Cleaner air equates to a longer life

Researchers in the U.S. say reductions in fine particulate air pollution appears to have an affect on mortality.

The Harvard Six Cities study (1979 to 1990) revealed an association between levels of fine particulate matter pollution and mortality risk.

The new study, which was conducted from 1990 to 1998, reports on this later period of reduced air pollution concentration.

Although the earlier studies showed a direct link between death rates and air pollution it was never clear if improvements in particle exposure would actually lead to better survival.

According to a report by Dr. Francine Laden, from Harvard Medical School in Boston and her colleagues reductions appear to translate into a survival benefit on a population level.

Laden's team analyzed data during a period when air pollution was declining in many of the cities studied.

The study population consisted of 8,096 white participants living in Watertown, Massachusetts; Kingston and Harriman, Tennessee; St. Louis, Missouri; Steubenville, Ohio; Portage, Wyocena, and Pardeeville, Wisconsin; and Topeka, Kansas.

The average age of participants at the start of the original study was 50, with women comprising 55 percent of the cohort.

As was found in previous studies the overall mortality in those cities rose steadily with each increase in PM2.5 of 10 microgram per cubic meter.

As PM2.5 levels fell during follow-up, so did overall mortality.

Laden says the drop in mortality was particularly noticed in deaths due to cardiovascular and respiratory disease but not from lung cancer, which is a disease with a longer latency period and less reversibility.

Investigators found that long-term exposure to ambient PM2.5 was associated with increased mortality.

The researchers conclude the results suggest that increases in mortality related to PM2.5 are "at least in part reversible.

The results appear in the second issue for March 2006 of the American Journal of Respiratory and Critical Care Medicine, published by the American Thoracic Society.

What is PM2.5?

Particulate matter is the term used for a mixture of solid particles and liquid droplets found in the air. PM2.5 refers to particulate matter that is 2.5 micrometers or smaller in size. 2.5 micrometers is approximately 1/30 the size of a human hair; so small that several thousand of them could fit on the period at the end of this sentence. The sources of PM2.5 include fuel combustion from automobiles, power plants, wood burning, industrial processes, and diesel powered vehicles such as buses and trucks. These fine particles are also formed in the atmosphere when gases such as sulfur dioxide, nitrogen oxides, and volatile organic compounds (all of which are also products of fuel combustion) are transformed in the air by chemical reactions. Fine particles are of concern because they are risk to both human health and the environment.

How does PM2.5 affect human health?

Because these particles are so small they are able to penetrate to the deepest parts of the lungs. Scientific studies have suggested links between fine particulate matter and numerous health problems including asthma, bronchitis, acute and chronic respiratory symptoms such as shortness of breath and painful breathing, and premature deaths. Most of these premature deaths are the elderly who's immune systems are weaker due to age or other health problems such as cardiopulmonary diseases.

Children are more susceptible to the health risks of PM2.5 because their immune and respiratory systems are still developing. The average adult breaths 13,000 liters of air per day and children breath up to 50 percent more air per pound of body weight than adults. The breathing of fine particles by children is believed to cause both acute and chronic respiratory problems such as asthma. Forty percent of all asthma cases are children who make up only 25 percent of the population.