The Human Health Effects of Outdoor Air Pollutants

It is now well established that breathing polluted air can have a dramatic influence on human health. In this section, the most important effects of outdoor air pollutants are described, and the variation in concentration of the dominant air pollutants in different countries is discussed.

The effect that pollutants have on human health cannot be deduced from general laws of biology or physiology; they must be established by experimentation. One can imagine experiments involving animals or human volunteers in which the health effects of exposure to brief periods of artificially produced high-level pollution are studied. However, the extrapolation of information gained from short-term studies of high-level pollution to the long-term exposures at low levels is difficult. In particular, for some pollutants, there may exist a threshold pollutant concentration, or an exposure below which a particular health effect does not occur. In such cases, predictions obtained by assuming simple direct proportionality between exposure and effect would be unwarranted. In addition, there could be deleterious effects of chronic exposure that do not come into play when exposure, even intense exposure, to pollutants occurs only for brief periods of time.

For these reasons, the best information regarding the effects of pollutants on health comes from the large-scale "experiment" in which we are all enrolled as "test animals"—namely, living in a society in which we are routinely exposed to these pollutants for our whole lives. Because the level of exposure to any given pollutant varies considerably from place to place, scientists can collect information on health and on pollution levels in different locations, then correlate them using statistics to establish the effect of one on the other.

As would be expected, the major effects on human health from air pollution occur in and through the lungs. For example, asthmatics suffer worse episodes of their disease when the sulfur dioxide or the ozone or the particulate concentration rises in the air that they breathe. In one U.S. study, it was established that asthma attacks increased by 3% for each increase of 10 ¡xg/m3 in the PM10 index (discussed in Chapter 3). A recent study in California found that asthma can be caused by air pollution, specifically by ozone and especially among highly active children, who naturally inhale more air into their lungs.

Another gaseous pollutant of some concern is 1,3'butadiene, which has the structure CH2—CH—CH=CH2. This hydrocarbon is known as an air toxic since there is evidence that it causes cancer—leukemia and non-Hodgkin's lymphoma especially—and may also negatively affect human reproduction. It is produced as a by-product of the incomplete combustion of fuels, is produced in forest fires and wildfires, and is a component of cigarette smoke.

The Human Health Effects of Smogs

In the middle decades of the twentieth century, several Western industrialized cities experienced such serious wintertime episodes of smog from soot and sulfur pollution that the death rate increased noticeably. For example, in London, England, in December 1952, about 4000 people died within a few days—plus 8000 more in the next few months—as a result of the high concentrations of these pollutants that had built up in a stagnant, foggy air mass trapped by a temperature inversion close to the ground. Those at most risk were young children and elderly persons already suffering from bronchial problems. A ban on household coal burning, from which most of the pollutants originated, has now largely eliminated such problems. Scientists are still unsure whether the main sulfur-containing agent that caused such serious problems in London was the S02, the sulfuric acid droplets, or the sulfate particulates.

Today, due to pollution controls, soot-and'sulfur smogs are no longer a major problem in Western countries. Deaths from bronchitis have fallen by over half in the United Kingdom, the result of changes in air quality (and smoking habits). However, the quality of winter air in some areas of what was the Eastern bloc of countries, such as southern Poland, the Czech Republic, and eastern Germany, until recently was very poor on account of the burning of large amounts of high-sulfur (up to 15% S) "brown coal" for both industrial and home-heating purposes. For example, although the acceptable limit for the concentration of S02 in air is 80 /xg/m3 in many countries, the level of this gas in Prague surpassed 3000 /u.g/m3 on occasion. Indeed, four out of five children admitted to the hospital in some areas of Czechoslovakia in the early 1990s were there for treatment of respiratory problems. However, the average S02 level in Prague decreased by about 50% from the early 1980s to the early 1990s, and overall in the Czech Republic S02 emissions are now only about 10% of 1990 levels. The tremendous improvement of air quality in eastern Germany since 1990, where mean S02 levels have dropped from 113 to 6 jug/m3, has resulted in a decrease in childhood respiratory infections and an increase in lung function.

The effects of sulfur dioxide are also evident in cities such as Athens, where the death rate is found to increase by 12% when the concentrations of the gas exceed 100 fig/m3. Detail on the ancient statues and monuments of Athens is also being seriously eroded by sulfur dioxide and its secondary pollutants. High levels of sulfur dioxide and of fine particulates, both mainly from diesel-fueled vehicles, caused about 350 premature deaths in Paris annually in the late 1980s. And the air in London is not so improved that it does not affect human health; a recent study concluded that one in every 50 heart attacks was triggered by outdoor air pollution, from a combination of smoke, CO, S02, and NOz,

European cities are not the only ones affected by air pollution. Both sulfur dioxide and particulate matter levels regularly exceed World Health Organization (WHO) guidelines in Beijing, Seoul, and Mexico City. In 2002, 13 of the 20 world cities having the highest averages for airborne particulate matter were located in China; the others were Cairo, Jakarta, and five cities in India. In many large cities in the developing world, coal is still the predominant fuel and in some cases diesel-powered vehicles substantially worsen the problem. In Beijing, high S02 emissions from coal burners that are used to heat buildings, plus smoke from smelters on the edges of the city, plus windblown dust and sand from the Gobi Desert combine to produce poor air quality. Haze over China, produced by air pollution, so reduces sunlight intensity that it may be cutting food production by as much as 30% across a third of the country. Indeed, there are a number of cities in China in which the air quality is among the poorest in the world. According to recent projections, if no attempts are made to reduce S02 emissions as industrialization increases, by 2020 the concentration of the gas in Bombay and the Chinese cities of Shanghai and Chongqing will be about four times the WHO maximum safe limit.

It is a historical characteristic that once an undeveloped country starts industrial development, its outdoor air quality worsens significantly. The situation continues to deteriorate until a significant degree of affluence is attained, at which point emission controls are enacted and enforced, and the air begins to clear. Thus, although the quality of air is now improving with time in most developed countries, it is worsening in the larger cities of developing countries. Mexico City and several urban areas in China, especially Beijing, are generally considered to have the worst urban air pollution in the world at present. Half the respiratory disease in China is caused by air pollution. A report by the United Nations Environmental Program estimates that deaths worldwide from all forms of air pollution amounted to 2.7-3.0 million in 2001, a figure that may rise to 8 million by 2020.

Although acute smog episodes from soot and sulfur-based chemicals have been eliminated in the West, many residents in these countries still are chronically exposed to measurable levels of suspended particles containing sulfuric acid and sulfates due to the long-range transport of these substances from industrialized regions that still emit S02 into the air. For example, research has shown a positive correlation between atmospheric concentrations of ozone and oxidized sulfur and hospital admissions for respiratory problems in southern Ontario. There is some evidence that the acidity of the pollution is the main active agent in causing lung dysfunction, including wheezing and bronchitis in children. Asthmatic individuals appear to be adversely affected by acidic sulfate aerosols, even at very low concentrations.

Photochemical smog, which arises from nitrogen oxides, is now more important than sulfur-based smog in most cities, particularly those of high population and vehicle density. As discussed in Chapter 3, it consists of gases such as ozone and an aqueous phase containing water-soluble organic and inorganic compounds in the form of suspended particles. In contrast to "London smogs," which chemically were reducing in nature due to sulfur dioxide, photochemical smogs are oxidizing.

Ozone itself is a harmful air pollutant. In contrast to sulfur-based chemicals, its effect on the robust and healthy is as serious as on those with preexisting respiratory problems. Experiments with human volunteers have shown that ozone produces transient irritation in the respiratory system, giving rise to coughing, nose and throat irritation, shortness of breath, and chest pains upon deep breathing. People with respiratory problems can often tell from their symptoms—such as the tightening of their chest or the beginning of a cough—when the air quality is poor. Even healthy, young people often experience such symptoms while exercising outdoors by cycling or jogging during smog episodes. Indeed, there is evidence that the daily race times of cross-country runners increase with increasing ozone concentration in the air that they inhale. A small percentage of the day-to-day fluctuations in mortality rate in Los Angeles is explained by variations in the concentrations of air pollutants. An analysis of 95 urban centers in the United States discovered that a period of high ozone concentrations increased daily cardiovascular and respiratory mortality by about 0.5% per 10 ppb increase following a few days of continuous exposure. It is not yet clear what, if any, long-term lung dysfunction results from exposure to ozone, and indeed this is a controversial subject among scientists. Exposure to ozone produces a number of indirect health effects as well—including a decrease in sperm count.

One anticipated effect of ozone is a decreased resistance to disease from infection because of the destruction of lung tissue. Many scientists believe that chronic exposure to high levels of urban ozone leads to the premature aging of lung tissue. At the molecular level, ozone readily attacks substances containing components with C=C bonds, such as occur in biological tissues of the lung. As discussed later, the fine particulates produced in the photochemical smog process can have a deleterious health effect on humans.

Most industrialized nations have enacted standards that regulate the maximum concentrations in air of sulfur dioxide, nitrogen dioxide, N02, and carbon monoxide, CO, as well as ozone and fine particulates (see Table 4-1), and in some cases total reduced sulfur, since all these pollutants cause health effects at sufficiently high concentrations. For example, several recent North American studies have statistically linked the rate of hospitalization for

Air Quality Standards, in Parts per Billion, for Pollutants

Air Quality Standards, in Parts per Billion, for Pollutants


Time Span to Average

United States


European Union



8 hr






1 hr



8 hr






1 day





1 year





1 year





PM2 5 (in ¡Jbg/m?)

1 day





*To be implemented by 2010.

*To be implemented by 2010.

congestive heart failure among elderly people to the daily carbon monoxide concentration in outside air. Mexico City currently has the highest levels of carbon monoxide among the world's most polluted cities. Both CO and N02 are usually more of a problem in indoor air and will be discussed in detail in a later section.

It has been speculated that pollution due to S02 and sulfates causes a decrease in resistance to colon and breast cancer in people living in northern latitudes. The suggested mechanism of this action is a reduction in the amount of available UV-B that is necessary to form vitamin D, which is a protective agent for both types of cancers. Since sulfur dioxide absorbs UV-B and sulfate particles scatter it, significant concentrations of either substance in air will reduce the amount of UV-B reaching ground level. Thus too little UV-B can have detrimental health effects, just as too much of it can—as was outlined in Chapter 2.

Finally, we note that there are some positive effects of air pollution on human health! For example, the rate of skin cancers in areas heavily polluted by ozone is probably reduced because of the ability of the gas to filter UV-B from sunlight.

Particulates as Health Risks

Particulate matter in the form of smoke from coal burning has been an air pollution problem for many hundreds of years, especially in the United Kingdom. John Evelyn wrote in his January 1684 diary that "London by reason of the excessive coldness of the air, hindering the ascent of the smoke, was so filled with the fuliginous [sooty] steam of sea-coal, that hardly could one see across the street, and this filling the lungs with its gross particles exceedingly obstructed the breast, so as one would scarce breathe." Indeed, unsuccessful attempts to control coal burning and punish offenders had begun in the thirteenth century in Britain. Perhaps Shakespeare was referring to this type of air pollution in the quotation from Hamlet that opened Chapter 3.

Although serious episodes of such soot-and-sulfur smogs have been largely eliminated in Western industrialized countries, the air pollution parameter that correlates most strongly with increases in the rate of disease or mortality in most such regions is the concentration of respirable (fine) particulates, PM, 5. It appears that particulate-based air pollution has a greater effect on human health than that produced directly by pollutant gases.

Substances that dissolve into the body of a particle are said to be absorbed by it; those that simply stick to the surface of the particle are said to be adsorbed (see Figure 4-6). An important example of the latter is represented by the adsorption of large organic molecules onto the surfaces of carbon (soot) particles, as discussed later in Chapter 12. Many insoluble airborne particles are surrounded by a film of water, which can itself dissolve other substances. The adsorption of metal atoms and organic molecules on the surface of airborne particles may give rise to some of the health hazards these particles represent.

Larger particles—coarse ones, according to the definition in Chapter 3—are of less concern to human health than are small (fine) ones for several general reasons:

• Since coarse particles settle out quickly, human exposure to them via inhalation is reduced,

• When inhaled, coarse particles are efficiently filtered by the nose (including its hairs) and throat and generally do not travel as far as the lungs. In contrast, inhaled fine particles usually travel through to the lungs (which is why they are called respirable), can be adsorbed on cell surfaces there, and can consequently affect our health.

• The ratio of surface area to mass of large particles is smaller than that of small ones; thus, gram for gram, their ability to transport adsorbed gas molecules to any parts of the respiratory system and to catalyze chemical and biochemical reactions there is correspondingly smaller.

• Devices such as electrostatic precipitators, spray towers, and cyclone collectors that are used to remove particulates from air are efficient only for coarse particles. Thus, although a device may remove 95% of the total particulate mass, surface area and respirable particles are reduced by a much lower fraction; see Problem 4-2. Baghouse filters, which are finely woven fabric bags through which air is forced, are highly efficient in removing fine particles in the 1-jum size range, as well as all the larger ones.

The exhaust from diesel engines has been classified as "likely to be carcinogenic to humans" by the U.S. Environmental Protection Agency (EPA). Studies in California and in Seattle conclude that 70% or more of the risk to health from air toxics arises from diesel exhaust. Following court decisions, the United States will institute by 2010 a series of new regulations limiting emissions from on-road diesel vehicles.



/ Particle


/ \ Absorbed


FIGURE 4-6 Contrast between adsorption and absorption of molecules on/in an airborne particle (schematic).

FIGURE 4-6 Contrast between adsorption and absorption of molecules on/in an airborne particle (schematic).


An air-filtering device is tested and is found to remove all particles larger than 1 /¿m in diameter, but almost none of the smaller ones. Calculate the percentage of the surface area removed by the device for a sample of particulates, 95% of the mass of which is particles of diameter 10 /xm and 5% of which is particles of diameter 0.1 jam. Assume all particles are spherical and of equal density, [Hint: Recall that the surface area of a sphere is 4th2. Calculate the surface areas of particles of each size from this formula,]

A number of studies have correlated day-to-day urban morbidity (sickness) rates, as measured by hospital admission rates, for respiratory problems against the pollution levels during the same short time period. For example, there have been several reports concerning the immediate effects on the population of southern Ontario of the pollutants—ozone gas and sulfate particulates—to which they are most exposed. In one study, the average number of hospital admissions for respiratory problems correlated best with the ozone level of the previous day, and to a slightly lesser degree with the sulfate level from the previous day, for the summers of 1983-1988. Air pollution was found to account for about 6% of summertime hospital respiratory admissions, a magnitude close to that found in previous investigations in Ontario and New York State. A recent study found that respiratory admissions correlated significantly with both PM10 and ozone concentrations in Spokane, Washington—an area where atmospheric sulfur dioxide is essentially nonexistent and therefore can be ruled out as the true culprit in causing the illnesses.

The strongest links between human health and exposure to airborne particulate matter are based on recent studies involving cities in the United States and are the subject of the online Case Study The Effect of Urban Air Particulates on Human Mortality at the website associated with this chapter.


The burning of wood in domestic fireplaces produces large quantities of particulates, which are emitted from the chimneys into outdoor air unless catalytic converters are fitted to the smokestack. Indeed, in residential neighborhoods where wood is the predominant fuel used for heating, wood stoves contribute up to 80% of the line particles in the air during the winter months. Outdoor wood-fired boilers, used to heat water for saunas and swimming pools, have grown so much in popularity that the particulates they emit have become a significant problem. Some newer wood stoves and boilers have catalytic converters or secondary combustion chambers in which particulates and unburnned gases are more fully oxidized, thus reducing their emission to outside air.

Serious episodes of smoky haze pollution over large areas of land have occurred in recent years in Southeast Asia, especially in Malaysia and Indonesia. The smoke originates mainly from forest fires that are intentionally started in order to clear land that can be subsequently used for agriculture and to grow trees for their rubber, palm oil, or pulp content. A secondary source of the smoke is the smoldering underground fires that slowly burn in underground coal and peat deposits. Indeed, there are estimated to be a quarter million individual coal fires currently burning in Indonesia, as well as many m China and India, and there are also many peat fires in Malaysia. The fires are initiated when an outcropping of coal, or of a peat deposit that has dried after draining, is ignited, typically during one of the fires set to clear the land. Fires can also be ignited in coal, once exposed to the air, by lightning strikes and even by spontaneous combustion when the surface pyrite is oxidized and the heat released by this reaction sets the carbon ablaze. These underground fires can continue to burn for decades after the original forest fires have stopped.

A so-called Asian brown cloud of particles and gases from forest fires, vehicle exhausts, and domestic cookers—especially in rural areas—that burn wood, dung, and agricultural waste overhangs most of eastern and southeastern Asia annually from December to May, the main season for home heating. The brown cloud over the Indian Ocean consists mainly of smoke from the burning of dried manure in cooking fires. This haze lowers sunlight levels up to 15%, with a corresponding decline in the yield of crops such as rice and an alteration to rainfall and monsoon patterns. In contrast to the pollution aerosol over North America and Europe, to which it is comparable in magnitude, the "black carbon" content of the Asian cloud is significant. The absorption of sunlight by this elemental carbon alters the local hydrological cycle and hence the weather over the northern Indian Ocean. The lack of nitric oxide produced in the low-temperature flames of burning biomass currently limits ozone production over the area, but that will likely be reversed in the future with increased use of fossil fuels for vehicles.

Large forest fires in northern Canada produce huge quantities of carbon monoxide and volatile organic compounds (VOCs), which have been found to travel as far as the U.S. Southeast and which may well increase ozone and particulate concentrations in the air of this region.

Coping with Asthma

Coping with Asthma

If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.

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