1. What are the health effects of particulate air pollution?

More than two dozen community health studies since 1987 have linked particulate pollution to reductions in lung function, increased hospital and emergency room admissions, and premature deaths. Recently, two major epidemiological studies (by the American Cancer Society and Harvard University) were published that showed that people living in more polluted cities had an increased risk of premature death compared to those in cleaner cities.

2. How does mortality attributable to particulate pollution compare to total cardiopulmonary mortality?

NRDC estimates that at current levels of pollution, approximately 64,000 premature deaths from cardiopulmonary causes may be attributable to particulate air pollution each year. That represents 6.5% of all cardiopulmonary deaths, which total 986,000 per year. The national estimate of mortality attributable to smoking is 418,690 for 1990.

3. Who is at greatest risk?

The elderly and those with heart and lung disease are at greatest risk of premature mortality due to particulate air pollution. Their lives might be shortened by one to two years on average in more polluted areas.

4. How do particles cause harm to human health?

The exact toxicological mechanisms are not well understood, but researchers have a number of theories. For instance, studies show that particulate matter causes respiratory symptoms, changes in lung function, alteration of mucociliary clearance, and pulmonary inflammation which can lead to increased permeability of the lungs. Increased permeability might precipitate fluid in the lungs in people with heart disease. In addition, mediators released during an inflammatory response could increase the risk of blood clot formation and strokes.

Particulate exposure might also increase susceptibility to bacterial or viral respiratory infections, leading to an increased incidence of pneumonia in vulnerable members of the population. Potential mechanisms could include impairment of clearance mechanisms or immune system function. In the presence of pre-existing heart disease, acute bronchiolitis or pneumonia induced by air pollutants might precipitate congestive heart failure.

Particulate air pollution might also aggravate the severity of underlying chronic lung disease, causing more frequent or severe exacerbation of airways disease or more rapid loss of lung function.

5. Has a cause-and-effect relationship been demonstrated?

Evaluation of epidemiological studies requires consideration of a number of factors such as strength of the association, consistency of the association, dose-response relationship, biological plausibility, and coherence with other known facts. Based on these factors, a number of prestigious international panels including a British Committee on the Medical Effects of Air Pollutants and a Committee of the Health Council of Netherlands have concluded that there is a cause-and-effect relationship between particulate pollution and mortality.

6. What exactly is particulate matter?

Particulate matter includes a wide range of pollutants -- road dust, diesel soot, fly ash, wood smoke, and sulfate aerosols that are suspended as particles in the air. These particles are a mixture of visible and microscopic solid particles and minute liquid droplets known as aerosols.

7. Where do fine particles come from?

Combustion of fossil fuels is the principal source of fine particle emissions, including the burning of coal, oil, diesel fuel, gasoline, and wood in transportation, power generation, and space heating. Old coal-fired power plants, industrial boilers, diesel and gas-powered vehicles, and wood stoves are the worst culprits. High temperature industrial processes such as metal smelting and steel production are also significant sources.

8. What level of exposure to particulates is considered unhealthy? Is there a threshold?

Epidemiological studies have reported a linear relationship between exposure and effects. In other words, the higher the concentration of particles, the greater the effect on the health of populations. Effects have been demonstrated at levels well below the current National Ambient Air Quality Standards. Scientists have not been able to identify a threshold below which health effects do not occur. While not a threshold, the long-term epidemiology studies show that the risk of premature deaths starts to increase at annual average concentrations of PM2.5 of 10 g/m3, according to the World Health Organization.

9. How did NRDC come up with its mortality estimates?

NRDC used a methodology suggested by prominent research scientist Dr. Joel Schwartz of the Harvard School of Public Health. We applied the findings of a 1995 study by the American Cancer Society (ACS) and Harvard Medical School to local data to gauge the extent of the particulate pollution problem. The ACS study is the largest, most comprehensive long term epidemiologic study examining the effect of ambient air pollution on human health. The study used statistical techniques to adjust for age, and to control for the effects of smoking, body weight, occupational exposure, and other risk factors.

There were four steps to NRDC's analysis: 1) Analysis of EPA particulate monitoring information for metropolitan statistical areas; 2) Tabulation of data from the National Center for Health Statistics on adult mortality rates from selected cardiopulmonary causes; 3) Calculation of a risk coefficient per microgram of particle pollution from data presented in the ACS study; and 4) Application of the risk coefficient to city-specific monitoring and mortality data.

Although NRDC's analysis relies on several assumptions, a sensitivity analysis based on alternative assumptions shows that the estimates are reasonable.*

10. What about other factors such as smoking, diet, occupational exposures and extreme weather?

The ACS study and the Harvard Six City Study tracked information about other risk factors such as smoking and occupational exposure in their study population and used statistical techniques to control for these risk factors. The risk ratios reported in the original studies factored out the effects of these other factors. Several reanalyses of the short-term epidemiological studies have dismissed the criticism that reported effects are due to confounding by weather.

11. What can individuals do to help reduce particulate pollution?

People cannot choose the air they breathe, but they can choose cleaner and more efficient energy sources for home heating and cooling, transportation, and appliances. Carpooling, recycling, maintaining automobiles, and insulating homes can make a big difference. Perhaps the most significant action an individual can take is to limit the use of fireplaces and wood burning stoves. Finally, citizen support for clean air programs can help counter industry pressure to weaken the Clean Air Act.

12. What would it cost to clean up particulate pollution?

While there are no exact estimates of the cost of clean-up, there are many practical, cost-effective measures that can be taken to reduce emissions of fine particulates, including the adoption of energy conservation measures and switching to cleaner fuels such as natural gas.

Many existing Clean Air Act programs, such as the acid rain control program, and the ground level ozone smog programs, if properly implemented, will do much to reduce concentrations of fine particles by controlling the precursor pollutants. Reductions in sulfur dioxide, the largest contributor to fine particle pollution in the East, have proven to be far less expensive than anticipated. The price of sulfur dioxide emissions allowances has fallen to $68 per ton, compared to the $300 to $750 per ton predicted when the Clean Air Act Amendments were passed.

Based on BREATH-TAKING: Premature Mortality Due to Particulate Air Pollution in 239 American Cities, a May 1996 report by the Natural Resources Defense Council. To view mortality data for individual cities and states, see the online version of the report.

last revised 5.7.96

Notes

* The 1996 findings are still significant. In July 2000, an independent analysis by the Health Effects Institute validated the methods and conclusions of the American Cancer Society study, on which the NRDC risk estimates are based. And a September 2002 National Academy of Sciences report endorsed the approach taken by NRDC in its "Breath-Taking" report, which used the risk rates to estimate air pollution-related mortality.