The Environmental Protection Agency’s analysis of soil near Louisville’s former Black Leaf Chemical Plant continues, but the agency still isn’t sure how much of the contamination of nearby land—including some private lawns—can be blamed on the plant. And now, an agency spokesman says some of the preliminary results suggest one of the most prevalent chemicals found might not have come from the facility at all.

The Black Leaf site is on 29 acres in the middle of Louisville’s Park Hill neighborhood. Up until the 1970s, it was home to numerous pesticide manufacturing companies, as well as a saw mill and a whiskey barrel manufacturer. In 2010, the EPA discovered high levels of heavy metals and chemicals on the site. Last year, the agency tested nearby homes to see whether the contamination had spread. Sixty-nine homes were tested, and all were found to have some level of contamination.

But here’s where it gets more complicated. A number of contaminants were found in properties near Black Leaf. There were heavy metals like lead and arsenic. There were traces of the pesticides the plant had manufactured, like dieldrin and DDT. And there were chemicals known as polycyclic aromatic hydrocarbons, or PAHs.

A number of PAHs are classified as probable human carcinogens. One of those is benzo(a)pyrene, which was found at some level in all of the 69 homes tested. The EPA’s Regional Screening Level for the chemical is 15 parts per billion; all of the homes tested exceeded this level. The next threshold the agency uses is called the Removal Management Level, which is much higher (1500 parts per billion). Only two of the homes tested exceeded that level for benzo(a)pyrene.

So clearly, benzo(a)pyrene on properties near Black Leaf is a concern to residents. At a community meeting in June, many were worried about years of eating food grown in backyard gardens and children who grew up playing in the yards. But what’s not clear is whether the benzo(a)pyrene levels that have been found in those 69 yards is actually significantly higher than levels in other areas of Louisville. Maybe the chemical should be a concern for all Louisville residents.

“The challenge at Black Leaf has been, yes, there’s been benzo(a)pyrene in every yard that we’ve sampled,” said EPA On-Scene Coordinator Art Smith. “The reality is, that it may be there due to influences other than due to a release from the Black Leaf site, in which case it’s problematic and may make it difficult to act on that.”

Benzo(a)pyrene comes from a lot of places. It’s found in emissions from wood burning and coal-fired power plants, vehicle exhaust, cigarette smoke and charbroiled food. It’s dangerous, and a probable human carcinogen. And it’s one of those remnants from our industrialized society that has made its way into the soil through air deposition and groundwater runoff, and now is ubiquitous in many places.

A 2002 paper from the Massachusetts Department of Environmental Protection tried to calculate the background levels for various PAHs in the state’s soil. Regulators estimated that the background level for benzo(a)pyrene in “natural soil” is 2000 ppb. The specific data from the soil testing on Black Leaf neighbors hasn’t been released yet, but the Massachusetts background level exceeds the EPA’s standard for remediation, and is higher than possibly all but two of the homes tested in Louisville.

There hasn’t been any study of background levels of benzo(a)pyrene in Kentucky, or in Louisville. Kentucky Division of Waste Management Assistant Director Tim Hubbard says regulators found PAHs—like benzo(a)pyrene at Black Leaf, so that’s why they sampled the residential yards for them. But that’s not something that would be routinely sampled.

“Had we done that before, we probably would have found similar concentrations that we’re finding now,” Hubbard said.

After testing the Black Leaf site itself—where they found benzo(a)pyrene levels of about 12000 parts per billion—regulators calculated an estimated background level for the chemical at the site. That level was 200 parts per billion. Because the data from the residential testing hasn’t been released in its entirety it’s impossible to know for sure, but there’s a possibility that only 2 of the 69 sites sampled were higher than this background level.

There’s also the issue of where the higher levels of benzo(a)pyrene were found. Art Smith says if the contamination came from Black Leaf, the levels would be higher in back yards than front yards, because the homes all back up to the plant’s property. But that doesn’t seem to be the case.

“We’re in the process of putting all of the data together, but we have seen examples of where benzo(a)pyrene and lead in front yards are at higher concentrations than they are in the back yards,” Smith said.

That means the chemical could have come from vehicle exhaust, or from coal piles that fueled the homes decades ago.

There’s no evidence that benzo(a)pyrene levels are higher in the homes that were tested than in any other home in the city. “Nobody wants to hear that ‘yeah, [benzo(a)pyrene] is there, and it’s above a risk level, but it’s okay because it’s everywhere else,” Hubbard said. “That’s a difficult statement to make.”

Regardless of where the benzo(a)pyrene in yards near Black Leaf came from, it’s still a concern. And there were other chemicals and heavy metals found, too. But the issue could be much more widespread than just the 69 homes near Black Leaf, and could be more an indication of the overall effect humans have had on the planet than just the toxic legacy of one pesticide company.