When a laboratory analyses a tar-containing sample for polycyclic aromatic hydrocarbons (PAH), the result is a table of individual values. The eye falls almost automatically on one number: the benzo[a]pyrene content. That is understandable, because benzo[a]pyrene is the best-studied compound of the group and the only PAH that the IARC classifies as carcinogenic to humans (Group 1; IARC 2012, Monograph 100F). As the sole measure of a material's hazard, however, this single value falls short. The toxicity equivalent shows by how much.
The principle: refer everything to one index compound
PAH always occur as a mixture, often of dozens of individual compounds of very different potency. For most of them there are no dedicated, robust cancer-risk criteria. Toxicology therefore uses an approach familiar from the dioxins: each carcinogenic compound is referred, via a toxicity equivalency factor (TEF), to the index compound benzo[a]pyrene, which by definition receives the factor 1. Multiply each measured concentration by its factor and sum, and you obtain the benzo[a]pyrene toxicity equivalent, BaP-TEQ for short: a single number that expresses the mixture's total carcinogenic effect in "benzo[a]pyrene units".
The benefit is immediate: a material can carry a high equivalent despite a moderate benzo[a]pyrene content, if other, highly potent PAH are abundant. Whoever looks only at benzo[a]pyrene misses that.
The factor set matters
There is not one set of such factors. Several working groups have proposed different values over the years, and the differences are not cosmetic. The most common is the set by Nisbet and LaGoy (1992); a frequently used alternative are the provisional factors of the US Environmental Protection Agency (US-EPA 1993). For the same measurement both sets yield different equivalents; how much depends on the material. For a certified coal-tar reference material they differ by roughly 29 percent (see the worked example in the box), and the gap grows the more dibenz[a,h]anthracene a sample contains.
The most important lever is a single compound: dibenz[a,h]anthracene. Nisbet and LaGoy assign it the factor 5; there it counts as even more potent than benzo[a]pyrene itself. Other sets set it lower. In samples where dibenz[a,h]anthracene is present in appreciable amounts, this one choice alone can decide whether dibenz[a,h]anthracene or benzo[a]pyrene dominates the equivalent. The practical consequence is plain: a toxicity equivalent without the factor set stated is not interpretable. A clean finding names the set.
For experts (or the expert-curious): the factor set in detail
The set by Nisbet and LaGoy (1992) assigns the EPA-relevant PAH the following factors:
| TEF | PAH |
|---|---|
| 1 | Benzo[a]pyrene |
| 5 | Dibenz[a,h]anthracene |
| 0.1 | Benz[a]anthracene, benzo[b]fluoranthene, benzo[k]fluoranthene, indeno[1,2,3-cd]pyrene |
| 0.01 | Anthracene, chrysene, benzo[g,h,i]perylene |
| 0.001 | Naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene, fluoranthene, pyrene |
The distinctive point is the factor 5 for dibenz[a,h]anthracene; the provisional US-EPA factors (1993) set the same compound at 1.
Running this scheme on the certified reference material NIST SRM 1597a, a PAH mixture isolated from coal tar with certified individual contents (NIST 2012), gives:
| Quantity | Value |
|---|---|
| Benzo[a]pyrene alone | 93.5 mg/kg |
| BaP-TEQ after Nisbet and LaGoy (1992) | ≈ 159 mg/kg |
| BaP-TEQ after US-EPA (1993) | ≈ 123 mg/kg |
Two things become visible. First, the toxicity equivalent is 1.7 times the pure benzo[a]pyrene value: whoever reads off only benzo[a]pyrene underestimates the total carcinogenic effect here by about 40 percent, and that for a material in which benzo[a]pyrene, at just under 59 percent, already provides the largest single contribution. Second, the two factor sets differ by roughly 29 percent. A good three quarters of the difference is due to dibenz[a,h]anthracene (factor 5 versus 1), although this compound makes up only about 7 percent of the benzo[a]pyrene mass; yet it contributes a good fifth of the equivalent. In tar-containing materials with a higher dibenz[a,h]anthracene share the effect is correspondingly larger.
Two subtleties for a careful reading: the certified contents refer to the reference material (coal tar in toluene), not to a solid building material; for the comparison of the factor sets this is irrelevant, because the ratio of the equivalents does not change on dilution. And what is meant is exclusively the isomer dibenz[a,h]anthracene of the EPA list, not the separately certified isomers a,j and a,c.
The sixteen EPA PAHs are not everything
Almost every standard analysis determines the "16 EPA PAH", a list that the US Environmental Protection Agency assembled in the 1970s for reasons of measurability and availability. It is a proven but incomplete selection. Precisely in pyrogenic and tar-containing materials, PAH outside this list contribute substantially to toxicity, foremost the dibenzopyrenes and 7H-benzo[c]fluorene. Richter-Brockmann and Achten (2018) show, on petrogenic and pyrogenic environmental samples, that non-EPA PAH can account for between roughly 69 and 95 percent of the total toxicity equivalent. An equivalent based on the list of sixteen alone therefore systematically underestimates the real burden of such materials, not by percentages but by a multiple.
For experts (or the expert-curious): beyond the EPA-16
The 16 EPA PAH contain, with dibenz[a,h]anthracene, only one of the highly potent more-highly-condensed systems. The dibenzopyrenes, in particular dibenzo[a,l]pyrene, as well as 7H-benzo[c]fluorene and individual methylated compounds (such as 5-methylchrysene) have equivalency factors partly well above 1, yet do not appear in the standard list. In tar-containing materials formed by high-temperature pyrolysis they are regularly present. Richter-Brockmann and Achten (2018) quantify their contribution at 69.3 to 95.1 percent of the total TEQ. Methodological consequence: a value limited to the EPA-16 is a lower bound, not a realistic estimate of the total toxicity; when assessing tar-like materials this must be noted explicitly.
An equivalent is not a risk statement
Useful as the toxicity equivalent is, it remains a concentration quantity, a property of the material, not of the person in front of it. It says how carcinogenic a kilogram of the substance acts compared to pure benzo[a]pyrene. It says nothing about whether and how much of it actually enters a body. That depends on the exposure pathway: whether the material is built in and sealed or exposed, whether dust is mobilised, whether volatile fractions such as naphthalene outgas, how long and how close people are present. A high equivalent marks a heavily burdened source and thus a reason to act; the actual risk assessment additionally needs the path from material to person.
What this means for assessment
From this follows why a robust PAH assessment is more than reading off a benzo[a]pyrene value. It states the sum and the toxicity equivalent, it names the factor set, it places what the EPA list may not capture, and it distinguishes cleanly between material burden and exposure. Only then can a finding be reliably placed within the legal framework, in Austria for example the classification of tar-containing waste as hazardous (Waste Catalogue Ordinance 2020; EU waste code 17 03 03*, hazard property HP 7).
Where PAH occur in buildings at all, and why not every black adhesive is tar-containing, is covered in the article PAH in old buildings; it also places the tar-containing damp-proofing in masonry. Which pollutants are to be expected in which building era is set out in Pollutants by construction year.
Sources
- IARC/WHO (2012), Monograph 100F: benzo[a]pyrene as a Group 1 carcinogen (the only PAH classified as such).
- Nisbet, I.C.T. & LaGoy, P.K. (1992): Toxic equivalency factors (TEFs) for polycyclic aromatic hydrocarbons (PAHs). Regulatory Toxicology and Pharmacology 16: 290–300.
- Richter-Brockmann, S. & Achten, C. (2018): Analysis and toxicity of 59 PAH in petrogenic and pyrogenic environmental samples including dibenzopyrenes, 7H-benzo[c]fluorene, 5-methylchrysene and 1-methylpyrene. Chemosphere 200: 495–503.
- US-EPA (1993): Provisional Guidance for Quantitative Risk Assessment of Polycyclic Aromatic Hydrocarbons (provisional equivalency factors, EPA/600/R-93/089).
- NIST (2012): Certificate of Analysis, Standard Reference Material 1597a: Complex Mixture of Polycyclic Aromatic Hydrocarbons from Coal Tar (certified mass fractions, Table 1): tsapps.nist.gov.
- Waste Catalogue Ordinance 2020 (Abfallverzeichnisverordnung, BGBl. II No. 409/2020), waste group 17 03, RIS: ris.bka.gv.at.
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