New York, Apr 14 (IANS): Children and young adults living in polluted megacities are at increased risk of developing Alzheimer's, a debilitating brain disease characterised by memory loss, a new study has warned.

"Alzheimer's disease hallmarks start in childhood in polluted environments, and we must implement effective preventative measures early," said one of the researchers Lilian Calderon-Garciduenas from University of Montana in the US.

"It is useless to take reactive actions decades later," Calderon-Garciduenas said.

The findings, published in the Journal of Environmental Research, indicate that Alzheimer's starts in early childhood, and the disease progression relates to age, pollution exposure and status of Apolipoprotein E (APOE 4), a well-known genetic risk factor for Alzheimer's.

The researchers studied 203 autopsies of Mexico City residents in the US ranging in age from 11 months to 40 years.

Metropolitan Mexico City is home to 24 million people exposed daily to concentrations of fine particulate matter and ozone above US Environmental Protection Agency standards.

The researchers tracked two abnormal proteins that indicate development of Alzheimer's, and they detected the early stages of the disease in babies less than a year old.

The scientists found heightened levels of the two abnormal proteins -- hyperphosphorylated tau and beta amyloid -- in the brains of young urbanites with lifetime exposures to fine-particulate-matter pollution (PM2.5).

They also tracked APOE 4 as well as lifetime cumulative exposure to unhealthy levels of PM2.5 -- particles which are at least 30 times smaller than the diameter of a human hair and frequently cause the haze over urban areas.

The researchers found hallmarks of the disease among 99.5 percent of the autopsies they examined in Mexico City.

In addition, the findings showed that APOE 4 carriers had a higher risk of rapid progression of Alzheimer's.

The researchers believe the detrimental effects are caused by tiny pollution particles that enter the brain through the nose, lungs and gastrointestinal tract, and these particles damage all barriers and travel everywhere in the body through the circulatory system.

The authors noted that ambient air pollution is a key modifiable risk for millions of people across the globe.

"Neuroprotection measures ought to start very early, including the prenatal period and childhood," Calderon-Garciduenas said.

"Defining pediatric environmental, nutritional, metabolic and genetic risk-factor interactions are key to preventing Alzheimer's disease," she added.