When you lift a parquet board in an old building, there is sometimes a black, viscous, tar-like layer beneath it. This adhesive is among the most common pollutant findings in renovations, and it has a geochemical backstory. Polycyclic aromatic hydrocarbons, PAH for short, are a group of more than a hundred compounds made of fused carbon rings. They form when organic material burns incompletely, and they are concentrated in coal tar, a distillation product of coal. It was precisely this tar that was used for decades as adhesive and sealant.
The most important qualification right away: not every black adhesive is tar-containing. Bitumen adhesive too is black and viscous, but contains considerably less PAH. The two cannot be distinguished by eye. A tar-like or mothball-like smell (naphthalene is the most volatile PAH) can be a hint, but is not proof. Only a laboratory analysis gives certainty.
Where PAH occur in buildings
- Parquet adhesive: the black layer under old strip parquet, glued with coal tar, especially in the 1950s to 1970s. Such tar adhesives reach very high PAH contents; values of over 20,000 mg/kg are documented.
- Roofing felt and seals: tar-containing membranes on flat roofs and in basements.
- Tar coatings: black coatings on basement masonry as a damp barrier.
- Mastic asphalt screeds: in some old buildings as a floor base.
Why PAH are dangerous
PAH do not stay in the material. They off-gas slowly over decades and accumulate in house dust. Benzo[a]pyrene, the best-studied individual compound, is classified by the IARC of the WHO as a Group 1 carcinogen, that is, as carcinogenic to humans. It is the only PAH with this classification and therefore serves as the marker substance for the whole group. In the body the liver converts PAH into water-soluble substances, and in the process reactive intermediates arise that bind to DNA and can trigger mutations. Mainly affected are the lung, skin and bladder. PAH are not only inhaled but also taken up through the skin.
When PAH become a problem
As with most building pollutants, the condition is decisive. An intact, sealed floor over tar-containing adhesive releases little. Exposed tar adhesive in living spaces is grounds for action. It becomes acutely dangerous when sanding, milling, drilling or ripping out without protective measures: this creates fine dust to which the PAH bind, it spreads throughout the dwelling and settles permanently in the house dust. Rooms that were previously unburdened are only then contaminated.
How to determine PAH
Since tar and bitumen look identical, the path leads through a material sample. A small piece of the adhesive or sealant is examined in an accredited laboratory; assessed are the content of the marker substance benzo[a]pyrene and the PAH sum. The result is available within a few days and says whether and at what level PAH are present; from this follow protective measures and disposal route. Sampling on one's own is not advisable: anyone scratching at suspect material creates dust and risks skin contact. Such work and the sampling belong in expert hands.
The legal framework
Tar-containing construction and demolition waste is recorded in Austria in the Waste Catalogue Ordinance 2020 (Abfallverzeichnisverordnung), in waste group 17 03 (coal-tar and tar-containing materials). The tar-containing fractions, such as bitumen mixtures containing coal tar (waste code 17 03 01*) and coal tar itself (17 03 03*), count as hazardous waste; the decisive hazard property is HP 7 (carcinogenic). For removal, worker protection and the Limit Values Ordinance apply. There is no binding limit for PAH in indoor air in Austria; in practice German assessment values serve as orientation, such as the PAH guidance and the specifications of the trade association, which provide for remediation relevance from 50 mg/kg benzo[a]pyrene in the material.
What to do
- Floor intact, no smell: no acute need for action. Document the location and take it into account at the next renovation.
- Before a floor renovation in an old building: if the building was erected before about 1980 and old floor coverings are to be removed, clarify beforehand what is stuck under the parquet. A sample is a fraction of what a subsequent remediation costs.
- Tar adhesive found: do not sand, mill or rip out yourself. Proper removal is done with containment, negative pressure, extraction and protective equipment by a specialist firm.
- Until clarified: cover the exposed area, do not work on it further, and avoid skin contact.
PAH is not the only pollutant that turns up in old-building renovations. In buildings from before 1990 there is often also asbestos in floor coverings and adhesives, and in houses from before 1970 lead in paints and pipes.
Sources
- IARC/WHO, Monograph 100F: benzo[a]pyrene as a Group 1 carcinogen (the only PAH classified as such)
- Waste Catalogue Ordinance 2020 (BGBl. II No. 409/2020), waste group 17 03 (coal-tar and tar-containing waste), RIS: ris.bka.gv.at
- German Environment Agency (UBA) and the Construction Industry trade association, PAH guidance and remediation guide values (benzo[a]pyrene as marker, remediation relevance from 50 mg/kg)
- allum.de, „Polyzyklische aromatische Kohlenwasserstoffe (PAK)": allum.de
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