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A Potent Pollutant is Tracked Indoors

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Detailed Description

Carcinogenic compounds in a popular pavement sealer are tracked indoors, where they show up in high concentrations in house dust.

Listen as USGS hydrologist Barbara Mahler explains how she and her team identified the link between polycyclic aromatic hydrocarbons in coal-tar-based pavement sealcoat and house dust.




Public Domain.



[Intro Music]

Heidi Koontz: Welcome and thanks for tuning in to this episode of CoreCast. I'm Heidi Koontz. A new journal article in Environmental Science & Technology contains an article about the occurrence of Polycyclic Aromatic Hydrocarbons or PAHs in house dust. I'm here with one of the article's authors, Barbara Mahler.

Welcome, Barbara.

Barbara Mahler: Thanks for inviting me.

Heidi Koontz: You bet. The new study that appears in the ES&T shows PAHs appear in house dust. How would scientists who study processes of the Earth think to look at dust that collects in people's homes?

Barbara Mahler: Well, it does seem like quite a distance from my principal area of research which is lake sediments to house dust but you could think of this as or simply working our way up the water shed.


To give you a little background on this, I work with the U.S. Geological Survey, National Water Quality Assessment Program, the NAWQA program, to -- is tasked with evaluating water quality in all of the nation's or across the nation in water sheds. And our team addresses one specific objectives which is to determine trends in water qualities, water quality getting better or is it getting worse. And the way we do that is we travel around to lakes across the United States and we collect sediment cores.

Now, sediment core you could think of -- it's really just a big chip of mud and you can think of it acting kind of like tree rings, where as different layers of mud are deposited down in the bottom of a lake, one on top of the other, they're bringing with them from the water shed all the chemicals that are being used in the water shed.

So we can look at the layers of mud and as we look deeper and deeper in the mud, we're going back in time. And we can reconstruct the history of contaminants in the water shed.


So that's kind of the background. What does this have to do with PAHs? Well, what we've noticed, one of the things we're looking for is are there contaminants where we can see a clear decrease or are there contaminants where we can see a fair increase. So this allows us to determine if public policy decisions such as banning DDT have been effective or are there other contaminants where we need to perhaps focus our interest because they're getting worse.

So we do see downward trends in decreasing trends in things like DDT lead which was removed from gasoline in the 1970s. We see decreases in lead. What caught our attention was that there were one group of contaminants, one group of contaminants that were increasing and that's the PAHs. The Polycyclic Aromatic Hydrocarbons were increasing primarily in urban lakes all across the United States.


So we as scientists, we asked ourselves why and that's when we started working our way up the water shed to determine where the PAHs are coming from and why they might be increasing.

Heidi Koontz: So how did PAHs, a known carcinogen, get into people's homes?

Barbara Mahler: Well, as we worked our way up the water shed and we looked, are PAHs coming from roof tops, no, they don't appear to be coming from roof tops, are they coming from vehicle emissions, no, they don't appear to be coming from vehicle emissions because when we started using catalytic converters on cars, decreased PAH emissions of 10 to 50. We don't see that in the cores. So that eliminated that. And then working with some scientists with the City of Austin, they had identified these parking lots sealcoat products as having very high concentrations of PAHs.


Now parking lot sealcoat is the black shiny substance that people spray or paint on their parking lots or driveways to improve the appearance and to attempt to increase the longevity of the asphalt. And what we found was that the PAH concentrations in the dust on these parking lots was extremely high in PAHs. It's much higher than we have seen from any other PAH source including things like used motor oil and tire particles.

So that is when I asked myself the question, well, we know, for example, that children in the '60s -- '50s and '60s were exposed to lead through ingesting lead in house dust. It made us wonder, are the PAHs from these parking lots getting trapped in the people's homes? There have been a lot of studies on PAHs on indoor dusts and established that there are indoor sources of PAHs but that also people trapped in all kinds of stuff from the outdoors.


So it seems logical for us to ask, are people walking on these parking lots and driveways with a very high concentrations of the PAHs and the little particles of sealcoat and are they bringing those into the homes on their feet.

Heidi Koontz: So coal-tar-based sealcoat is used around the country, correct?

Barbara Mahler: Well, interestingly, it's used primarily east of the continental divide, so in the midwest, the northeast and the southeast United States, coal-tar sealcoat is primarily used but west of the continental divide asphalt-based sealcoat is used. And the asphalt-based product has concentrations of the PAHs are about 1,000 times less than are in the coal-tar based products. So the difference is that coal-tar itself is about 50% were more PAHs by weight and the coal-tar-based products contain anywhere from typically 20 to 35% coal-tar.


Heidi Koontz: OK. So if dust in similar locations that used coal-tar-based could probably occur where those sealants are used?

Barbara Mahler: Yes, where I've -- on the bases of our data, it looks very much as though if you live in an apartment complex that has a coal-tar-based sealcoat on the parking lot, that we would expect to see elevated concentrations of PAHs in the dust in your apartment.

Heidi Koontz: OK. So if a person lives near a driveway or a parking lot that is covered by coal-tar-based sealcoat, what should they do?

Barbara Mahler: Well, we haven't done any studies that directly test any of these methods but I have read some other studies that say that using a doormat to wipe your feet will certainly go a long way in decreasing the particles that are or being trapped in from the outdoors. Also, obviously, taking your shoes off would prevent you from tracking particles indoors; that would be another strategy and in fact that would just decrease in general the amount of dust and outdoor particles that are in your residence.


If you have a residence with a coal-tar sealcoat on the driveway, I know that here in the City of Austin there are some folks that have started up business that actually govern that sealcoat off and then another possibility of course would be to put another layer of a different type of product over that sealcoat that doesn't contain PAHs. But, again, I'm not an engineer. I'm not a pavement engineer. These are just kind of some common sense approaches that people in Austin have been using.

Heidi Koontz: OK. So if this has human exposure potential, then indeed fish and fowl would be affected as well, correct?

Barbara Mahler: That's correct. That's in fact one of the motivations that we got into this to the USGS NAWQA program, the focus is on contaminants in water and sediments and exposure of aquatic biota to those.


So we have shown that there's a high concentration of PAHs in particles that are storm-run-off that comes off of these parking lots. And those wash into urban rivers and streams and there are some of the biologists in the City of Austin that have investigated this process and have found a link between run-off from coal-tar sealcoat particles and some impairment of the aquatic biologic community.

Heidi Koontz: Where's your research taking you next?

Barbara Mahler: Well, we would like to pursue this question of human health exposure. We are hoping to collaborate with some others perhaps at the CDC or any academic community that are human-health risk professionals.


As USGS researchers, we can do the environmental-type analysis but we're hoping to link up with people that can really focus on whether these might have some important exposure effects for human. One of the questions we'd like to address is, what is the health risk for adults or children that are playing on the sealcoated surfaces themselves.

So for example, if kids are playing basketball on a sealcoated parking lot or driveway or a playground -- now there are some playgrounds that are treated with the coal-tar-based sealcoat or young children that are sitting on and making chalk drawings or skateboarding, is there a human health risk associated with those activities? That's one are that we think would deserve some investigation. And then another avenue that we're moving to is air quality.


PAHs are large group of compounds that range from low molecular weight to high molecular weight. And a lower molecular weight PAHs are very volatile. That means they evaporate easily. You might have noticed if you walk across a parking lot that is basically been sealcoated, you can smell it. It smells kind of like mothballs, and mothballs contain naphthalene which is a PAH.

So we're doing a volatility study to try and look at how much of the PAHs from a sealcoated parking lot are making it into the air that we breathe.

Heidi Koontz: Thank you for your time, Barbara. And we appreciate you speaking with us today.

Barbara Mahler: Thank you for inviting me.

Heidi Koontz: This podcast is a product of the U.S. Geological Survey, Department of the Interior. I'm Heidi Koontz. Thanks for tuning in.

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