Women who drink water contaminated with low levels of the weed-killer atrazine may be more likely to have irregular menstrual cycles and low estrogen levels, scientists concluded in a new study.

The most widely used herbicide in the United States, atrazine is frequently detected in surface and ground water, particularly in agricultural areas of the Midwest. Approximately 75 percent of all U.S. cornfields are treated with atrazine each year.

The newest research, which compared women in Illinois to women in Vermont, adds to the growing scientific evidence linking atrazine to altered hormones.

The women from Illinois farm towns were nearly five times more likely to report irregular periods than the Vermont women, and more than six times as likely to go more than six weeks between periods. In addition, the Illinois women had significantly lower levels of estrogen during an important part of the menstrual cycle.

Tap water in the Illinois communities had double the concentration of atrazine in the Vermont communities’ water. Nevertheless, the water in both states was far below the federal drinking water standard currently enforced by the U.S. Environmental Protection Agency.

The amount of water consumed also seemed to make a difference: Women who said they drank more than two cups of Illinois tap water daily reported an even greater occurrence of irregular periods.

In recent years, some tests on lab animals have linked the herbicide to fertility issues, including altered hormone levels, delayed puberty and pregnancy loss.

Co-author Lori Cragin, an epidemiologist with Colorado State University at the time of the study, said the new findings fit with the results of the animal studies, as well as with some limited research that reported human effects.

In 2009, a study tied atrazine in drinking water to low birth weight in Indiana newborns. And in a study of more than 3,000 women enrolled in the Agricultural Health Study, those who described using atrazine and other pesticides had an increased risk of missed periods and bleeding between periods. The Agricultural Health Study is a nationwide project sponsored by the National Institutes of Health.

The manufacturer of atrazine says some unknown factor – not atrazine – might have caused the menstrual irregularities.

“Many things can cause changes to a woman’s menstrual cycle – stress, exercise, diet,” said Tim Pastoor, principle scientist for Syngenta, the Switzerland-based company that makes atrazine.

Pastoor noted that the company’s mice studies have not found reproductive effects, even at atrazine levels far greater than those found in the drinking water in the new study.

The researchers did not test the water for other contaminants.

“It is possible that the difference we found is due to pesticide exposure in general or another, unmeasured chemical in the drinking water,” said Cragin, who now is an epidemiologist at the Vermont Department of Public Health.

Cragin and her team, which included researchers from Colorado State University, Emory University and the Centers for Disease Control and Prevention, collected questionnaires from 102 premenopausal women in the Illinois farm towns of Mount Olive and Gillespie, and in the Vermont towns of Waterbury and Fair Haven, where atrazine is not used. The authors said they considered lifestyle factors, such as physical activity, weight and foods, and found no significant differences between the two groups of women.

The findings, which were published in the journal Environmental Research earlier this month, were based on municipal tap water tested between July and September of 2005.

Cragin was surprised to see a significant effect in the women whose water contained atrazine levels far below the EPA’s standard of 3 parts per billion.

In Illinois water, the average concentration of atrazine was 0.7 parts per billion, several times lower than the averages recorded in previous and subsequent summers. Cragin said drought conditions in 2005 would have slowed runoff from farm fields.

The researchers did not examine whether the menstrual and hormonal changes reduced the women’s ability to become pregnant. However, estrogen levels and menstrual cycle characteristics are known to affect fertility.

“These types of changes to hormone concentration and ovarian function could potentially lead to problems with fertility,” said Emily Barrett, a reproductive health scientist at the University of Rochester in New York.

Hormonal changes also have been associated with greater risk of certain diseases such as osteoporosis, diabetes, heart disease and some cancers.

Though scientists are not sure how atrazine would disrupt hormone levels, some studies suggest that the chemical may block the production of estrogen in the body.

The small number of women involved in the study could increase the possibility that the differences between the two communities are due to chance. However, “to find a profoundly higher incidence of reproductive irregularities in such a small group of people suggests that something is definitely going on here,” said Laura Vandenberg, a reproductive scientist at Tufts University.

First registered as an herbicide in 1958, atrazine is used primarily to eliminate weeds on land where crops including corn and sorghum grow, but it is sometimes used on lawns and golf courses too.

In 2003, the EPA reevaluated the safety of atrazine and determined that the current safety standard of 3 parts per billion is sufficient to protect against hormonal effects of atrazine.

At the time, the EPA mandated Syngenta to begin monitoring roughly 100 community water systems nationwide for levels of the chemical in drinking water. Environmental groups criticized the decision because they said it allowed the chemical company to oversee itself.

Since 2003, more than 150 new studies raising concerns about the potential health effects of atrazine have been published. The European Union has since banned it due to safety concerns.

Studies in frogs suggest that atrazine, even at low levels, may affect development of the male reproductive system, decreasing fertility and in some cases leading to hermaphroditic frogs.

“In frogs, atrazine disrupts the balance between what it means to develop and function as a male or a female,” said Tyrone Hayes, a scientist from the University of California, Berkeley who studies the reproductive effects of pesticides in frogs.

In 2009, the EPA ordered another review in light of new studies. The agency is currently awaiting the results of an evaluation by a scientific advisory panel.

In the meantime, use of the herbicide continues to rise. In the first half of 2011 alone, Syngenta reported double-digit growth in sales, with atrazine as a high performer.

Some environmental groups and scientists are frustrated with the pace of the regulatory process when it comes to evaluating chemical safety.

“We can’t continue to allow the use of this chemical when we are seeing adverse effects on animals and people,” said Vandenberg.

This article originally ran at Environmental Health News, a news source published by Environmental Health Sciences, a nonprofit media company.