Companies with a financial interest in a weed-killer sometimes found in drinking water paid for thousands of studies federal regulators are using to assess the herbicide’s health risks, records of the U.S. Environmental Protection Agency show. Many of these industry-funded studies, which largely support atrazine’s safety, have never been published or subjected to an independent scientific peer review.

Meanwhile, some independent studies documenting potentially harmful effects on animals and humans are not included in the body of research the EPA deems relevant to its safety review, the Huffington Post Investigative Fund has found. These studies include many that have been published in respected scientific journals.

Even so, the EPA says that it would be “very difficult for someone to put a thumb on the scale” to slant the outcome.

Atrazine is one of the most widely used herbicides in the U.S. An estimated 76 million pounds of the chemical are sprayed on corn and other fields in the U.S. each year, sometimes ending up in rivers, streams, and drinking water supplies. It has been the focus of intense scientific debate over its potential to cause cancer, birth defects, and hormonal and reproductive problems. As the Huffington Post Investigative Fund reported in a series of articles last fall, the EPA failed to warn the public that the weed-killer had been found at levels above federal safety limits in drinking water in at least four states. Some water utilities are suing Syngenta to have it pay their costs of filtering the chemical.

Now the EPA is re-evaluating the health risks of atrazine, which was banned in the European Union in 2004 due to a lack of evidence to support its safe use. That ban includes Switzerland, where atrazine’s manufacturer, Syngenta, is headquartered. The EPA expects to announce results of its re-examination of the herbicide in September 2010. It could take action ranging from restrictions on its use on crops to an outright ban. Or it could permit continued use without additional restrictions.