (See Section D). In this chapter, air pollution refers to the general or community air pollution that affects a general population.

WHAT MAKES UP GENERAL AIR POLLUTION?

Many different particles and gases make up what we commonly refer to as air pollution. For convenience they can be divided into six major groups:

1. Carbon Monoxide. This is a colorless, odorless, invisible gas, the product of internal combustion engines. It combines with hemoglobin in red blood cells and displaces oxygen. In small amounts carbon monoxide can cause headaches and fatigue. Large amounts can be lethal.

2. Particulates. This is a general term for a mixture of solid and liquid particles in the air, usually produced by stationary fuel combustion and industrial processes. Particulates include small bits of soot and ashes that emanate from incinerators and smokestacks.

3. Sulfur oxides. These are acrid, corrosive, poisonous gases that come from burning sulfur­containing fuel such as coal and oil. They are produced mainly by industrial plants that burn fuel containing sulfur as an impurity.

4. Hydrocarbons. So named because they contain both hydrogen and carbon, these compounds come from the incomplete burning of fuel, mainly gasoline. Although hydrocarbons are not very harmful themselves, they can react with sunlight to form smog, which is irritative. In industrial areas most hydrocarbons are from automobiles.

5. Nitrogen oxides. These include nitric oxide and nitrogen dioxide (NO₂), produced when fuel is burned at very high temperatures. This occurs mainly in automobiles, electric utilities, metal­fabricating plants, and chemical plants. NO₂ is a yellow­brown gas that can combine with hydrocarbons and sunlight to form smog and ozone. Ozone is a specific photochemical oxidant that can be irritative to the lungs and eyes.

6. Miscellaneous. A host of other pollutants, including lead, arsenic, asbestos, mercury, beryllium, plutonium, cadmium fluoride, and organic pesticides, may enter the atmosphere from various sources. Airborne lead, mainly from leaded gasoline, has decreased considerably since the conversion by many cars to unleaded gasoline.

WHAT lS SMOG?

There are actually two kinds of smog: photochemical smog due to the action of sunlight on air pollutants, and a combination of smoke and fog (hence smog) that has nothing to do with sunlight. This combination, the original smog, used to be common in London when coal was widely used to heat homes. The term smog has since been adopted to describe the haze that occurs when sunlight reacts with the hydrocarbons and nitrogen dioxide that are part of automobile exhaust.

The reaction of sunlight with hydrocarbons and NO₂ results in a variety of chemical products. One of these is ozone. Ozone is a molecule made up of three atoms of oxygen, abbreviated O₃. Regular oxygen, which of course is vital to life, is a molecule of 2 oxygen atoms, abbreviated O₂. Adding an extra oxygen atom to form ozone results in an irritative, noxious gas. Other undesirable chemicals, such as aldehydes, also result from this photochemical reaction and account for the harmful effects of smog: eye irritation, cough, and, for some, trouble in breathing. Because sunlight is essential to this type of smog, the concentration of ozone and other measurable chemicals is maximum around noon and falls off considerably at night. (See Section M for a discussion of the harmful effects of Los Angeles smog on patients with heart disease).

WHAT RESPIRATORY DlSEASE IS CAUSED BY COMMUNITY AIR POLLUTION?

The effects of air pollution should be viewed in two different groups: healthy people and people with chronic heart or lung disease. Most studies demonstrating harmful air pollution effects have been done on chronic disease patients and the very elderly, two groups more susceptible to air pollution than the general population.

The most notorious example of air pollution in the United States occurred in 1948 in Donora, Pennsylvania. Weather conditions caused the already heavily polluted air to stagnate for several days. Out of a population of 14,000, twenty died and over 6,000 became ill. All of the deaths and most of the serious illness occurred in people with underlying heart or lung disease.

Various epidemiologic studies in the United States, Canada, and England have associated increased levels of air pollution with:

• an increase in the hospitalization rate of children with pre­existing asthma
• diminished lung function tests in children when compared with tests in unpolluted areas
• an increase in the hospitalization rate for acute respiratory illness in the
• general population
• an increase in deaths when compared to less polluted areas of the same city
• an increase in the incidence of severe emphysema in cigarette smokers when compared with smokers in unpolluted areas

In London, attacks of bronchitis associated with air pollution led to the British Clean Air Act of 1965. Subsequently, there was a marked decrease in mortality and morbidity from bronchitis.

On a year to year basis there is no doubt that air pollution, as exists in and around our cities and industrial plants, makes the lives of chronic heart and lung disease patients more difficult. Various local governments now publicize a "pollution standard index," or PSI to warn such patients to stay indoors when the air is unsafe (Table 1). The PSI scale ranges from 0 to 500 and is based on the air pollutant with the highest concentration at the time the test is done. Five major pollutants are measured at various points throughout the area: nitrogen dioxide, sulfur dioxide, carbon monoxide, photochemical oxidants (mainly ozone), and particulate matter. The PSI was originally developed by the U. S. Environmental Protection Agency to provide consistency in reporting on air quality. The PSI is published every morning in large city newspapers or is available from your local pollution control agency.

Common sense dictates that anyone suffering chronic cardiac or pulmonary disease – asthma, bronchitis, emphysema, cystic fibrosis, coronary artery disease, heart failure, and so forth – should avoid air pollution to the maximum extent possible, including altering lifestyle, residence, or occupation if necessary and economically feasible.

DOES GENERAL AIR POLLUTION LEAD TO SPECIFIC DISEASE IN HEALTHY PEOPLE?

It is difficult to pin down specific examples of cause and effect between general air pollution and diseases such as asthma, chronic bronchitis, emphysema, and lung cancer. This question, although studied for many years, remains largely unanswered. Data have been published from England linking chronic bronchitis to heavy sulfur oxide and particulate exposure, and most investigators seem to accept that severe air pollution of this type can cause chronic bronchitis.

Apart from this link, there is no body of data, analogous to the data for cigarette smoke, proving cause and effect between current levels of air pollution and specific diseases.

Many of the compounds listed in the earlier section (in concentrated form) have been shown to cause skin cancer in laboratory animals. Although lung cancer is definitely more prevalent in urban than in rural populations, there are many differences between urban and rural populations besides levels of air pollution that might cause differences in lung cancer rates. These differences include total smoking history, exposure to passive smoke (see Section S), population density, and work environment. At present there is no accepted epidemilogic proof that lung cancer is caused by general or community air pollution.

We do know that air pollution is definitely irritative, particularly to the eyes and bronchial tubes. The watery eyes and coughs experienced by many otherwise healthy people during heavy bouts of smog are but two examples. There is no reason to doubt that repeated exposure to heavy concentrations of air pollution can cause major pulmonary disease. Factors such as cigarette smoking (Section S) and work-related exposure (Section Q) are known causes of specific illnesses. It is likely that general air pollution, if severe enough, can also lead to specific diseases in otherwise healthy people.

A summary of effects of community air pollution is given in Table 2. Next to each effect is the section in this site where the reader will find a more general discussion of the specific condition or disease.