Eight studies of air pollution in U.S. cities have now shown that fine particles (the invisible soot emitted by incinerators, automobiles, power plants and heating units) are presently killing about 60,000 Americans each year. [1] More than a dozen studies have, in one way or another, confirmed this relationship. Furthermore, there appears to be no threshold, no level below which effects disappear. This means that people are being killed by air pollution levels well within existing federal standards.

To summarize bluntly, any increase in fine particles in the atmosphere kills someone. The victims remain nameless, but they have been deprived of life all the same. Mere compliance with federal standards does not protect the public. Any increase in the number of small particles in the air elevates the death rate. This has obvious implications for certain technologies: incinerators and fossil-fuel-powered machines (automobiles and trucks, power plants and heating units). To protect public health, these technologies must be avoided, or fitted with expensive control equipment, or replaced by cleaner alternatives.

People have known for a long time that particles in the air can kill. In 1952, a dense smog killed 4000 people during one week in London, and since then no one has doubted the cause-and-effect relationship. The question, therefore, isn't whether airborne particles can harm humans, but rather, how much pollution causes how much damage, and, secondly, is there a threshold, an amount below which no effects are seen?

Throughout the '50s and '60s, complacent authorities assumed there was a threshold --some amount that was safe. However, after 1975, a revolution took place in scientific understanding of fine particles and health. In 1979, the National Research Council of the National Academy of Sciences, [2] and the United Nations, [3] both published book-length studies of the dangers of small particles to humans. Here is the current view: humans evolved in an environment where dust was made up of large particles. Humans therefore evolved means for protecting themselves against large particles. Large particles are filtered out by hairs inside the nose, mucous membranes in the throat and airways, and other mechanisms. However, modern combustion machines produce small particles which pass right by these natural protections and then enter the deep lung. In the deep lung, air comes into contact with a person's blood stream; this is where oxygen passes into the body and carbon dioxide passes out with each breath we take. Putting tiny particles of pollution directly in contact with the surface of the deep lung is a recipe for trouble. Because of their origin in combustion processes, most fine particles are coated with toxic materials --metals like lead and mercury, or toxic organics like polycyclic aromatic hydrocarbons (PAHs). So fine particles provide a uniquely efficient carrier, giving dangerous toxins direct entry into the blood stream.

Armed with new knowledge, in 1987, U.S. Environmental Protection Agency (EPA) established new, stricter standards for particles in the air. The 1987 standard, which governs today, is expressed in terms of small particles (also called particulate matter) that measure 10 micrometers or less in diameter. (A meter is 39 inches and a micrometer is a millionth of a meter.) These are called respirable or inhalable particles because, as we saw above, they are small enough to get into the deep lung where they cause various kinds of damage. The shorthand way to refer to these pollutants is PM10 (meaning Particulate Matter 10 micrometers or less in diameter). Current U.S. standards say that the ambient air (the general air we all breathe) may contain no more than 50 micrograms of PM10 particles per cubic meter of air as an annual average, and the one-day average should exceed 150 micrograms per cubic meter only one day each year. (A gram is 1/28th of an ounce and a microgram is a millionth of a gram.)

Since 1987, evidence has been accumulating, showing that the 1987 standards do not protect human health. The question about the existence of a PM10 threshold was addressed first by Joel Schwartz of U.S. Environmental Protection Agency (EPA). Schwartz reviewed data on air pollution and deaths from London, 1958-1972, and showed there was no threshold down to the lowest observed levels of air pollution. [4] A study published last month in the NEW ENGLAND JOURNAL OF MEDICINE, of six U.S. cities, including several that are not heavily polluted, such as Portage, Wisconsin and Topeka, Kansas, shows death rates increasing with just 15 micrograms per cubic meter of PM10 pollutants. [5] In all, at least 8 studies have now shown that PM10 at any level kills people. It seems clear there is no threshold.

A study of people in Steubenville, Ohio, showed that each increase of 100 micrograms per cubic meter of total suspended particles (of which PM10 represents about half) is associated with a 4% increase in the death rate, with no threshold. [6] Interestingly, the Steubenville study showed that the death rate changes as fine particle levels change, but not as sulphur dioxide levels change.

In Philadelphia, a close relationship between PM10 pollutants and the death rate was observed. [7] Once again sulfur dioxide levels did not correlate with the death rate, but particle concentrations did. Here each increase of 100 micrograms per cubic meter of total suspended particles (of which PM10 makes up half) was associated with a 7% increase in the death rate. There was no threshold.

A study of people in Detroit showed that a 6% increase in the death rate was associated with each increase of 100 micrograms per cubic meter of total suspended particles (of which PM10 makes up half). [8] There was no evidence of a threshold. Sulfur dioxide levels were not significantly associated with increases in the death rate. Studies [9] of St. Louis, Missouri and Kingston, Tennessee, showed that the death rate increased 16% (St. Louis) and 17% (Kingston) with each addition of 100 micrograms per cubic meter of PM10 pollutants to the air. Associations with gaseous pollutants --sulfur dioxide, nitrogen oxides and ozone --did not come close to achieving statistical significance.

In the Utah Valley, a study of the population of Provo revealed that the daily death rate was closely associated with levels of PM10 pollution. [10] The Utah Valley is unique because PM10 is the only pollution present there in significant quantities (contributed chiefly by a steel mill). For every increase of 100 micrograms per cubic meter of PM10 pollutants, there was a 16% increase in the death rate, and no threshold was observed.

In all cities, the increase in deaths was most notable among people older than 65 and in people with chronic obstructive pulmonary disease (COPD) or cardiovascular disease.

There is a remarkable consistency apparent in all these studies: a 100 micrograms per cubic meter increase in PM10 is always accompanied by an 8% to 17% increase in the death rate. Joel Schwartz, the only EPA employee ever given a "genius award" by the MacArthur Foundation, re-analyzed data from London's 1952 killer smog and showed that the death rate increased 6.4% for each increase of 100 micrograms per cubic meter total suspended particles, or about 13% for each 100 micrograms per cubic meter increase in PM10 pollutants--again, remarkably consistent with the other studies.

No epidemiological study can prove a cause and effect relationship because it is always possible that some key factor was not considered. Until last month, skeptics could say smoking might explain why death rates increase as PM10 concentrations increase. But the study published last month in the NEW ENGLAND JOURNAL OF MEDICINE looked at 8111 adults in six American cities and showed that smoking did not explain the increased death rate observable when PM10 concentrations rise. [5] Smoking has now been ruled out.

Joel Schwartz recently quoted the British researcher, Bradford Hill, saying, "All scientific work is incomplete... All scientific work is liable to be upset or modified by advancing knowledge. That does not confer upon us a freedom to ignore the knowledge we already have, or to postpone the action that it appears to demand at a given time." Then Schwartz added: "At this given time, the knowledge we already have seems to demand a reduction in population exposure to airborne particles." [1]
                                                                         --Peter Montague, Ph.D.
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[1] Seven studies are reviewed by Joel Schwartz, "Particulate Air Pollution and Daily Mortality: A Synthesis," PUBLIC HEALTH REVIEWS 1991/1992 Vol. 19 (1992), pgs. 39-60. For the 8th, see footnote 5. The 60,000 figure is taken from "Air Pollution in Typical U.S. Cities Increases Death Risk," press release dated May 13, 1991, from the Harvard School of Public Health, Boston, Mass. describing findings later reported in Joel Schwartz and Douglas W. Dockery, "Increased Mortality in Philadelphia Associated With Daily Air Pollution Concentrations," AMERICAN REVIEW OF RESPIRATORY DISEASE Vol. 145 (1992), pgs. 600-604. Two million deaths occur in the U.S. each year; according to Schwartz and Dockery, fine particles account for 3%. See also, Michael Weisskopf, "Particles in the Air Help Kill 60,000 a Year, Study Says," WASHINGTON POST May 13, 1991, pg. A13.

[2] National Research Council, AIRBORNE PARTICLES (Baltimore: University Park Press, 1979).

[3] United Nations, FINE PARTICULATE POLLUTION (NY: Pergamon Press, 1979).

[4] Joel Schwartz and Allan Marcus, "Mortality and Air Pollution in London: A Time series Analysis," AMERICAN JOURNAL OF EPIDEMIOLOGY Vol. 131 (1990), pgs. 185-194.

[5] Douglas Dockery and others, "An Association Between Air Pollution and Mortality in Six U.S. Cities," NEW ENGLAND JOURNAL OF MEDICINE Vol. 329 (1993), pgs. 1753-1759; see also pgs. 1807-1808.

[6] Joel Schwartz and Douglas Dockery, "Particulate Air Pollution and Daily Mortality in Steubenville, Ohio," AMERICAN JOURNAL OF EPIDEMIOLOGY Vol. 135 (1992), pgs. 12-19; see also pgs. 20 and 23 for discussion of the Steubenville study.

[7] Philadelphia study cited in note 1, above.

[8] Joel Schwartz, "Particulate Pollution and Daily Mortality in Detroit," ENVIRONMENTAL RESEARCH Vol. 56 (1991), pgs. 204-213.

[9] Douglas W. Dockery and others, "Air Pollution and Daily Mortality: Associations with Particulates and Acid Aerosols," ENVIRONMENTAL RESEARCH Vol. 59 (1992), pgs. 362-373.

[10] C. Arden Pope III and others, "Daily Mortality and PM10 Pollution in Utah Valley," ARCHIVES OF ENVIRONMENTAL HEALTH Vol. 47 (1992), pgs. 211-217.

Descriptor terms: air pollution; morbidity statistics; mortality statistics; fine particles; particulates; particulate matter; fossil fuels; coal; oil; natural gas; automobiles; trucks; soot; smoke; exhaust; electric power; steubenville; oh; philadelphia; pa; detroit; mi; st. louis; mo; kingston; tn; provo; ut; utah valley; pm10; joel schwartz; douglas dockery; bradford hill; regulations; ambient air standards;