Let the dust settle

Household dust is one of the principal sources of indoor pollution. Floating dust can be breathed in by the occupants but much settles on floors and surfaces, where it is more of a threat to young children. They spend a great deal of their time on the floor and their frequent hand-to-mouth contact means that they ingest greater amounts of dust.

Settled house dust (SHD) consists of deposited indoor dust, biological material and particles trodden in from outside. It can contain all sorts of chemicals but one group of particular concern is the ubiquitous polycyclic aromatic hydrocarbons (PAHs). Several common PAHs are probable carcinogens and they have been detected in SHD.

Despite a broad knowledge of PAH sources, particularly the incomplete combustion of wood, fossil fuels, tobacco and food, it has been difficult to pinpoint the major source of PAHs in SHD. Now, a new research study carried out in the USA may have come up with the answer and it is all to do with parking lots.

Barbara Mahler and colleagues from the US Geological Survey in Austin and Denver, the Watershed Protection Department of the City of Austin, and Designs4Earth in Manchaca, Texas, were not worried about emissions from vehicles but from the lots themselves. Many parking lots are surfaced with asphalt which is treated after application with a sealant to improve the look and increase the lifetime of the surface.

Of the two common types of sealant used in the USA, one is based on an asphalt emulsion and the other on coal tar. It is known that the two have strikingly different PAH contents. The asphalt-based sealant has relatively low total PAH levels of 50 µg/g but the coal tar-based sealant is far more potent, containing median values of more than 50,000 µg/g PAHs.

This discrepancy is reflected in the PAH contents of dust swept up from parking lots, which contains typically 2.1 and 2200 µg/g PAHs, respectively. So, this is where the researchers focussed their efforts. They analysed dust collected from apartment parking lots with various surfaces and compared the results with SHD from the ground floor apartments adjacent to the lots. Dust collection took place in Austin, Texas, and more than 50% of the parking lots examined were coated with the coal tar-based sealant before it had been banned in 2006.

The dust was extracted for PAH analysis by GC/MS with electron ionisation in full-scan and SIM modes. The contents of the 16 PAHs defined in the US EPA priority pollutants lists were summed, as well as those of 7 PAHs recognised as probable carcinogens.

Dust from coal tar-sealed lots contained median concentrations of total PAHs of 4760 µg/g, about 530-fold higher than any other type of lot tested, in agreement with published work. The asphalt-coated, asphalt-uncoated and concrete-uncoated surfaces had statistically similar low PAHs contents.

The SHD from apartments adjacent to coal tar-coated parking lots had a median total PAH content of 129 µg/g. This value was about 25 times higher than SHD from apartments adjacent to the other types of lot and exceeded reported values for total PAHs in most other studies.

In previous work, some researchers have recognised the high PAH levels in SHD but have been unable to figure out their sources. Mahler and the team thought that coal tar sealcoat "might be that source."

In support of that prediction, a statistical analysis revealed that the absence or presence of an associated coal tar-sealed parking lot accounted for 48% of the variance in total PAHs in SHD. A number of other factors were examined to try and make up the difference and a second factor explained 60% of the difference when considered with the coal tar sealant. This was the intensity of urban land use that considered the contribution of residential buildings, commercial premises, offices, warehouses, streets and roads.

Other factors such as the vehicle emissions, the frequency of vacuuming, indoor fires, the amount of carpets, and the presence of PCs appeared to have insignificant contributions.

The 7 PAHs that are probably carcinogenic also had relatively high median concentrations of 47 µg/g in SHD from apartments beside coal tar-sealed lots. This has serious health implications.

Published work on the non-dietary exposure of preschool children to these PAHs in the USA was based on levels of 1-2 µg/g in SHD, far lower than this new data, greatly under-estimating exposure levels. PAHs in settled household dust represent the greatest source for infants living in apartments in these particular locations and should be taken into account in future models.

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Environmental Science and Technology 2010, 44, 894-900: "Coal-tar-based parking lot sealcoat: An unrecognized source of PAH to settled house dust"