PAHs/Coal Tar Sealants Effects on Environment & Human Health

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Webinar on USGS research findings on Polycyclic Aromatic Hydrocarbons (PAH) contamination associated with coal-tar-based sealants and implications for environmental and human health. Dr. Spencer Williams explains how these sealants are an important source of PAH exposure for children in affected environments, and may create excess cancer risk for persons who live in these areas.


Date Taken:

Length: 00:38:00

Location Taken: Reston, VA, US


Barbara Mahler:  The USGS, we're the sole earth science agency for the US Department of the Interior. The USGS is tasked with providing impartial information on the health of our ecosystems and our environment, among other things. One of our many goals is to communicate that information to other federal and state agencies.
We're not a regulatory agency. I'm not advocating for any particular public policy. My goal today is to share with you what we've learning in our about 10 years of research on this subject.
How did the USCS get involved in PAH and pavement sealants? It might seem like a bit of a stretch, but, it was a very natural outgrowth of the work that we've been doing with the National Water Quality Assessment program evaluating contaminant trends in time using lake sediment cores. So, using lake sediment cores to go back in time to look contaminant histories in watersheds.
One of our principle goals is to identify trends, and then explain why those trends are occurring. Some of the trends that we've identify are things like downward trends, across the United States, in lakes for contaminants like DDT and PCBs that were banned. We see a lot of...each one of these symbols is a lake that we've cored, and the downward arrows indicate a statistically significant downward trend.
A surprise to us was the upward trends that we saw in PAHs, polycyclic aromatic hydrocarbons. These were primarily in urban lakes across the United States. We were very curious to find out why PAHs were increasing in concentration.
Coincidentally and simultaneously with the research that we were doing on lake sediments and PAHs...I'm located at Texas Water Science Center here in Austin. The city of Austin was using a grant from EPA to collect and analyze sediments from small urban streams and drainages.
These were not in heavy industrial or innercity areas. These were in areas of light commercial, multifamily, and singlefamily residential housing. They were analyzing a very suite of contaminants, and they saw something that was very surprising when it came to PAHs.
They were measuring concentrations in the thousands of PPM in some of these areas. This was a real eyeopener, because these concentrations are on par with what are typically measured in soils at Superfund sites. They're not at all what we would expect to be seeing in residential areas.
To put these into context, the probable expect concentration, in other words the concentration at which we would expect to see adverse effects on lentic biota is only 23 parts per million. So these were extremely elevated concentrations.
They shared this data with us because we have a number of collaborate, cooperative programs with the city. It was a real eyeopener for us, at first we thought that the laboratory had put a decimal point in the wrong place.
It turned out that they weren't. A very astute staff member with the City of Austin was walking watersheds and he walked upstream and he noticed that the most contaminated sediments were just below parking lot drainages.
The parking lots were coated with a black substance, and that substance is coaltarbased sealcoat. Sealcoat is a consumer product that is marketed as enhancing the appearance of asphalt pavement, and preserving or extended the longevity of the underlying asphalt.
There are two brands, excuse me, two formulations of pavement sealer. There is one with an asphalt base which is used primarily west of the continental divide, and there's one with a coaltar base which is used primarily east of the continental divide.
These products are sprayed or painted on. They can be applied by homeowners or property owners, or you can hire someone to do it. It is not an intrinsic part of the pavement process.
It is an optional product that can be used after paving. It's not used on roads, it's primarily used on parking lots, on driveways, and even on some playgrounds and sidewalks.
This turns out to be a concern because the coaltarbased pavement sealants contain anywhere from 15 to 35 percent coal tar pitch, crude coal tar, one or the other, either coal tar pitch or now some of them are starting to use crude coal tar.
Both crude coal tar and coal tar pitch are known human carcinogens. These have the potential to be very potent sources of PAHs to the urban and residential environments. Coal tar and coal tar pitch are a concern because among other compounds they contain very high concentrations of polycyclic aromatic hydrocarbons, or PAHs.
PAHs are compounds that have the benzene ring as a building block, and as we combine different numbers of benzene rings in different geometric configurations each one of this is a PAH. These can be modified by adding additional constituents like nitrogen or sulfur to one of the carbons and create heterocyclic compounds that also have important environmental and health considerations. Seven of the PAHs are probable human carcinogens, and that number is growing as the EPA is modifying what they have probable cause to believe are human carcinogens.
There are a very large number of sources of PAHs in the urban environment. PAHs are formed whenever we burn or combust organic matter. When we burn a cigarette, or we char meat, or we heat up motor oil in our cars, we're creating PAHs.
Similarly, products that involve organic matter combustion like tires also contain PAHs. We're going to add to this list of urban PAH sources then, the coaltarbased pavement sealants.
The fact that there are so many urban sources has been one of the reasons that for a long time it's been very difficult to determine which of these sources are the most important in the environment.
One thing that's very helpful is to put the concentrations of PAHs in these sources into some context. Here we're looking at concentrations of PAHs in urban sources. You can see that while asphalt does contain PAHs it has relatively low concentrations.
That's why when the City of Austin was looking around to determine where these PAH concentrations were coming from, we were discarding the possibility that they were coming from either fresh or weathered asphalt surfaces because these concentrations are far lower than the 1,500 milligrams per kilogram which were measured in the sediment.
In fact, even used motor oil which is one of the major sources of PAHs in urban environments, if you mix used motor oil with some of the sediments in Austin, you'd be cleaning it up.
If we take a look at the two different formulations of sealcoat products, the asphaltbased product, like asphalt itself, has relatively low concentrations, but the coaltarbased product has very high concentrations.
These can range anywhere from 50,000 parts per million all the way up into the hundreds of parts per million, depending on the manufacturer and the particular brand of sealcoat that's in question. To put this into context, a bucket of coal tar sealcoat off the shelf contains about 100 times more PAHs than a similar volume of used motor oil.
These products are used extensively in primarily in the Eastern United States, I mentioned east of the continental divide, so that's the Great Lakes region, the North East, the South East, the Mid West, and here in Texas.
That rule is not hard and fast, there is some use of the coaltarbased products in the West, and there is some use of the asphaltbased products in the East. We first heard about this geographic difference as anecdotal information from applicators and our investigation since then, that information has held up.
About 85 million gallons of coaltarbased sealcoat are used every year in the United States. Enough to cover about 170 square miles. Many applicators recommend that sealcoat be reapplied everywhere from two to every five years, and many home owners prefer to reapply to their driveways on a yearly basis.
If you live anywhere in the United States and you look around at parking lots and driveways, you will see sealcoat. It's used on commercial properties, it's used as schools, it's used at churches, it's used in shopping centers, universities, it's a very commonly used product in the United States.
The issue is that the PAHs and the sealcoats themselves don't stay where they're put. After sealcoat is applied it makes a black shiny surface, and it makes the pavement look like new.
After a few months to years of abrasive action from car tires, and in many parts of the country snow plows, it doesn't take long to start seeing some of the underlying asphalt showing through as the product abrades and is removed. Ultimately after a few years, an asphalt parking lot or a private drive will start to look something like this.
If we go out and we sweep up some of the dust that's on these sealed surfaces, you'll see all these little black bits. These are little bits of sealcoat. They are loose, they are mobile, they can be blown by wind and inhaled, they can be washed off by storm water runoff, they can stick to our skin, they can stick to our shoes and be tracked to other locations.
There's lots of places that these little particles can go. Part of our research has focused on identifying the effects of coal tar sealcoat on all of these different environmental compartments to which these particles can be transported.
Some PAHs also are volatile, meaning they can evaporate into air. Some of our work also has focused determining how important coal tar sealcoats is as a contributor to PAHs to air. We're going to walk through some of these compartments. We're going to start with the dust itself. As part of our work for the National Water Quality Assessment Program we go around to lakes across the United States and we collect sediment cores. In the course of that work we also swept parking lots.
I love my job, we get to get out and sweep up parking lots all over the United States. We measured the concentrations of PAHs in that parking lot dust. What you're seeing here confirms what we had heard from the industry about the use of the low PAH asphalt products used in the West, and the use of the high PAH coal tar products used in the East.
These concentrations are very consistent with what was measured in some of those stream beds in Austin, Texas, suggesting that some of those very small stream beds are getting almost completely undiluted runoff from some of these parking lots. They are similar to what we see at Superfund sites.
We also swept up dust from unsealed parking lots, and we saw a very different story. We saw much lower concentrations in PAHs, even in the East.
These parking lots that we swept are all in the same watersheds, they're in the same airsheds, and so the difference between these numbers are pointing out that these high concentrations really are the result of the sealcoat, because these unsealed parking lots are getting all the same other urban PAH sources like car tire fragments, and dripping motor oil, and even atmospheric emission.
The mobile particle then can get washed down storm drains and storm drains end up in streams which ultimately flow to lakes.
One of the big questions that we were interested in as part of the NWQA program, is are these products contributing to the upward trends that we're seeing in PAHs in urban lakes.
The approach that we used is called an environment forensic approach. In other words, we used the profiles, the PAH profiles or fingerprints, of different PAH urban sources and we compared those to the PAH fingerprint in the sediment samples that we were collecting.
What do I mean by PAH fingerprint? There's a whole range of different sizes and shapes of PAHs, and different sources contain different proportions of those PAHs. We used a statistical model that was developed by USEPA that says, "What is the optimal way that I can combine different PAH sources and best replicate the PAH profile or the PAH fingerprint that we see in the sediments themselves?"
When we do that, we tested 22 different urban PAH sources and we divided those into five large categories. Sealcoat products, vehiclerelated PAH sources, coal burning, oil burning, and wood burning.
What we're looking at here is urban lakes that we sampled across the United States, this is the PAH concentration on the Yaxis, the total PAH concentration, and the different colors of the bars are telling us a portioning the PAHs to one of these different five PAH source groups.
What it's showing us is that on the basis of the PAH fingerprints, we're estimating that overall about 50 percent of the PAHs in the urban lakes that we've sampled are coming from coaltarbased sealcoat. Furthermore, if we look at the new urban lakes where we see the most pronounced upward trends in PAHs, overwhelmingly those PAHs are coming from coaltarbased sealcoats.
I want to draw your attention here to the probable effect concentration.
We saw this a little earlier, 23 milligrams per kilogram, this is the concentration at which we would expect to see adverse effects on biota, and many of these lakes have concentrations of PAHs that exceed that PEC.
There has been some work since we've first identified this as a source, there has been some research looking specifically at effects of PAHs associated with coaltarbased sealcoat on different organisms and on ecological communities.
I'm sure you're aware that there's a vast amount of research out there on PAHs in general, or individual PAHs on a wide variety of test organisms. These research projects looked specifically at PAHs from coaltarbased sealcoat.
They saw chronic and sublethal effects on a couple of different amphibians that were tested, they also saw measurable effects on ecological communities changing the number and variety of species that were present.
Very recently, a paper has come out looking at DNA damage on Japanese Medaka associated with runoff from coal tar sealed parking lots.
The next question then is, "Well, what about human health exposure"? Because there clearly are a lot of ways that humans can come into contact with the dust on these surfaces whether they are coming into contact during play activity, or whether they're breathing the fumes that are being volatilized from the surfaces, or whether they are coming into contact with the PAHs where it contaminates soil or house dust.
In fact, one of the interests that we had in determining are PAHs from parking lots and driveways as they abrade and adhere to the bottoms of our shoes, are they being tracked into homes in the same way that the other contaminants are.
We did a study in 2010, we looked at 23 apartments in Austin, Texas. 11 of those had coaltarbased sealcoat on the adjacent parking lot, these were all ground floor apartments, and 12 of these either had asphaltbased sealcoats or unsealed asphalt.
We measured the PAHs concentrations both in the dust on the parking lot and in the apartments themselves. For the parking lots we saw very similar concentrations to what we'd measured previously in other parts of the country. Concentrations, median concentrations, were in the thousands on the parking lot.
We also saw higher concentrations in the house dust. The concentration of PAHs in this were about 25 times higher. This is the sum of the carcinogenic, the B2PAHs, and the sum was about 25 times higher in those apartments that had coaltarbased sealcoat.
Dr. Williams has looked at a couple of studies using this data along with data for PAHs in affected soils, and looked at some implications for human health and that's what he'll be talking about in just a few minutes.
Another pathway for human exposure of course is air, it's inhalation. We have been investigating also volatilization of PAHs from coal tar sealed parking lots.
The way that we've done this, we've used an approach that was developed by Environment Canada, using something called a "hat sampler," apparently people in Canada think this is what a sombrero looks like, so they call this a hat sampler.
Essentially the approach is to use a puff up here at about 1.25 meters above the surface, and compare concentrations between what's in the puff and what is collected in a puff that is underneath the hat sampler.
We use the gradient in concentrations and other environmental factors to estimate a flux. In other words, a mass of PAHs leaving a given surface area per unit time. The first work that we did was looking at parking lots that had been sealed anywhere from months to years earlier. These are parking lots that have been in use, have been sealed for quite a while, have been exposed to sun, and wind, and rain, and all those environmental factors.
We compared unsealed asphalt to coal tar sealed lots. When we compare the flux between the air over the unsealed parking lot and the flux coming off the coal tar sealed parking lot, there's about a factor of 60 difference, even years after application.
It appears that coal tar sealed pavement continues to be a source of PAH to the atmosphere years after application.
Once you remember this number, because the next study that we did investigated PAH fluxes from sealcoat directly after application and how those fluxes change over time following application.
We had two test plots sealed in Austin, Texas. We compared the concentrations of PAHs volatilizing off a coal tar sealed lot, and compared that to what was volatilizing off an asphalt sealed lot.
We measured the concentration starting pretty much as soon as we could get out on the pavement, within hours of application. The applicator went nuts, we weren't supposed to walk on their nice wet sealcoat. We went out there anyway, and we continued to measure for first starting with hours, and then days, and weeks following application.
Remember this 88 number from the coal tar seal coated parking lots that were sealed a few years earlier, that concentration is way out here. If we take a look at the flux from seal coated parking lots during about the first two weeks after application, we have concentrations that are in the tens of thousands of micrograms per square meter every hour.
Looking at that more closely, so here's the first two weeks following application, and we have a diurnal cycle it's hotter during the day so we have more volatilization and it's a little less hot at night. Remember this is Texas in the summer, so it's pretty hot out there.
We compute the area under the curve and we come up with a total PAH loss over the two weeks after application of about 2.5 grams per square meter of sealed lot. If we put that into context, we're going to do a little calculation here for national PAH emissions. Just during the first two weeks after application.
We have our numbers for annual sealcoat use and area covered, and our emission rates that we've measured of 2.5 grams per square meter, and if we multiply that through, we find that PAH emissions on an annual basis are about a thousand mega grams, or a thousand metric tons, which is on par or exceeding estimated vehicle emissions. It turns out that this is a potentially important source of PAHs to urban air.
I'm going to circle back now before I pass the microphone over to Spencer, and go back to our original question, because we've just had some research published this week that I want to share with you.
The question is, "Are the PAHs that are ending up in those storm drains, are they really like our models and our forensic evidence says, an important source of PAHs to lake sediments?"
To investigate this, we had the opportunity to do a study in Austin, Texas, which was the first jurisdiction to ban use of coaltarbased sealants. They banned them back in 2006.
Prior to the ban, back in 1998, we had collected a sediment core from Town Lake. It was one of those urban lakes where we saw upward trends in PAHs. We went back to Town Lake which has been changed to be called Lady Bird Lake now, we went back to Lady Bird Lake in 2012 and 2014, and we collected more sediment cores from the downstream end of the reservoir here.
We also collected some bed sediment samples which we could compare to bed sediment samples that had been collected back in 2000 before the ban. Here are the trends from the sediment core we collected in 1988.
I apologize, this graph is from a publication and it's showing micrograms per kilogram, so actually parts per trillion not parts per million. Divide by a thousand to get backs to parts per million like we've been talking about. They range from two up to about ten ppm.
Here's the trend in the lake core that we collected in 98. We have a very statistically significant upward trend, an increase of about 20 times in the 40 years of deposition. Here are the results from the lake sediment cores that we collected in 2010, excuse me, 2012 and 2014, following the 2006 ban on coaltarbased sealcoats. Here's our bed sediment before and after the ban.
If we compare the average concentration in the years directly preceding the ban with those from 2012 and 2014, we're seeing almost a 60 percent decrease in PAH concentrations following the ban. That was published this week in ES&T, that's something I wanted to share with you. If we use our source apportionment model on those sediments collected just before the ban, it was telling us that most of the PAHs were coming from the coaltarbased sealcoat dust.
We did the same thing for the sediments collected recently. We see a decrease in concentration but we still see that most of the PAHs are coming from sealcoat as those existing stocks on parking lots and in stream bed sediments continue to be depleted, we expect to see that PAH concentration in Lady Bird Lake should continue to decrease.
If you'd like more information about the USGS research, you can find it on this Web page here, or you are welcome to contact either me or Dr. Van Meter, we've coauthored a lot of papers.
There have been quite a few publications now by researchers with other government agencies, and also researchers with academia. All of this independent research has come to similar and consistent conclusions about the importance of coaltarbased sealcoat as a PAH source to the urban environment.
There are some publications that have come out recently that are by consultants funded by the PCTC which is the lobby group, which lobbies for the coal tar sealcoat industry, and they are disagreeing with our results. If anyone would like access to any of these documents, please let me know and I would be happy to share them with you.
Spencer Williams:  My name is Spencer Williams, I'm at Baylor University. Prior to being at Baylor University I was a consultant, I worked for a small firm called ChemRisk, I was located in Houston. In the course of my work there I've worked for a long time with PAHs, and dioxins, and a number of organic pollutants. I happened to come across a publication from Barbara and Peter on sealcoat,
I thought it was very interesting, and I continued to follow their work. Then a few years later I had an opportunity to come here at Baylor and do some scientific research, teaching students, and that very fall I just happened to go into a room at the Society for Environmental Toxicology and Chemistry annual conference, and who was standing up there giving a talk but Barbara.
After the meeting, I walked right up to her and I said, "I've been following your work, I'm very interested, do you guys have time to chat?" We started chatting about it, I told them I was a human health risk assessor.
They said, "How coincidental, we've been looking for someone just like you." Since then we've been having what I think is a really interesting and productive conversation, and one that I hope is going to carry us further forward. In my previous life as a consultant, I became interested in contaminants in house dust, I think that's first how I came across the work.
For those of you that don't know, house dust is a very important medium for environmental contact. It's approximated that most people in the United States spend about 90 percent of their time indoors.
Anybody who's worked on environmental lead, one of the great success stories for public health over the last 40 years is aware that dust is probably the most important way in which children come into contact with lead. Probably the most, soil is a close second.
The relationship between these two media is very complicated.
Barbara and Peter and I started talking about this, and we started talking about it as, their interest was primarily environmental chemistry and environmental forensics, mine being human health and also ecological health.
I was very curious, starting off with, what these might look like in terms of how we know people are exposed to PAHs. Real quick, I'll give a quick primer on that.
Barbara told you quite a bit about PAHs. They are ubiquitous; you guys all had them for breakfast this morning if you had just about any version of foodstuff that you can get. They would be in your coffee, they would be in about everything you eat. If you go and have a hamburger, if you go and have barbecue, you get a dose of PAHs, for certain.
One of the things that we have been studying over a long period of time is how people come into contact with these, by and large. Most studies have demonstrated over time, it appears that our primary route of exposure for the general public to PAHs is through their diet, as I said.
This has been pretty well characterized through a number of authors; Charlie Menzie, the incoming President of SETAC has done some work on this.
In particular, we became very interested in a couple of very robust studies that were done in North Carolina on children. How they did the study was, they had the parents prepare duplicate meals.
The children would eat one meal, and then the other meal would be taken back to the lab for analysis, so they were able to approximate  a pretty good approximation has to be  the PAH content of these children's diet. We wanted to look at that as a point of comparison.
We also have some pretty good ideas about how much dust children tend to ingest on a daily basis. For those of you who have kids, you know that this is how children are learning about their environment.
They move around and they stick their hands on everything, and then they stick their hands in their mouth, and they stick their mouth on window sills and everything, trying to come to grips with their environment. That brings them into contact with dust.
This is certainly the case for dust that's contaminated with leadbased paint, so that's an important one.
The same sort of exposure would be expected for the dust that Peter and Barbara were seeing in people's homes, so we decided to start off with the simplest way of looking at how much dust we think children ingest, on average, and then how much the reasonable maximum might be  the 95th percentile  what do we think children are coming into contact with?
There have been a number of studies done on this. It's a very complicated question to answer because if you're trying to divide soil and dust, there's a lot of similarity. In some settings, most of the dust will essentially be trackedin soil. In other settings, that's just not the case.
Starting with what we knew...there was a recent study done by a group  I was going to ask, you guys may know these authors  using the Consolidated Human Activity Database to generate a couple of estimates of dust from handtomouth and objecttomouth behavior.
Go on ahead, Barbara.
You can see here, we started on the left by showing what your exposure to B2 PAHs, so these are the ones that are listed as B2, or "probable human carcinogens," and what that would look like for your dietary ingestion. Those come from the studies in North Carolina.
Then we decided to compare it with what we knew about the concentrations of these chemicals in residences with coal tarsealed parking lots, and then asphalt parking lots that were not sealed with coal tar.
We're looking at these two bars in the middle. On the left you can see that just having 27 milligrams of dust per day is going to give you an incidental ingestion exposure to PAHs that is significantly higher than typical dietary ingestion for children of the same age.
We felt like that was pretty remarkable. Most studies of PAH exposure show that it's rare to find a source for the general public that will exceed that kind of an ingestion. This is for children that are in the average  children that eat the average amount of dust.
Then if you look at the right, it's about 100 milligrams per day. I should say that 100 milligrams per day was sort of a default value for dust not that long ago. There are a lot of different values that people have settled on, but we feel like these are the most robust that are available to us right now.
As you can see, we're getting up towards 240 nanograms per kilogram per day, compared to in the range of about 24 or right around 25 for dietary ingestion so we did believe, from this initial set of calculations that we could be looking at a significant source of exposure for PAHs for children in residential settings.
As you guys know, PAH exposures, generally you would think that most of your PAH exposures for the general public come from dietary. But if you're looking at PAH exposures that are relevant and likely to cause significant problems, you'd be thinking about smokers. Then on the occupational side, you're thinking about people who work at coke ovens, stuff like that.
We decided to take the next step, and go ahead and take the data that we had and start thinking about what this might mean for people in terms of their cancer risk. We followed the default parameters for Superfund, knowing that coal tar and coal tar pitches, there's quite a bit of epidemiological evidence to support the idea that these are human carcinogens.
Of course as you all know, EPA has had a float factor for BaP and related compounds for quite a while. This is still undergoing revision. There's still an ongoing robust scientific discussion about this topic, but the bottom line is, all of the evidence we have seems to center on the idea that these things do cause cancer in people.
Go on ahead, Barbara, we'll go to the next one.
The next thing we decided to do is  this is a simplification of a larger manuscript  so what we decided to do was set up a hypothetical scenario in which people who lived in these residences adjacent to coal tarsealed pavement were compared to the people who were exposed to concentrations of PAH consistent with parking lots that weren't sealed with coal tar.
Here you can see the differences in the risk. What you see over here on the left is something we would generally refer to as in the area of de minimis. It was mostly right around 10^(6), so right around one in a million. Right around one in a million, as high as three in a million. If you consider upperrange exposures, then you can get something a bit higher.
It is completely dwarfed by the theoretical lifetime cancer risk you would expect to encounter if you lived in an environment that was contaminated with coal tar, or affected by coal tar.
What we're seeing here, is concentrations in soil tend to be a bit higher. Soil adjacent to these spaces tends to be higher. That's not a huge surprise; much more clean as you go inside the home. You can see that the predominant source of risk here is associated with soil.
Even with house dust, we're reaching a level that is in excess of 10^(5), and 10^(4) is generally the kind of range for a Superfund risk assessment which EPA would say, "We're going to have to do something about this and we need to start thinking about it right now." 10^(5) for a general population is kind of a remarkable number.
I should also say, the number that you're looking at right now, this is an average. This is not the reasonable maximum exposure. This is essential tendency exposure, so the average of what we'd expect people to be exposed to.
If you look at the reasonable maximum exposure, which means people are maybe just eating a little more dust or coming into contact with a little more soil, it winds up being about 5x10^(4), which is an eyeopening number, or it would be in a Superfund risk assessment.
What we're also looking at here is a lifetime exposure. Because of the nature of people, some people do live in the same place for their entire life, so we wanted to have a look at this with regard to shorter spans of time.
One of our exposure scenarios, we were looking at children who lived in these kind of settings, basically from birth until right before they turned six years old. We found at that time that these children, in average, were in the 4x10^(5) range, so that's 4 in 100,000.
If you think about reasonable maximums, then we're talking in the area of 3x10^(4), so even just over six years of life in a reasonable maximum exposure, you would expect children to be encountering what most regulatory agencies would consider an unacceptable risk.
We should also note at this time...Barbara mentioned that there's an ongoing scientific debate, and certainly there are individuals out there who disagree with some of our conclusions along these lines, and that's certainly a conversation we're happy to have.
The bottom line is, we did try to be very conservative in the way we've looked at these potential risks. We did a number of things which we felt were consistent with what we felt the actual situation was.
We were encouraged at several points to use exposure parameters and risk assessment parameters  characterization parameters  that would have tended to increase our numbers, but we felt like the ones we were using were probably more reflective of the real situation.
With regard to that, ultimately what the purpose of these two papers in environmental pollution and environmental science and technology  the entire purpose of them  was to point out that the exposures associated with these environments.
These environments that have clearly been affected by coal tar use in adjacent spaces, seems to be associated with a strong likelihood of high PAH exposure for general population, for children in particular. Then we feel like there, under certain circumstances, are significant concerns for additional human health risk.
What we're trying to do here is not to say that many children are going to wind up coming down with leukemia or liver cancer or so forth from this, but simply that at this point we've got to learn more about what we're looking at here.
There is truly not enough data. This is an area that is in dire need of further research. This was a screeninglevel risk assessment based on theoretical exposures, and we would very much like to be able to fully characterize the exposure of children. That's a difficult question for lots of reasons, but that's something that we feel is going to be a critical area of research over the next several years.
With that, I think I'll stop talking about human health. I think at this time Barbara and I will be ready to answer some questions from the audience.
Barbara:  Spencer, I'd like to add one point here, which is that this human health risk analysis did not take into consideration either inhalation of PAHs in the air over the parking lots, nor did it take into consideration dermal contact or nondietary ingestion of the sealcoat particles on the parking lot itself.
This doesn't take into consideration children's play activities, for example, on the seal coated driveway or parking lot.
Spencer:  Yeah, that's absolutely right. This was purely from oral ingestion. The reason for that is that was our strongest dataset. It's also the strongest dataset on exposure parameters as well.
Thank you very much, Barbara.