Decades after the factories of Kensington and Northern Liberties began closing, the area's industrial legacy remains in plain sight. Last year's Toxic City investigation — in which Inquirer and Daily News reporters found lead-tainted soil in parks, playgrounds, and yards across the River Wards — made clear the hazards remain as well.

Now, new research reveals that those hidden threats extend far beyond that small corner of Philadelphia and that upward of 93 percent of hazardous industrial sites are not tracked by regulators.

Sociology professors Scott Frickel of Brown University and James R. Elliott of Rice University mined more than 50 years of business directories to build a database of potentially hazardous industrial sites and discovered that they are as thick as three per square block, on average, across Philadelphia — located in every historical census tract, but concentrated in many gentrifying neighborhoods.

In addition to more than a thousand sites officially designated for cleanup efforts, Philadelphia is home to 3,500 more or less uncharted industrial sites that were active at some point since the 1950s but were not regulated and have never been officially remediated. Many have since been replaced by businesses, homes, and even playgrounds, with little regard for what lies below.

Frickel and Elliott spoke with the Inquirer and Daily News about their database, their new book, Sites Unseen, published by the Russell Sage Foundation, and what's lurking beneath our city.

In Sites Unseen, you describe a collective amnesia about hazardous industrial sites. Why do we have such a poor memory about what's under our neighborhoods?

Frickel: One explanation is that we forget because industrial or manufacturing activities are constantly churning through time and space. These businesses open up for a while and close down, and something else comes to take their place — often within the span of a decade. The physical visible evidence of those sorts of facilities will disappear from the landscape. Another piece of our argument is that people who live in those neighborhoods disappear over time; they move, communities change, and individuals move in and out, so whatever place-based or cultural memory of those earlier facilities erodes. And the third part of the argument is that the vast majority of these kinds of activities simply don't get recorded by anybody in a regulatory position. That adds up to a lot of missing information and lost knowledge about the industrial past that cities inherit.

So, these hazards are basically everywhere in Philly?

Frickel: In Philly and the other cities we studied, there are real, intense former industrial areas that have very dense concentrations of these legacy sites.

But every city also has a complementary dynamic: Over time these things spread out spatially, and the vast majority are small or medium-scale facilities that are not obligated to report their hazardous releases. Those are precisely the kind of smaller facilities that also turn over more quickly and disappear form the landscape.

How concerned should we be about that legacy?

Elliott: The broader point is there's not a lot of testing. If there's no testing, we generally assume it's safe. And the part we want to emphasize is the EPA and other organizations — local departments of environmental quality — although well meaning, are often very under-resourced, so they can't go out and investigate all these sites. They typically investigate what we call the worst first, the places that are known to locals to be a cause of concern. We're trying to draw attention to the invisible landscape that is not subject to regulatory policies and practices.

Our intent is not to create a gloom-and-doom story but to reveal what's happening and to expose the systemic nature of it. What we're trying to do is open up a conversation about urban sustainability. To build more sustainable cities in the future, we have to look back and develop context-specific strategies — places to do more testing and look at whether some of this should be entombed by concrete or other barriers, making sure children or the elderly are not exposed.

You cite one risk analysis that found between 75 percent and 95 percent of these sites are likely contaminated. So if a site's in your database, does that mean it's a hazard?

Frickel: The analysis they did sheds a very stark light on the potential for danger coming from these hazardous facilities. A city like Philadelphia is going to have thousands of those kinds of sites — thousands of shops that are doing metal processing or building transportation equipment. I think the concern is real.

What's interesting to us is that is the only study of its kind we've been able to find. There is almost nobody doing the kind of work that's necessary to understand the extent to which these legacy sites present a real and present threat to urban populations. There's nothing in the regulations. This is not a story about an agency not following the letter of the law. The letter of the law doesn't require them to ask the kinds of questions we've posed in this book.

Is anyone doing anything about this? If not, what should be done?

Elliott: If we can encourage local governments and groups to think about how it is that this is a fundamental feature of our environment, hopefully we can  have a policy conversation. It happens, often, through private ownership — but the response is often not to divulge that information or to take action, but to encourage people to sign nondisclosures.

Frickel: In our book, we lay out a handful of ideas for using this work to make positive changes. One is to remind urban planners and environmentalists who are focused on building green cities that new technology and energy infrastructure is great, but we shouldn't forget there's a history to all cities and those histories come with hazardous legacies we need to pay attention to.