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AIR pollution may cause brain damage similar to Alzheimer's disease, as well as heart problems, two new studies suggest.

Dogs exposed to air pollution were found to develop damaged brain cell genes in as little as four weeks, according to research presented April 15 at the Experimental Biology 2003 conference in San Diego.

The animals were exposed to the highly polluted air in different parts of Mexico City, and compared against a control group of dogs kept in less-polluted rural parts of Mexico. Mexico City is considered one of the worst cities in the world for air pollution.

More than 200 dogs were involved in the study, which lasted for more than a year. The dogs in the highly polluted environment suffered lung and upper respiratory damage, which let particles enter the central nervous system, leading to gene and DNA damage in their brain cells. Even dogs less than 1 year old were found to have brain lesions similar to those of human Alzheimer's patients, the researchers say.

Lead researcher Dr. Lilian Calderon-Garciduenas, of the University of North Carolina, says exposure to air pollution causes inflammation in the respiratory tract, which lets tiny airborne particles and metals enter the central nervous system and brain. This, in turn, causes oxidative damage and DNA changes in brain cells.

Air pollution breaks down the vital blood-brain barrier that usually keeps toxic substances away from the brain, she says.

"This is extremely important," says Calderon-Garciduenas, "because once you break down the barriers, you have an entrance for pollutants directly to your brain."

The researchers also found signs of lung damage in children as young as 4 years old who were raised in Mexico City.

"The same breakdown in the respiratory system we're seeing in dogs is happening in children and adults in Mexico City," Calderon-Garciduenas contends, "and it probably also happens in cities like Los Angeles."

A separate study presented at the same symposium found a link between air pollution and heart problems in humans.

Exposure to air pollution raised levels of certain peptides in the bloodstream that can constrict blood vessels and decrease blood flow to the heart muscle, the researchers found.

The study was conducted at the Gage Institute of the University of Toronto, where healthy volunteers were exposed to air pollution in a laboratory setting. The volunteers were subjected to air pollution about two to three times the level normally found in Toronto, which is considered one of North America's less-polluted major cities.

The study focused on endothelin, a naturally occurring peptide that plays an important role in blood vessel health.

"If we expose healthy humans to airborne particulates, we can document a doubling of endothelin in the blood," says Renaud Vincent, one of the researchers and head of Health Canada's Inhalation Toxicology and Aerobiology Section.

"We now have at least one mechanism that could plausibly explain how someone with a heart condition exposed to a low level of air pollution could die or come down with severe symptoms, such as congestive heart failure," Vincent says.

Recent epidemiological studies have found higher rates of death and hospitalization in cities with high levels of air pollution. Vincent says the culprit appears to be airborne particulates.

When test subjects breathed polluted air for as little as two hours, the level of vasoconstrictive peptides in their blood rose sharply and stayed at abnormally high levels for as long as 24 hours, even without further exposure. The changes in peptide levels were proportionate to the concentration of particles to which the subjects were exposed.

"The picture is starting to come together of why we see these spikes in mortality associated with air pollution levels," says Fred Miller, a researcher with CIIT Centers for Health Research, an independent, non-profit research organization based in North Carolina's Research Triangle Park.

"The mortality may be coming about because you have this exposure, and how well can your system handle this added stressor?" he says.

Elevated levels of endothelin can reduce blood flow by as much as 50 percent, particularly in people with atherosclerosis, high blood pressure and diabetes, Vincent says.

Further study needs to be done on which specific particulates and their components produce the rise in vasoconstrictive peptide levels, he says.