Air pollution and scientific certainty


Peter Montague

Air pollution from the combustion of fossil fuels (oil, coal, and natural gas) in cars, trucks, and power plants, is killing roughly 60,000 people in the USA each year, according to researchers at Harvard University's School of Public Health (D.W. Dockery and C.A. Pope III, "Acute Respiratory Effects of Particulate Air Pollution", Annual Review of Public Health, Vol. 15 (1994), pp. 107-132).

This represents about 3% of all US deaths. Every combustion source is contributing to the death toll; none is benign. Diesel vehicles and oil- and coal-burning power plants seem to be the worst offenders.

The culprit in every case is the fine particles — invisible soot — created by combustion. Fine particles are not captured efficiently by modern pollution-control equipment. Furthermore, they are not visible except as a general haze. They are far too small to be seen.

According to more than a dozen studies, there seems to be no threshold, no level of fine-particle pollution below which no deaths occur. Even air pollution levels that are well within legal limits are killing people, especially older people, and people with chronic heart and lung ailments, the Harvard researchers have found.

Furthermore, studies indicate that fine-particle pollution is causing or exacerbating a wide range of human health problems, including: initiating, and worsening, asthma, especially in children; increasing hospital admissions for bronchitis, asthma, and other respiratory diseases; increasing emergency room visits for respiratory diseases; reducing lung function (though modestly) in healthy people as well as (more seriously) in those with chronic diseases; increasing upper respiratory symptoms; sinusitis; sore throat; wet cough; head colds; hay fever; burning or red eyes); and increasing lower respiratory symptoms; and heart disease.

Since 1987, the US Environmental Protection Agency (EPA) has been measuring fine-particle air pollution, calling it PM10, which means "particulate matter 10 microns or less in diameter". A micron is a millionth of a metre.

EPA measures PM10 pollution by weight — the total weight of all particles with a diameter of 10 microns or less in each cubic meter of air. The legal limit is 50 micrograms of PM10 particles in each cubic metre of air, as a year-round average. Many US, Canadian and European cities from time to time will have as much as 100 to 200 micrograms of PM10 particles in each cubic metre of air.

The size of the particles is what's most important from a health viewpoint. Particles larger than 10 microns generally get caught in your nose and throat, never entering the lungs. Particles smaller than 10 microns can get into the large upper branches just below your throat, where they are caught and removed (by coughing and spitting or by swallowing). Particles smaller than 5 microns can get into your bronchial tubes, at the top of the lungs; particles smaller than 2.5 microns can get down into the deepest (alveolar) portions of your lungs. These are the really dangerous particles because the deepest portions of the lung have no efficient mechanisms for removing them. If these particles are soluble in water, they pass directly into the blood stream within minutes. If they are not soluble in water, they are retained in the deep lung for long periods (months or years).

About 60% of PM10 particles (by weight) have a diameter of 2.5 microns or less. These are the particles that can enter the human lung directly. (They also enter homes; indoor air and outdoor air typically contain the same quantities of fine particles, so buildings provide no refuge from these invisible killers.)

In a modern US city, on many days, the air will contain 100 billion one-nanometre (0.001 microns) diameter particles in each cubic meter of air, all of them invisible. By weight, these 100 billion particles will only amount to 0.00005 micrograms (one ten-thousandth of 1% of the 50-microgram legal limit), yet they may be responsible for much of the health damage created by fine-particle pollution. For this reason, in 1979 the National Research Council said that measuring particles by weight, without regard to particle size, has "little utility for judging effects."

The study of fine particles and their effects on human health has been under way in earnest since 1975. During the past 20 years, studies have been able to rule out sulphur dioxide and ozone pollution as the cause of the observed deaths. This year a new study of 552,138 adults in 151 US metropolitan areas confirmed once again that there is a clear relationship between fine-particle air pollution and human deaths, and it ruled out smoking as a cause of the observed deaths. (C. Arden Pope III and others, "Particulate Air Pollution as a Predictor of Mortality in a Prospective Study of US Adults", American Journal of Respiratory and Critical Care Medicine, Vol. 151 (1995), pp. 669-674.)

This study is particularly important because it didn't simply match death certificates with pollution levels; it actually examined the characteristics (race, gender, weight and height) and lifestyle habits of all 552,138 people. Thus the study was able to rule out tobacco smoking; exposure to passive tobacco smoke; occupational exposure to fine particles; body mass index (relating a person's weight and height); and alcohol use. The new study also controlled for changes in outdoor temperature. The study found that fine-particle pollution was related to a 15% to 17% difference in death rates between the least polluted cities and the most polluted cities.

One might ask why steps weren't taken long ago to prevent the disease and death associated with fine-particle pollution. The National Academy of Sciences said as early as 1979 that fine particles retained in the deep lung cause long-term lung disease. The Academy said at that time, "In summary, particulate atmospheric pollutants may be involved in chronic lung disease pathogenesis as causal factors in chronic bronchitis, as predisposing factors to acute bacterial and viral bronchitis, especially in children and cigarette smokers, and as aggravating factors for acute bronchial asthma and the terminal stages of oxygen deficiency (hypoxia) associated with chronic bronchitis and/or emphysema and its characteristic form of heart failure".

If the Harvard researchers are correct in their estimate that 60,000 people in the US die each year from fine-particle pollution, and tens of thousands more are made sick (especially children), then we can calculate that, since 1979, nearly a million have been killed by fine particle pollution, and millions more have been made sick.

Why can't we act to prevent this important problem? US regulatory agencies — and the courts — have lost their way, searching for the holy grail of scientific certainty. Regulators and judges now insist that science has to "prove harm" before regulatory control can begin. Philosophers of science know that science cannot "prove" anything. It often takes science decades — sometimes centuries — to reach a clear majority opinion, and there will always be uncertainties, giving rise to nagging doubts, which can only be laid to rest by further study.

The science of 1979 was sufficient to tell us that people are dying and children are getting sick because of fine particles. The precise mechanism of harm is, even today, not fully understood, but the harm itself has been clear beyond any reasonable doubt for many years. Common sense says that the National Academy's conclusions back in 1979 should have been sufficient for regulators to clamp down in earnest.

When did our society first turn away from the common sense, weight-of-the-evidence, preventive approach which, theoretically at least, guides public health decisions? What role did corporate lawyers and scientists play in convincing scientifically illiterate judges and politicians that scientific certainty was required before society could take prudent steps to protect public health and safety?
[From Rachel's Environment & Health Weekly (USA).]