If you talk to its proponents, and there are lots of them, sewage sludge fertilizer is a great way to divert human waste from landfills and to grow crops, despite the unappealing picture it may conjure. The US Environmental Protection Agency has a nicer name for the muck that’s left over after processing our shit—“biosolids”—and has encouraged its widespread use as a cheap, effective way to fertilize crops and recycle human waste. But while the EPA requires that bacteria and viruses are killed off before sludge is applied to farm fields, other contaminants, like pharmaceuticals and metals, are only minimally regulated, if at all.

New research suggests this could be a problem, as contaminants are now showing up in treated sludge—and, in lower levels, even in some animals that have fed off the plants it fertilizes.

“I don’t think the present rules are even remotely adequate,” Murray McBride, a soil contaminant researcher at Cornell University, told me. “There are a lot more toxic metals on the periodic table [and potentially in the soil] than what they decided to regulate.”

According to him, the EPA’s rules are outdated. They regulate only nine metals with known health risks—including lead, cadmium and arsenic. And metals are just the beginning. Pharmaceuticals and other organic chemicals found in biosolids are cause for even greater concern, he said. Others agree. “If you look at what [could be] potentially regulated by EPA, it’s just a tiny fraction of the universe of the chemicals we live in,” David L. Lewis, a former EPA scientist who is now a fierce critic of the agency, told me.

“What the EPA regulates is negligible.”

EPA officials haven’t said whether the rules will be revised, but agency spokesperson Robert Daguillard noted in an email that the agency plans to assess the risks posed by pharmaceuticals in sludge. While they haven’t yet determined these risks, they do know what’s in it. An EPA sludge survey, Daguilllard said, includes “92 pharmaceuticals, steroids, and hormones.” But none of those are actually subject to enforceable limits under current rules.

For many farmers, biosolids are a cheaper alternative to synthetic fertilizers. Municipal governments like them, too: since some cities divert as much as 50 percent of sludge to farms, they can reduce the amount of waste they have to pay to landfill. San Diego, Portland and Edmonton, among many others, process their waste to produce biosolids.

It makes sense, then, that demand has been increasing. While the EPA hasn’t recently estimated the size of the market, it put biosolids production at 7.2 million tons in 2004, up four percent from six years earlier. If that growth rate has continued roughly apace, production would be near 8 million tons today. Organic farmers can’t use biosolids as fertilizer without risking their organic certification, but it’s relatively easy for others to get, often via local organizations.

Biosolids boosters say the fertilizer is effective. “If there are any small negative impacts [from contaminants], they are overwhelmed by the positive effects” on crop yields, said Ned Beecher of Northeast Biosolids and Residuals Association, which promotes its use. He believes that the EPA rules are adequate as-is, noting that the agency has already considered the risks posed by dozens of sludge contaminants, and judged them minimal. “Just because there is no limit set, doesn’t mean risk assessment hasn’t been done,” Beecher told me.