Science News - February 27, 2010

Atrazine at the surface The U.S. Geological Survey is developing models that estimate pesticide
levels based on monitoring data and watershed characteristics. The map shows the highest 21-day moving
average modeled for atrazine concentrations in stream water for 2007. The tables reveal that atrazine and
deethylatrazine, a breakdown product, are among the most frequently detected pesticide compounds.
SOURCE: USGS WATERSHED REGRESSIONS FOR PES TICIDES ATRAZINE MODEL AT INFOTREK.ER.USGS.GOV/ WARP
Atrazine
Deethylatrazine Metolachlor Cyanazine Alachlor
Acetochlor
Simazine
Prometon
Tebuthiuron
2,4-D Diuron
Diazinon
Chlorpyrifos
Carbaryl
SOURCE: “PESTICIDES IN THE NATION’S STREAMS AND
GROUND WATER.” 1992–2001/USGS

summer showed a 3 percent higher rate of birth defects, such as spina bifida, cleft lip, urogenital defects and Down syndrome. And while a mix of farm pesticides usually showed up in that seasonal runoff into waterways, Winchester observes that “far and away, the most prevalent pesticide — and the one which exceeds safety limits most often — is atrazine.” The weed killer’s statistically significant association with birth defects is intriguing, he says, and deserves further exploration.

A related investigation linked atrazine concentrations in Indiana drinking-water supplies to the chance that a baby would be very small at birth. “Atrazine concentrations above 0.1 part per billion were associated with, on average, a 17 percent increase in the risk of having a small baby,” says Hugo Ochoa-Acuña of Purdue University in West Lafayette, Ind. Such contamination, well below the EPA’s 3-ppb limit, can be common. Biweekly drinking-water data suggested that in Fort Wayne, Ind., for example, atrazine concentrations exceed 0.2 ppb on 265 days a year, and exceed 0.5 ppb more than one out of every three days.

Each 1 ppb increase of atrazine in drinking water, for consumption averaged throughout a pregnancy, increased by 15 percent a woman’s chance of giving birth to a baby in the lowest 10 percent

of weight for its gestational age. Such babies have a poorer chance of survival.

The weed killer’s correlation with
low birth weight proved most robust for
contamination during a woman’s third
trimester, the Purdue team reports
in the October Environmental Health
Perspectives. “And that makes sense,”
Ochoa-Acuña explains, “because most
of a baby’s growth occurs during the last
trimester. So a small effect there would
produce a big difference.”
As Winchester’s group found, many
other pesticides tended to coexist with
atrazine in water, but their levels weren’t
nearly as high.

At his agency’s February Scientific Advisory Panel meeting, the EPA’s Aaron Niman reviewed these and additional new studies correlating the levels of atrazine in drinking water to birth defects and low birth weights. The strength of Winchester’s study, he said, “is that it provides for an overall snapshot of trends in both birth defects and atrazine levels in the environment. For this reason, it’s useful in hypothesis generation … [but] can only be used to demonstrate correlation” — not causation.

The study by Ochoa-Acuña’s team
is “probably the strongest” linking the
weed killer to birth outcomes, Niman
said, because it offers individual expo-
sure estimates and is able to adjust
for several potentially confounding
factors, such as seasonality. Still, Niman
acknowledged, there are limitations to
even this study. But he also noted that
this is to be expected when looking at
complex, real-world exposures — not the
controlled environments of lab rats.