Indoor Air Pollution
The levels of some common air pollutants often are greater indoors than outdoors, although pollutant concentrations do vary significantly from one building to another. Since most people spend more time indoors than outdoors, exposure to indoor air pollutants is an important environmental problem and may cause more problems to human health than does outdoor air. Indeed, the inadequate ventilation practices encountered in developing countries that burn coal, wood, crop residues, and other unprocessed biomass fuels create smoke and carbon monoxide pollution that produces respiratory problems and ill health among huge numbers of people in these countries. Women and young children are particularly affected since they spend more time indoors. Cooking smoke from biomass fuels increases asthma rates among elderly men and women. The particulate emissions from traditional cook stoves used indoors in developing countries can be reduced by 90% by switching from wood to charcoal. It is estimated that over 400,000 premature deaths are caused in China annually owing to exposure to dirty household fuels and to other pollutants in the air.
In the material that follows, we investigate the various indoor air pollutants that are thought to have the most serious effects on human health. To ensure that the relevant background has been covered, a discussion of several other indoor air pollutants of interest to human health is deferred until later chapters: Radon is discussed in Chapter 9, pesticides in Chapter 10, and poly-cyclic aromatic hydrocarbons (PAHs) in Chapter 12. Chloroform in indoor air is considered when water purification is discussed, along with indoor air contamination by chlorinated organic solvents, in Chapter 14-
The most controversial indoor organic air pollutant gas is formaldehyde, H2C=0, It is a widespread trace constituent of the atmosphere since it occurs as a sta ble intermediate in the oxidation of methane and of other VOCs. While its concentration in clean outdoor air is too small to be important— about 10 ppb in urban areas, except during episodes of photochemical smog— the level of formaldehyde gas indoors is often orders of magnitude greater, in certain cases exceeding 1000 ppb (1 ppm). A survey of U.S. homes in the late 1990s found that the indoor formaldehyde concentration usually was in the 5-20 ppb range.
The chief sources of indoor exposure to this gas are emissions from cigarette smoke and from synthetic materials that contain formaldehyde resins used in urea formaldehyde foam insulation and in the adhesive employed in manufacturing plywood and particleboard (chipboard). Many useful resins (which are rigid polymeric materials) are prepared by combining formaldehyde with another organic substance. Formaldehyde itself is used in the dyeing and gluing of carpets, carpet pads, and fabrics. In the first few months and years after their manufacture, however, such materials release small amounts of free formaldehyde gas into the surrounding air. Consequently, new prefabricated structures such as mobile homes that contain chipboard generally have much higher levels of formaldehyde in their air than do older, conventional homes. Many manufacturers of pressed-wood products have now modified their production processes in order to reduce the rate at which formaldehyde is released.
The rate of formaldehyde emission from synthetic materials increases with temperature and relative humidity and declines as the materials age. Initially, formaldehyde temporarily trapped as a gas or simply adsorbed onto the materials is released into the surrounding air. There is also release of formaldehyde due to the rearrangement and dissociation of amide end-groups on resin polymers, from R—NH—CH2OH to R—NH2 + H2CO. Later, slow but continuous reactions of water vapor in humid air with the methylene bridges joining amide groups within the polymer backbone provide a continuing emission of formaldehyde:
R—NH—CH,—NH—R + H70-» 2 R—NH, + H.CO
Formaldehyde has a pungent odor, with a detection threshold in humans of about 100 parts per billion, i.e., 0.1 ppm; its odor is often noticeable in stores that sell carpets and synthetic fabrics. At somewhat higher levels, many people report irritation to their eyes, especially if they wear contact lenses, and to their noses, throats, and skin. The formaldehyde in cigarette smoke can cause eye irritation. Common symptoms of acute (i.e., short-term, high-level) formaldehyde exposure include coughing, wheezing, bronchitis, and chest pains. Chronic exposure to low levels of formaldehyde produces similar effects and respiratory symptoms. Formaldehyde in air may cause children to develop asthma and to have more respiratory infections and allergies and asthma attacks, although evidence for these effects is controversial. Dampness in homes, allowing the proliferation of dust mites, fungi, and bacteria, also plays a large role in increasing lower respiratory tract illnesses, especially in children.
Formaldehyde is thought to be the most important VOC in producing what is known as sick building syndrome. This term is used to describe situations in which the occupants of a building experience acute health effects and discomfort that seem to be linked to the time they spend in a particular building, though no specific illness or cause is apparent. Complaints commonly include
• irritation of eyes, nose, or throat; dry cough;
• dizziness and nausea; fatigue;
• difficulty in concentrating; and
In addition to VOCs emitted from indoor sources, other factors that contribute to the syndrome include inadequate ventilation, pollutants entering from outside the building, and biological contamination of the air from bacteria, molds, pollen, and viruses that have bred in stagnant water that has accumulated in air vents, etc.
The related compounds acetone, (CH3)2C=0, and 4-butanone (also called methyl ethyl ketone, MEK) are the ketones most commonly present in indoor air in U.S. homes, owing to their use as solvents in nail polish and paint removers, etc.
Formaldehyde is established as a carcinogen (a cancer-causing agent) in test animals and may also be carcinogenic to humans; it was classified as a probable human carcinogen by the U.S. EPA in 1987. The expected cancer sites are in the respiratory system, including the nose; cancers at these sites have been found for some people who are exposed to the gas in occupational settings. However, studies of human populations exposed to formaldehyde have led to no clear-cut conclusions concerning an increase in cancer frequency arising from nonoccupational exposure. From animal studies, an upper limit to the possible effect in humans can be estimated: It corresponds to an increase in the cancer rate of one or two cases per 10,000 people after 10 years of living in a house or trailer with high formaldehyde levels. However, the lower limit to the effect could well be a zero increase in the cancer rate. In summary, no scientific consensus has yet been reached on the dangers to human health of low-level exposure to formaldehyde.
Benzene and Other Gasoline-Related Hydrocarbons
Like formaldehyde, benzene is classified as a hazardous air pollutant, HAP, sometimes known as air toxics. Benzene, C6H6, is a stable, volatile liquid hydrocarbon that through the modern age has found a variety of uses. It is a minor constituent of gasoline and was commonly used as a solvent for many organic products, including paints and inks. The public is exposed to benzene vapor indoors from the use of solvents and gasoline, through smoking (mainly for the smoker but to a lesser extent for those inhaling second-hand smoke), and from the importing of benzene from outdoor air into the house. The levels of benzene generally are smaller outdoors and in large buildings than in individual homes, especially those with smokers living in them. A significant fraction of benzene vapor exposure occurs while riding in motor vehicles and refueling them at gas stations.
Benzene is classified by the U.S. EPA as a known human carcinogen. Chronic exposure at high occupational levels increases the rate of leukemia to individuals. Indeed, there were many deaths among workers in the first half of the twentieth century from exposure to benzene from petroleum-based solvents, such as those used in the rubber and glue industries; in the making of paints, adhesives, and coatings; and in dry cleaning. It also causes aplastic anemia, a condition in which an individual is chronically tired and is especially susceptible to infections because the bone marrow produces insufficient red blood cells. There continues to be some uncertainty, however, about whether occupational or domestic exposure to low levels of benzene vapor does indeed increase the risk for leukemia and multiple myeloma. Recently, it was estimated that benzene accounted for one-quarter of cancer deaths caused by air toxics in the United States.
Because of the serious health problems it causes, the use of benzene as a solvent has largely been phased out. In addition, its maximum allowable level in gasoline has been reduced. Benzene can be replaced in many applications by toluene, Q^CHj, which consists of molecules of benzene with one hydrogen atom replaced by a methyl group. The —CH3 group in toluene provides liver enzymes with a site that is much easier to attack and thereby initiate metabolism than any of the very strong bonds in benzene itself. Toluene and the corresponding dimethylated benzenes called xylenes, the 1,2,4-trimethylated benzene, and ethylbenzene are all present in modern unleaded gasoline; they are very commonly detected in indoor air, as are the nonaromatic hydrocarbons cyclohexane and decane. The concentration of toluene usually greatly exceeds that of benzene itself. However, there is evidence that methylated benzenes are demethylated in catalytic converters and that, as a consequence, additional benzene is emitted into the air under some operating conditions.
Indoor concentrations of N02 often exceed outdoor values in homes that contain stoves, space heaters, and water heaters that are fueled by gas. The flame temperature in these appliances is sufficiently high that some nitrogen and oxygen in the air combine to form NO, which eventually is oxidized to nitrogen dioxide. In one study, it was established that N02 levels in homes that use gas for cooking or that have a kerosene stove average 24 ppb, compared to 9 ppb for homes that have neither. Peak concentrations near gas cooking stoves can exceed 300 ppb.
Some nitric oxide is also released from the burning of wood and other biomass fuels since these natural materials contain nitrogen. However, the flame temperature used in burning such fuels is much lower than in burning gas, so little thermal NO is produced from nitrogen in the air.
Nitrogen dioxide is soluble in biological tissue and is an oxidant, so its effects on health, if any, are expected to occur in the respiratory system. There have been many studies of the effects on respiratory illness in children owing to exposure to low levels of N02 emitted by gas appliances, but the results of different studies are not mutually consistent and are inadequate for establishing a cause-and-effect relationship. One study found that a 15-ppb increase in the mean NO? concentration in a home leads to about a 40% increase in lower respiratory system symptoms among children aged 7 to 11 years. Nitrogen dioxide is the only oxide of nitrogen that is detrimental to health at concentrations likely to be encountered in residences.
Nitrogen dioxide is probably responsible for the finding that indoor concentrations of nitrous acid, HN02, exceed those found outdoors, since the gas reacts with water to form nitrous and nitric acids:
2 NOz + H2O-»HNO, + HNO3
Indoor nitrous acid concentrations were found to correlate inversely with ozone gas concentrations, presumably because the acid is oxidized to nitric acid by the gas.
Carbon monoxide, CO, is a colorless, odorless gas whose concentration indoors can be greatly increased by the incomplete combustion of carbon-containing fuels such as wood, gasoline, kerosene, or gas. High indoor concentrations usually are the result of a malfunctioning combustion appliance, such as a kerosene heater. Even properly functioning kerosene or gas heaters in poorly ventilated rooms can result in CO levels in the 50-90-ppm range.
Average indoor and outdoor CO concentrations usually amount to a few parts per million, though elevated values in the 10-20-ppm range are common in parking garages due to the carbon monoxide emitted by motor vehicles. Exhaust fumes containing high levels of CO and other poll utants can enter homes having attached garages. In developing countries, carbon monoxide poisoning is a serious hazard when biomass fuels are used to heat poorly ventilated rooms in which people sleep.
The major danger from carbon monoxide arises from its ability, when inhaled, to complex strongly with the hemoglobin in blood and thus to impair its ability to transport oxygen to cells. Hemoglobin's affinity for CO is 234 times that for oxygen, and once one CO is bound to a given hemoglobin molecule, the rate of release of its remaining oxygen molecules to cells is reduced. Recent research has found that mental functioning is reduced during short-term exposure to high levels of CO and perhaps also as a result of long-term exposure to low concentrations, because the brain, like the heart, is a body organ with a high requirement for oxygen.
One important feature of the reduction in vehicular pollutant emissions over the last few decades has been the substantial decline in accidental death from acute carbon monoxide poisoning. In the United States alone, it has been estimated that more than 11,000 deaths have been avoided as a result.
Smoking tobacco is a significant source of carbon monoxide indoors. Although nonsmokers usually have less than 1 % of their hemoglobin tied up as the complex with CO, the figure for smokers is many times this value because of the carbon monoxide that they inhale during smoking. Studies have shown that increased mortality from heart disease can result even if only several percent of hemoglobin is chronically tied up as the CO complex. Exposure to very high concentrations of CO results in headache, fatigue, unconsciousness, and eventually death (if such exposure is sustained for long periods).
Low-priced, easily installed carbon monoxide detectors suitable to warn residents in homes and offices when high CO levels occur are now widely available. However, scientists have begun to worry about the poorly known health effects of chronic exposure to low levels of CO and the fact that such exposure may be quite common.
Environmental Tobacco Smoke
It is well established that smoking tobacco is the leading cause of lung cancer and is one of the main contributors to heart disease. Nonsmokers are often exposed to cigarette smoke, although in lower concentrations than smokers since it is diluted by air. This environmental tobacco smoke, or ETS ("secondhand smoke"), has been the subject of many investigations in order to determine whether or not it is harmful to people who are exposed to it.
ETS consists of both gases and particles. The concentration of some toxic products of partial combustion is actually higher in sidestream smoke than in mainstream, since combustion occurs at a lower temperature—and so is less complete—in the smoldering cigarette compared to one through which air is being inhaled. Since the sidestream smoke is usually diluted by air before being inhaled, however, the concentrations of pollutants reaching the lungs of non-smokers are much lower than those reaching the lungs of smokers themselves.
The chemical constitution of tobacco smoke is complex: It contains thousands of components, several dozen of which are carcinogens. The gases in smoke include
• carbon monoxide and carbon dioxide;
• formaldehyde and several other aldehydes, ketones, and carboxylic acids;
• nitrogen oxides, hydrogen cyanide, ammonia, and a number of organic nitrogen compounds;
• methyl chloride;
• toluene, benzene, and several hundred different PAHs, to be discussed in Chapters 8 and 12; and
• cadmium and radioactive elements such as polonium (see Chapter 9).
Included in the nitrogen compounds are several nitrosamines, organic nitrogen compounds of formula R2N—N=0, which, together with the PAHs, are probably the most important respiratory carcinogens in the smoke.
The particulate phase of cigarette smoke is called the tar, and much of it is respirable in size. The zone in a cigarette that actively bums, as occurs when a smoker inhales a puff, is quite hot (700-950°C) and produces CO and H2 as well as the expected C02 and water vapor. Immediately downstream of this area is a cooler zone (200-600°C) where smoke constituents such as nicotine distill out of the tobacco. When this vapor cools farther along the cigarette path toward the smoker, much of it condenses to aerosol particles that constitute the particulate phase of the smoke.
Many people experience irritation of their eyes and airways from exposure to ETS. The gaseous components of ETS, especially formaldehyde, hydrogen cyanide, acetone, toluene, and ammonia, cause most of the odor and irritation. Exposure to ETS aggravates the symptoms of many people who suffer from asthma or from angina pectoris, chest pains brought on by exertion. ETS, particularly when it originates from maternal smoking, is known to induce new cases of asthma in children, especially those of preschool age. Some recent studies have established correlations between the rate of acute respiratory illness and the level of indoor PM2 5 (which would include the total amount of respirable particulates from all sources, including tobacco smoke). Passive smoking—which involves inhalation of sidestream as well as already exhaled smoke—is believed by scientists to cause bronchitis, pneumonia, and other infections such as those of the ear in up to 300,000 infants, as well as several thousand instances of sudden infant death, in the United States each year. Second-hand smoke may even reduce the cognitive abilities of children, whether exposed prenatally or when young. Being exposed to second-hand smoke, whether on the job or by living with a smoker, approximately doubles a nonsmoker's chance of developing asthma.
In 1993, the U.S. EPA classified ETS as a known human carcinogen and estimated that it causes about 3000 lung cancer deaths annually. ETS is also considered to be responsible for killing as many as 60,000 Americans annually from heart disease. In a study of American nurses, it was found that nonsmoking women regularly exposed to ETS had a 91% greater rate of heart attacks than women who had no exposure. Apparently the smoke leads to hardening of the arteries, a main cause of heart attacks. An analysis of all recent studies on passive smoking led to the conclusion that the risks of developing lung cancer and heart disease each are increased by about one-quarter for nonsmoking spouses of smokers. Longtime workers in bars and restaurants in which smoking is permitted also have an increased rate of lung cancer, even if they themselves do not smoke. A British study estimated that ETS kills 140,000 Europeans annually through cancer and heart disease.
The term asbestos refers to a family of six naturally occurring silicate minerals that are fibrous (see Chapter 16). Structurally, they are composed of long double-stranded networks of silicon atoms connected through intervening oxygen atoms; the net negative charge of this silicate structure is neutralized by the presence of cations such as magnesium.
The most commonly used form of asbestos, chrysotile, has the formula Mg3Si205(0H)4. It is a white solid whose individual fibers are curly. Chrysotile, mined mainly in Russia, China, Brazil, Canada (Quebec), and Kazakhstan, is the principal type of asbestos used in North America. It has been employed in huge quantities because of its resistance to heat, its strength, and its relatively low cost. Common applications of asbestos include its use as insulation and spray-on fireproofing material in public buildings, in automobile brake-pad lining, as an additive to strengthen cement used for roofing and pipes, and as a woven fiber in fireproof cloth.
The use of asbestos has been sharply reduced since the 1970s in developed countries because it is now recognized from studies on the health of asbestos miners and other asbestos workers to be a human carcinogen. It causes mesothelioma, a normally rare, incurable cancer of the lining of the chest or abdomen. In addition, airborne asbestos fibers and cigarette smoke act syner-gistically: Their combined effect is greater than the sum of their individual effects (in this case, equal to the product of the two) in causing lung cancer.
There is much controversy concerning whether chrysotile should be banned outright from further use and whether or not existing asbestos insulation in buildings should be removed. Many experts feel that existing asbestos should be left in place unless it becomes damaged enough that there is a chance that its fibers will become airborne. Indeed, its removal can dramatically increase the levels of airborne asbestos in a building unless extraordinary precautions are taken. One scientist stated: "Removing asbestos is like waking up a pit bull terrier by poking a stick in its ear. We should let sleeping dogs lie." Some environmentalists, however, feel that existing asbestos is a ticking time bomb—that it should be removed as soon as possible, as one can never predict when building insulation will be damaged.
Most of the initial concern about asbestos was related to crocidolite, blue asbestos, and amosite, brown asbestos. Evidence implicating crocidolite in causing cancer in humans was already well established several decades ago. It is a material with thin, straight, and relatively short fibers that more readily penetrate lung passages, making it a more potent carcinogen than the white form. Crocidolite and amosite are mined in South Africa and Australia; they were not used much in North America but were used in many areas of Europe, including the United Kingdom.
More than 15 countries, including the European Union and Australia, have now banned all forms of asbestos. Some environmentalists and physicians worry that although workers in developed countries wear masks and overalls and handle white asbestos properly to greatly minimize their exposure to it, these practices are not yet common in developing countries. Canada, among other countries, has resisted efforts by agencies of the United Nations to place chrysotile on the list of most hazardous substances.
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