(CNN) Pollution affects us even in the womb: Women who are exposed to air pollution during pregnancy have babies with shorter telomeres (a genetic biomarker), a study published Monday in JAMA Pediatrics found.

The length of telomeres -- caps at the end of chromosomes, similar to shoelace tips -- is considered a marker of biological aging.

Based on their results, the researchers theorize that pre-birth exposure to air pollution may lead to negative health consequences later in life.

Vulnerable period

The researchers looked at 641 mother-newborn pairs, who were recruited between February 2010 and December 2014 to participate in the Belgium-based Environmental Influence on Ageing in Early Life study. This ongoing research project explores the interaction of human aging and environmental factors.

Only women who had delivered a single child at full-term (at or after 37 weeks of gestation) were selected as participants. To measure exposure to air pollution, the research team relied on readings from monitoring devices calibrated to estimate particulate matter (PM2.5) at the mother's address.

Particulate matter " is the term used for a mixture of solid particles and liquid droplets found in the air, while 2.5 refers to those inhalable particles with diameters of 2.5 micrometers or smaller. (PM2.5 is about 30 times smaller than the average diameter of a human hair, which is about 70 micrometers.)

To measure telomere length, the researchers extracted DNA from each baby's cord blood and placental tissue.

The researchers found that mothers with higher residential exposure to PM2.5 had newborns with significantly lower telomere length -- and that this could not be explained by other factors including the mother's body mass index, ethnicity or smoking status.

Each increase of 5 micrograms per cubic meter in exposure was associated with 9% shorter cord blood telomeres and 13% shorter placental telomeres, the researchers estimated. The second trimester was a particularly vulnerable period, the researchers found.

The researchers theorize that exposure to particulate matter generates more reactive oxygen species in utero, which in turn increases the rate of shortening. Reactive oxygen species , or free radicals, are a type of unstable molecule that contains oxygen and easily reacts with other molecules. Within cells, a buildup of these free radicals may cause damage to DNA, RNA and proteins, which could eventually lead to cell death and so too cardiovascular disease.

As a result of their work, the researchers suggest that a reduction of environmental fine-particle air pollution levels may promote longevity.

However, we should be careful when interpreting these conclusions, some experts warn.

'Exercise caution'

Further studies are essential, according to an accompanying editorial by Pam Factor-Litvak of the Mailman School of Public Health at Columbia University, Dr. Ezra Susser of the New York Psychiatric Institute and Dr. Abraham Aviv of New Jersey Medical School at Rutgers.

They note that the researchers have a "sound basis" for proposing that shorter relative telomere length in cord blood might have ramifications for adult cardiovascular disease risk and longevity.

"One should exercise caution, however, about interpreting this finding in terms of a broader hypothesis about telomere length and the aging process," the editorial authors wrote. Telomere length is mostly inherited from our parents, they explain.

Still, this does not rule out that environmental exposures might impact telomere length in utero and ultimately at birth. It may be that the most intense phase of telomere length growth is during this period of development.

Jan Karlseder, director of the Paul F. Glenn Center for Biology of Aging Research at the Salk Institute, said one strength of the new study is the wealth of data. Karlseder was not involved in the research.

However, he said the study has some flaws, including the fact that no direct cause-and-effect relationship between pollution and telomere can be established.

"Telomere length can be influenced by many parameters, among them stress," Karlseder said. "Just to make up a possible scenario, it could be that mothers that live in more polluted areas are subjected to higher stress, which then leads to slightly shorter telomeres."

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Another issue is that the method the researchers used to measure the telomeres provides no information about their integrity.

"Telomere length correlates with aging," Karlseder said, though "telomere integrity might be a more important characteristic."

Professor Michael Davies, a reproductive epidemiologist at the Robinson Research Institute at the University of Adelaide, believes the study raises "important questions" for continued research. He added the new work also enlarges the range of known negative health outcomes "associated with exposure to very small sized particulate air pollution."